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Missouri Master Gardener Core ManualPatricia Hosack and Lee MillerDivision of Plant Sciences

The first and best defense against plant diseases is a healthy plant, which is the main task of an accomplished gardener. Preventing and managing plant disease begins even before planting, with site preparation and plant selection.

When a plant does not look normal, or as expected, a gardener may assume that the plant is diseased and control measures are needed. To properly diagnose plant problems, the gardener needs to have background knowledge about the plant, the current environment, and the typical diseases or other problems to which the plant is susceptible. Such information can help prevent an inaccurate diagnosis that may lead to unnecessary pesticide use, wasted time and expense, and continued plant decline.

This publication provides gardeners with information on how to establish and maintain healthy plants, and describes a systematic approach to identifying and solving problems that do occur.

A plant disease is defined as a malfunction in the plant in response to continuous irritation by an infectious causal agent, also known as a pathogen. A plant disease can cause many types of symptoms that may affect the plant's ability to yield, reproduce or grow properly.

Diagnosing a disease can sometimes be difficult, and differentiating between a true disease and an abiotic disorder is crucial to developing an effective management plan. The causal agents of plant disease are biotic, or living, and are called pathogens. Abiotic disorders are caused by abiotic, or nonliving, factors. Understanding the difference between the two is crucial to diagnosing the cause of plant damage.

Even if a disease is confirmed, the problems caused may be cosmetic or cause minor yield reduction, making costly control measures unwarranted and not worth the expense or bother. In other situations, a disease might weaken a young plant but have little effect on older, well-established plants.

Plant diseases often provide helpful clues to the underlying problems that made a plant susceptible. These problems might include poor site selection, nutrient imbalance, water stress, or improper mulching, irrigation or pruning practices. In many cases, if you can address the underlying cause of the plant's problems, the disease process will be thwarted, and the plant can regain its health and vigor to resist such problems in the future.

When control measures are required, you must decide which management techniques are most appropriate. An integrated pest management, or IPM, strategy is most prudent and effective because it involves employing a combination of management techniques. Cultural practices and plant selection are the first line of defense. Pesticides may be required and can be a part of an IPM program, but should be viewed as a last resort. Pesticides are often overused, particularly when one simply wants to solve a pest problem quickly rather than understand why it occured. When pesticides are needed, select the least toxic product that is designed for that specific plant and disease.

When pesticides are necessary, follow the recommended application methods and rates described on the pesticide label. A little extra is definitely not better when it comes to the application rate. Repeated use of some pesticides can cause the target organisms to develop resistance, which could make future applications less effective. In some cases, pesticides can also harm human health, the environment, or nontarget organisms, including birds and beneficial insects that might help keep other plant problems in check.

A triangle is often used to illustrate how plant diseases occur. A disease will only occur when three conditions are present, as represented by the three sides of the triangle (Figure 1):

A disease will only develop in the presence of all three conditions. The presence of the pathogen is the first condition, but there is considerably more to disease development. The likelihood for disease on a resistant plant is greatly minimized, so plant selection can be a key factor in disease management. Lastly, environmental conditions must be conducive for the disease to occur. These conditions allow for pathogen growth and reproduction while reducing plant vigor and predisposing the plant to infection. For example, a sun-loving plant grown in shade will be less vigorous and therefore susceptible to attack, and the shade will extend the leaf wetness period, creating favorable conditions for foliar disease.

The best management approach is to exclude any of the three conditions that form the triangle sides. Keeping these conditions in mind help will help you gain insights into plant diseases and their control.

Figure 1Plant disease occurrence triangle.A disease that has a biotic cause is only likely to occur when three conditions are present.

The disease cycle is another important concept that describes the life cycle of a pathogen and the chain of events involved in disease development (Figure 2). If the spread of inoculum can be prevented, the disease can often be managed.

A typical disease cycle includes the following events:

Depending on the disease, inocula are most commonly fungal spores, or mycelium; bacterial cells; viral particles; or individual nematodes. These can reside in seed, crop residue, soil, weeds or other crops. Inoculum may be spread by the wind, by water splashing during irrigation or rainfall, or by a human action such as pruning with infected shears. Inoculum may also be carried by vectors, often insects, that feed on an infected plant and transmit the disease to a nondiseased plant.

Pathogens in temperate climates must have a way to survive the winter when their host plants are dormant or absent. Considering how these pathogens overwinter can help identify what control measures will be most effective. In perennial plants, some pathogens can live through the winter in infected plant parts, such as roots, bulbs, stems and bud scales. Pathogens that infect annual plants must form resistant resting structures, survive in seeds or vectors, or spread from warmer regions where the host plants grow during the winter.

Common rust of sweet corn, Puccinia sorghi, is an example of a disease spread by wind. This fungal disease does not survive long outside of living plant tissue. Because sweet corn plants do not live through cold Midwestern winters, most of the sweet corn rust inoculum (as fungal spores) blows north each season from living corn plants in the South. Thus, an understanding of how much inoculum is present in the South influences management decisions farther north.

Insect pollinators aid the spread of fire blight of apple and pear caused by the bacterium Erwinia amylovora. The bacteria overwinter in the margins of old cankers, and exude from the stem in rain. A bee or other pollinator species may pick up the bacteria and serve as a vector, introducing the pathogen to a new plant through the flower. Although trying to control the vector is unwise, the old cankers can be carefully pruned out and disposed of to limit the initial source of inoculum.

Figure 2Example of a disease cycle.Typical disease cycle of anthracnose caused by Gnomia spp.

Abiotic plant disorders are not directly associated with a living organism, but instead are damage caused by a physical, environmental or chemical factor. Often, samples received by plant diagnostic centers have problems that are primarily caused by an abiotic factor. Many other plant samples do have a plant disease or pest problem, but also have an underlying abiotic disorder that made the plant more susceptible. For example, many plants have distinct habitat preferences and will easily develop problems if grown in an unsuitable location. In such circumstances, abiotic factors will make a plant more susceptible to infection by the biotic disease organisms discussed in the next section.

People, rather than insects or diseases, are often responsible for a plant's problems. Plant problems caused by people can be categorized as physical or mechanical. These problems include poor planting methods that allow limited area for root growth, improper mulching, construction-related injury, soil compaction, girdling of stems or trunks, or improper pruning. For example, plants should be pruned in the fall, just prior to dormancy. Pruning during the growing season can injure the plant and, if infected purning shears are used, introduce a pathogen to the open wound.

Storms that produce high winds, heavy snow, or ice can cause considerable tree damage. Damage from hail or lightning strikes can kill trees, crops and ornamental plants. However, plant death resulting from a moderate weather event is often the sign of a preexisting condition.

In some cases, physical problems can be corrected and the plant will recover. For example, proper pruning to remove torn limbs might allow a tree to recover from minor damage after a storm. Plants that were given a bad start through incorrect planting methods, however, often cannot be saved. By the time symptoms appear, you may be unable to address the cause and restore plant vigor.

Environmental factors are the most common source of a plant disorder. Often, symptoms develop on one side of the plant, or group of plants, based on where stress occurred (Figure 3). Other times the entire plant may be affected.

Extremes in temperature and moisture are common environmental culprits. Drought stress can cause leaf scorch, leaf drop or even branch dieback. Cold injury in winter can cause leaf burn and dieback of evergreens. When the soil is saturated for many days during the growing season, plants may develop yellowed foliage because of the lack of oxygen in the soil or poor nutrient uptake from nonfunctioning roots.

Too much or too little shade is a typical problem. For example, hydrangeas commonly wilt and scorch when they are not mulched and watered carefully to keep the soil moist during dry conditions. They do best in a location with afternoon shade that alleviates the effect of high summer temperatures. In contrast, lilacs or junipers will be stunted if planted in too much shade.

Certain plants have a fairly specific range of soil conditions in which they thrive. These plants will have problems if grown in soil that has a nutrient imbalance or an improper pH. The interplay between soil pH and nutrient availability is important for plant growth, so a complete soil test can be helpful in diagnosing a potential plant disorder. Pin oak and blueberries, for example, like acidic soils and commonly develop leaf chlorosis when the soil pH is neutral or alkaline. Conversely, soils that are exceedingly alkaline may become deficient in nutrients such as iron and zinc. Also, if you fertilize every year with a complete fertilizer containing nitrogen (N), phosphorus (P) and potassium (K), the P or K may eventually build up in the soil and interfere with uptake of other micronutrients, such as magnesium, manganese and iron. Thus, fertilizer applications of most nutrients (other than nitrogen) should be made based on a soil test.

Generally, plants have a limited geographic range where they will grow and perform well. Many plants are simply poor choices for temperate Midwestern growing conditions or for the specific site where they are planted. In such cases, manipulating environmental conditions, applying pesticides or attempting other control measures may still not result in a healthy plant. Selecting plants well suited to the local environment gives you the best chance of having thriving, disease-resistant plantings. Become familiar with the plant zone you live in (see "plant hardiness zone maps" under related websites). In Missouri, these zones have shifted significantly in the past 15 years, with most of Missouri now in Zone 6.

Figure 3Symptoms of abiotic plant injury.Injury from nonliving, environmental factors typically occurs on one side or area of a plant or group of plants.

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Nontarget effects from chemicals in the environment may also cause abiotic disorders in plants. Pesticide and, more specifically, herbicide injury is the most common cause of phytotoxicity, with symptoms varying depending on the product used. The most common symptoms are leaf cupping and distortion caused by either spray drift on foliage or root uptake by ornamentals and vegetables. Broadleaf weed killers applied to nearby lawns or crop fields can cause sudden decline in sensitive crops, such as tomatoes or peppers, and can cause leaf curling in ornamental trees when applied incorrectly (Figure 4).

Other chemical causes of abiotic injury include ice-melting salts or air pollutants. Ice-melting salts that wash off sidewalks and streets onto plants and soil often cause severe wilting or browning of leaf margins of trees, shrubs or turfgrass. Air pollutants that damage plants include sulfur dioxide and hydrogen fluoride from industrial sources. Incompletely burned hydrocarbons released from automobiles in heavily populated areas can result in production of both ozone and peroxyacetyl nitrate, known as PAN. These harmful gases enter plants through the stomata and cause a characteristic flecking or bronzing of leaves.

Figure 4Growth regulator herbicide injury to a maple following an improper herbicide application.

Fungi are the most common causal agent of plant disease. These microscopic organisms lack chlorophyll and are visible as mats of threadlike filaments called hypha that make up the mycelium, which are "resting structures" that include rhizomorphs and sclerotia. Many fungi reproduce by spores and produce conspicuous fruiting bodies that can aid in identification. These fruiting bodies are called the signs of the pathogen.

In the diagnostic lab, fungi are often identified by their growth patterns, spores or other structures. The first step is to examine infected plant tissue for signs of the pathogen with a hand lens or under the microscope. Because fungi are not always visible on plant surfaces, a lab may then test a sample by placing the affected tissue on a petri plate that contains a nutrient medium. If fungi are present, they may grow and produce the signs necessary for identification.

Fungal organisms cause various types of injury to plants. Typical fungal symptoms include seed rot, seedling blights, root and crown rots, vascular wilts, leaf spots, rusts, cankers, and stem and twig blights.

On leaves, fungi often cause lesions, or spots. The appearance within the lesions of mycelium, spores or small black dots visible with a hand lens indicates a potential fungal disease. Not all leaf spots require control measures. Fungal leaf spots can be managed by growing resistant cultivars or using cultural practices that limit the development of disease. Limiting overhead irrigation, and therefore leaf wetness duration, is an effective cultural practice for minimizing the occurrence of leaf spot diseases.

Blights the complete death of a plant structure, such as leaves, flowers or stems may result from many lesions that form quickly and merge. Blight diseases often occur rapidly and cause severe damage. One well-known historical example is late blight, a disease of tomatoes and potatoes that attacks stems and leaves, potato tubers and tomato fruits. This disease played a major role in the Irish famine that caused a wave of emigration during the 1800s. Cultural control measures, resistant varieties and fungicides are used to manage fungal blights.

Rots can occur on most plant parts but are most commonly seen in roots, stems and fruits. The rot that results from seedlings being attacked by soilborne fungi is commonly called damping-off. Damping-off occurs most frequently in a contaminated growing medium that is too wet. Careful watering practices and the use of sterile pots and uncontaminated, soilless seedling mixes are the most practical and effective preventives for root and stem rots. Do not reuse potting soil.

Cankers appear as sunken areas or spots where the bark is rough, missing or swollen. Sometimes sap will ooze from these areas, and a raised ring of callus material appears as the plant tries to protect the damaged area and limit disease spread. If the canker surrounds, or girdles, the stem completely, the stem or branch above will die. Canker diseases are difficult to manage. To slow their development and spread, practice good horticultural care to reduce plant stress and remove affected branches.

Vascular wilts are caused when fungi grow inside the plant vascular, or fluid-conducting, tissue, causing these tissues die. The leaves and branches. wilt and die from a lack of nutrients and water, with symptoms similar to those caused by drought. Dark streaks may be visible in the vascular tissues where fungi are active. Plants with severe vascular wilt infections usually cannot be saved, but adjacent plants can sometimes be protected with fungicide injections.

Bacteria are single-celled organisms that lack chlorophyll and reproduce by cell division. Bacterial cells often multiply quickly and clump together to form colonies. Thus bacterial diseases can begin suddenly and quickly become severe. Some types of bacteria are easily moved around in leaves and cause leaf spots. Others can multiply rapidly in the vascular system and plug it up, causing wilting and dieback. Bacterial diseases are difficult to manage because few chemical controls antibiotics, in this case are available and bacteria often rapidly develop resistance to them.

Some types of bacteria cause tumorlike galls. Crown gall is a common disease of many plants that occurs when soilborne bacteria cause lumpy swellings on roots and the lower stems (Figure 5). It may be seen on euonymus, grape vines, roses and fruit trees. Infection often occurs where a plant has been wounded or weakened. Cultural practices such as good sanitation and avoidance of wounding by mower or weed trimmer use can help prevent crown gall.

Several foliar diseases can be caused by bacteria. For example, leaf lesions and blight can develop, and sometimes a yellow halo may form around the margin of leaf lesions indicating a plant toxin. In some instances, the tissue falls out of the leaf, which gives it a shot-hole or ragged appearance. Under a microscope, bacterial ooze from a lesion may be evident. This ooze contains millions of bacteria that easily splash to healthy leaves in water droplets. The bacteria may enter plants through natural openings such as stomata or hydathodes in or through wounds.

Some bacterial diseases cause blights, such as fire blight, a common disease of apples, pears and related species in the Midwest. Typical symptoms include wilted shoot tips, where succulent new shoots droop, forming a characteristic shepherd's crook, and tips turn black as they dry. Fire blight infections are typically most active in spring when insects spread the infection as they pollinate flowers. The bacteria can enter plants through nectarthodes in blossoms. Splashing rain and pruning or other wounding events can also spread the disease. To manage fire blight, use good cultural practices and select resistant cultivars.

Bacteria can also cause soft rots of fruits, vegetables, tubers and bulbs. Rots can cause rapid decline in crop quality. Affected plants often have a strong odor and mushy tissues that appear melted. Avoid mechanical injury both before and after harvest, and practice strict sanitation to help reduce the incidence of soft rots.

Plant pathogenic bacteria can be difficult to kill when protected inside the plant. To protect healthy plants, you can manipulate environmental conditions, remove infected plants, or apply protectant pesticides. Good sanitation practices are especially important to prevent problems, because a single infected seed can result in an entire tray or even an entire greenhouse of diseased plants. Antibiotics are not normally recommended for home garden use because of the potential for antibiotic resistance.

Figure 5Crown gall symptoms. Crown gall is caused by Agrobacterium and related bacteria. The lumpy swellings typical of the disease are usually seen on roots or lower stems.

Virus particles consist of a small amount of genetic material within a protective protein coat called a capsid. Viruses are so small that individual particles cannot be seen with a common light microscope. When a plant cell becomes infected with a virus, that cell replicates new viral particles that prevent normal plant cell function.

Diagnosis of viral diseases can be challenging. Visual identification is difficult, and advanced identification techniques are expensive. Typical viral symptoms include stunting and chlorosis, as well as mottling, puckering, ring spotting and mosaic patterns in leaves. In the lab, virus species may sometimes be identified by their physical characteristics when viewed at extreme magnification with a high-powered electron microscope. In other cases, advanced serological or genetic testing of plant sap is needed to confirm diagnosis.

Viruses are spread by infected seed or pollen, poor sanitation when handling or pruning plants, or vectors. The mode of transmission depends on the type of virus, but most commonly arthropods, such as aphids or mites, serve as a vector. Unfortunately, there is no cure for viral diseases. If the virus causes severe symptoms and has potential to spread to nearby plants of the same species, the infected plants should be destroyed. Other control measures include destroying nearby weedy hosts, practicing good sanitation techniques during pruning and propagation, and managing insect vectors.

Phytoplasmas are essentially tiny, specialized bacteria that lack cell walls. They can be difficult to identify because they only survive and reproduce in living plant tissue. They cannot be isolated and cultured in a laboratory. An electron microscope is needed to detect structures of phytoplasmas in the cells of host plants. For many years, diseases caused by these organisms were thought to result from viruses, because the symptoms appear very similar.

Aster yellows is a phytoplasma-caused disease that affects many landscape and garden plants. Affected plants often develop stunted, malformed plant structures and appear chlorotic, or yellowish. Unfortunately, like viral diseases, plant diseases caused by phytoplasma have no cure. Control measures include removal of the infected plants and nearby weedy hosts, and control of leaf hoppers and other insects that may act as vectors.

Nematodes are unsegmented, microscopic roundworms that generally have a threadlike form. Nematodes are the most numerically abundant animal on Earth, and luckily not all nematodes are parasites or harmful to plants. Some are beneficial and kill plant pests, whereas others feed on bacteria or decaying matter. A plant parasitic nematode has a needlelike stylet, which is a tubelike structure that can pierce plant cells to withdraw nutrients.

Some nematodes live inside plants. One example is the pine wilt nematode that is responsible for the death of many Scots pines across the Midwest. Trees become infected when the vector, the pine sawyer beetle, feeds on the tree and also transmits the nematode. Affected trees quickly turn brown and should be destroyed to prevent infection of nearby healthy trees. To confirm the presence of pine wilt nematodes, a plant diagnostic clinic can test a portion of a large branch or tree trunk.

Nematodes that live in the soil sometimes cause severe plant damage. In Missouri, the root-knot nematode is prevalent in the southeast area of the state. In recent years, this nematode has been found farther north into central Missouri, perhaps because winters have been mild by historical standards. This nematode causes swollen knots at infected sites on the roots of a wide variety of plants, including certain fruits, vegetables and ornamentals.

Commercial growers can use soil fumigants to manage nematodes in the soil, but homeowners have few management options. Sanitation is important because nematodes are easily spread with infested soil or plant material. Dirty gardening tools, such as shovels or tillers with infested soil, can spread nematodes to new areas. Luckily, nematode damage is not a widespread problem for home gardeners in Missouri.

To accurately diagnose a plant problem and find its remedy may seem like a daunting task. In some cases, identification may require help from plant disease specialists. Before turning to the experts, however, attempt to make a diagnosis yourself. At the very least, gather evidence on potential symptoms, signs and potential abiotic stress. Even if the result is not definite, the process is a learning experience that will provide useful information.

When diagnosing plant problems, pay close attention to detail when collecting information, like a detective attempting to solve a crime. Items that are most helpful include a 10-times-magnification hand lens, digital camera, trowel, pruning shears, pocketknife, flashlight and something to keep notes on. Establish a location to keep records and reference materials.

Determine the most likely cause by following these five steps:

First, know the plant. Every species, variety or cultivar has a unique set of characteristics that often provide important clues to identifying the source of a problem. For potential abiotic disorders, consider the plant's preferences for soil and climatic factors such as pH, nutrient requirements, soil type, moisture level, light intensity, and temperature. Also realize that each plant species, and even different cultivars, may have plant diseases that are specific and troublesome.

If the identity of a plant is unknown, you can consult references such as those suggested at the back of this guide, or any available gardening or landscape records. Garden centers usually have someone who can help identify a plant if you bring in a stem with several leaves. Local extension centers or the University of Missouri Plant Diagnostic Clinic can also be a good resource. Most states provide similar services.

References can help you determine whether a plant is located on a site that matches its requirements. For example, a flowering dogwood tree is adapted to a woodland understory environment with excellent drainage. It is unlikely to thrive if planted in a poorly drained soil or on a south-facing slope in full sun. If the tree survives in such circumstances, it is likely to develop leaf scorch and damage from dogwood borers attracted to the stressed tree. Such problems often result from an unsuitable planting site and are unlikely to be resolved with pesticides or other treatments.

Read plant descriptions and observe other plants of the species, variety or cultivar to determine the normal appearance for the plant. Sometimes a natural feature of the plant is mistaken for a symptom. For example, someone unfamiliar with the 'Golden Vicary' privet might mistake this cultivar's yellow leaf color for a sign of nitrogen deficiency. Similarly, a plant with a splotchy pattern on a leaf may be a variegated cultivar. A gardener unfamiliar with paperbark maple might be alarmed to see sheets of bark peeling from the trunk of a specimen, though it is a normal process for this plant. Conversely, bark peeling from the lower trunk of a red maple would be a legitimate cause for concern.

It can also help to observe other plants of the same species of roughly the same age and at the same time of year as the sample being evaluated. For example, during hot, dry weather, mature river birch trees often drop a significant portion of their leaves as a drought-survival mechanism. For pines, yellowing of the interior needles in the fall is likely to be part of the normal process of shedding 2- or 3-year-old leaves.

Like all living organisms, plants have life spans, with some having longer ones than others. A bur oak may live 300 years, but it is relatively rare to find a redbud older than 30. Trees late in their expected life spans often succumb to trunk decay, root rots, stem-boring insects or other pests that normally do not attack young, vigorously growing plants. If a plant has reached its normal life expectancy, you can only do so much before having to remove and replace it.

Learn the common problems that affect the plant in question. Good reference materials can help as you match your observations with descriptions or photographs of typical plant diseases, and their related symptoms and signs.

A diagnostician learns to look for indications of problems that commonly affect certain species. Tall fescue is commonly damaged by brown patch, whereas Kentucky bluegrass is more frequently damaged by Pythium blight or dollar spot. Austrian pine trees are often affected by Diplodia pinea (also known as Sphaeropsis sapinea), a fungal tip blight that kills needles near the tips of lower branches. Zinnias, lilacs and zucchini are all commonly afflicted by powdery mildew. Red maple trees often display a leaf distortion caused by leaf hoppers. They also frequently suffer from chlorosis, indicated by yellow leaves with green veins, a condition that is frequently due to high-pH soil with little available manganese and iron. Learning these relationships may come from online or library research, discussion with someone at your garden center or plant source, or from the hard teachings of experience.

Observe carefully to determine whether a plant problem has been caused by a living, biotic, organism or by some type of nonliving, abiotic, factor. By studying the cultural preferences of plants and looking for patterns in the landscape, you may be able to determine the cause of the plant damage.

Other than a characteristic plant symptom or pathogen sign, several clues may help determine if the problem is the result of a plant disease rather than an abiotic disorder (Table 1).

Table 1Distinguishing between biotic and abiotic factors in plant damage.

Understanding symptoms and signs and the differences between them will help with disease diagnosis and allow for discussion with others. Symptoms are the plant's response to infection, or the signals that a plant is not functioning properly. Typical symptoms include leaf lesions, chlorosis, or malformed plant tissues. Signs are the visible parts of the pathogen or pest that caused the symptoms. Signs of a pathogen may include mold on the plant surface; spores; pycnidia, which are small flask-shaped structures that contain spores; or bacterial ooze.

Consider a typical blue spruce in Missouri. A common disease of this species is Rhizosphaera needlecast. To confirm the disease, you would first look for symptoms. Specifically, you would see the dead needles at the lower portions of the branches, because the disease attacks mature needles. Other diseases can also cause older needles of blue spruce to drop, so at this point, you would use a hand lens to further examine the brown or dropped needles, looking for signs of the fungus. Healthy spruce needles have rows of stomata that appear as white dots. In a tree with Rhizosphaera needlecast, pycnidia, which appear as small black bumps, emerge from the stomata (). Another fungal disease, Stigmina needle blight, also produces fungal structures in the pycnidia. However, these two diseases are managed similarly, with pruning and, in severe cases, preventive fungicides.

Figure 6Example of symptoms and signs.This spruce has browning needles and defoliation (left). The pathogen attacks mature, at least 1-year-old, needles, so the new growth at the tip is unaffected. On close inspection of the needles, numerous pycnidia can be seen emerging from the stomata (right).

Using the five steps described above to diagnose plant problems is like putting together a puzzle (Figure 7). If you can find enough pieces and fit them together, you will often see a logical picture emerge. Sometimes this process is called the guess-and-confirm method. With practice and experience, diagnosis becomes progressively easier.

Good reference materials can be a great help in the process. Sources of information and pictures include websites, textbooks, extension publications and professional and trade journals. Related MU Extension publications are listed at the end of this publication.

If a plant disease problem still has you stumped after following the steps to diagnosis, you might decide to call on experts. You could take a sample to a local garden store or extension center, where a quick consultation might answer your questions. You could also send a sample to a plant diagnostic laboratory.

To identify plant diseases and disorders, diagnostic labs use a variety of techniques. In many cases, diagnosis will be relatively simple because the lab is familiar with the problem, having previously seen many plants with the same disease.

With a more challenging sample, or when identifying an unfamiliar disease, a diagnostician may use a taxonomic approach that includes the main steps of isolating the suspect pathogen, identifying it, and then confirming it is the causal agent of disease. Using this approach can be time-consuming and incur additional testing fees. Sometimes a lab uses other advanced testing methods or sends a sample for retesting at another plant clinic that specializes in certain techniques or specific pathogens.

Figure 7Plant disease puzzle.To accurately diagnose a plant disease, a gardner must consider many factors that could be causing the disorder.

Most states have a university or state plant diagnostic lab. The University of Missouri has the Plant Diagnostic Clinic (see related websites). You can obtain the appropriate submission form to submit with a sample on the clinic's website or from your local extension center. The form asks for detailed information. To aid in a quick and accurate diagnosis, fill out the form as completely as possible.

The quality of the sample is crucial. When submitting small plants, it helps to include several samples that show a range of symptoms from the healthy to the severely damaged. When possible, submit an entire plant. If that is not practical, examine the different parts of the plant for all possible symptoms and signs, and submit portions that represent the observed problems. Sometimes the problem is different or more extensive than it first appears to be. For example, an accurate diagnosis of a problem first observed as foliar damage on leaves could result from an impairment of other parts of the plant such as the trunk or roots.

To aid in accurate diagnosis, keep the plant material as fresh as possible during shipping. To prevent decay of the sample, ship samples early in the week to avoid delay over the weekend. Most diagnostic clinics are located on university campuses that do not receive mail on the weekend. If you are collecting a sample over the weekend, store it in a cooler and ship it early the next week. Fresh samples sent through the mail generally arrive in good condition when they are wrapped in dry paper towels or newspaper and enclosed in a box with packing materials to prevent movement. Do not wrap samples in damp packaging material, as doing so frequently results in a moldy mess by the time the sample reaches the clinic, which wastes time and money for the sender and the recipient alike.

The National Plant Diagnostic Network was created to address concerns about bioterrorism after the events of Sept. 11, 2001. The goal is to establish a national network of diagnostic laboratories to rapidly and accurately detect and report pathogens, pests and weeds of national interest.

The plant diagnostic clinic at the University of Missouri is part of this network and receives funding and training opportunities to improve detection and identification of pests and pathogens. Every diagnosis made by the clinic, and by the other labs in the network, is collected in a national database. This database allows scientists to quickly determine where a specific pest or pathogen is occuring and how widespread that organism has become.

Integrated pest management, known as IPM, is considered the best approach to maximize the success of management techniques and to minimize costs including economic, environmental and potentially even health costs. Methods to manage plant disease primarily depend on the biology of the specific pathogen and the host plants.

The gardener who inspects plants frequently and identifies problems when they first begin to develop will often have a wider selection of effective management options. Keep in mind that more than one method may be needed to effectively manage a specific problem.

Common approaches to manage plant diseases include five main types of controls:

A regulatory approach to managing plant diseases is often based on exclusion, or using a quarantine to prevent the spread of a disease into new areas. Exotic diseases or pests pose a significant threat to wildlife in a new region. Pathogens and hosts coevolve, meaning as they change over time, each affects the other's evolution. Coevolution allows for some innate immunity in the host population so a disease does not wipe it out. When a new pathogen is brought into an area, the native plants may have no defenses against it. Similarly, an introduced pest does not have natural predators in the new area to keep its population in check. For these two reasons, exotic or invasive pests can have a profound and damaging impact on an ecosystem.

For example, if you have ever flown to a location such as California or Hawaii, you may have noticed measures taken at airports to prohibit transport of fruit and other agricultural or horticultural products that could harbor pests and diseases. So far, successful quarantine efforts have kept an aggressive strain of the bacterial wilt pathogen Ralstonia solanacearum from entering the United States. This disease could severely impact the country's production of solanaceous crops, including tomatoes and potatoes. Bacterial wilt inoculum was accidentally brought into the U.S. on flower cuttings shipped from Kenya and Guatemala, but the disease was quickly detected and eradicated before it could begin to become established here.

Another current quarantine aims to check the spread of sudden oak death, a new disease on the West Coast that has been damaging forests in California. In addition to killing oaks, it causes a blight of many other trees and shrubs, and it has infected nursery stock. Whenever infected stock is found, the plants must be destroyed and nearby plants must be isolated and watched for symptoms. If the disease should arrive in the Midwest, it could severely damage our landscapes.

Breeding for disease resistance uses genetics to prevent disease. Resistance refers to an ability to exclude or overcome infection by a particular pathogen. Many crops and ornamental plant species are bred for disease resistance, creating a new cultivar or variety that provides better performance. Gardeners can select varieties that can resist common diseases, such as roses with black spot resistance or crab apples with resistance to apple scab.

Keep in mind, however, that a plant considered resistant to one disease might be highly susceptible to other diseases or pest problems. For example, certain roses that are highly resistant to the common fungal leaf disease black spot are often still susceptible to other leaf spotting diseases, viruses and other problems.

Also, a disease resistant plant may still be infected if a genetic variant, or race, of the pathogen is present. Plant resistance may also break down if environmental stresses are present that limit the plant's defense mechanism. For example, tomato varieties resistant to Fusarium wilt may still develop the disease under highly favorable environmental conditions or when another race of the fungus is present in the soil.

Although the interactions may be complex, host resistance is the most long-lasting and environmentally responsible method of disease control, and therefore one to strive for. Proper plant selection, whether it be a different variety within a species, or a completely different plant species altogether, can save management or replacement costs in the long run. Before planting, do research to determine the best plant to establish, weighing the potential environment, disease and pest pressures that may be placed on it.

Cultural disease management strategies are long-practiced methods that prevent the conditions for diseases and other pests to become established. These practices, based on good sanitation and husbandry, often rely on a general knowledge of plants and their problems. Combining a variety of cultural control techniques often works better than using a single method.

Abiotic disorders are caused by the environment, and therefore cultural practices that mitigate or remove that stress should be employed. For example, if a lawn is being scalped or stressed by low mowing, raise the mower deck.

Controlling plant diseases with cultural practices involves a combination of preventing the conducive environment for pathogen growth and improving the growing conditions for maximum plant health. In most cases, gardeners should employ a multitude of cultural practices to produce healthy, disease-free plants that grow vigorously. A few examples are noted below.

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Preventing and Managing Plant Diseases | MU Extension

Missouri Master Gardener Core ManualDavid Trinklein, Division of Plant Sciences

Plants are living organisms that contain chlorophyll and use it to manufacture their own food. Their cell walls are more or less rigid and support both the individual cells and the whole structure. Even when plants have reached what we regard as their full, mature size, they continue to expand and develop new leaves, flowers, fruit and shoots.

Unlike animals, plants cannot move when the environment changes. They are at the mercy of the climate and the gardener because they are rooted in place. Even though it appears that many plants, especially larger ones, are quite tolerant of change, they sometimes do not show adverse effects until long after the event. For example, tree roots are often damaged or killed by suffocation during building projects or flooding. An established tree may still have strength to leaf out and may appear to thrive for several years. But in its weakened state, the tree is more likely to blow down, become infested or simply decline.

To understand why plants respond as they do to natural influences and to cultivation, gardeners must understand something about their structure and how they grow. This publication provides such an introduction.

Gardeners tend to group plants by their horticultural uses: fruits, vegetables, flowers, trees, shrubs, turf and so on. These categories are a convenient way to think and learn about plants.

Plants can also be categorized by the length of their life cycles. Annual, biennial and perennial are terms that describe how long a plant will live and also indicate when it is likely to bloom.

AnnualAn annual plant's entire life cycle from seed germination to seed production occurs in one growing season, and then the plant dies. Many flowering plants that we consider to be annuals are not annuals in their native habitats. They would continue to grow and flower in future years if freezing temperatures did not kill them. Tuberous begonia (Begonia tuberhybrida) is an example of an ornamental plant treated as an annual in the Midwest, although it is a perennial in the southern states.

Annuals may be further subdivided into summer and winter annuals:

BiennialA biennial plant starts from seed and produces vegetative structures and food storage organs in its first full season. A rosette of basal leaves persists through winter. During the second season, the plant's life cycle is completed with flowers, fruit and seed. The plant then usually dies. These plants will often reseed themselves. Examples of biennials are carrots, beets, cabbage, celery, onions, hollyhock, Canterbury bells and Sweet William.

Sometimes plants that typically develop as biennials may complete their entire cycle of growth from seed germination to seed production in only one growing season. Conditions of drought, unusual variations in temperature or other climatic changes can cause the plant to pass through the physiological equivalent of two full growing seasons in one year.

PerennialA perennial plant is a plant that lives for more than two years. Typically, perennials die back in the fall and return in the spring because of some sort of overwintering structure, such as a rhizome or crown. Examples include flowers such as daylilies, blackeyed-susan and coneflower; and vegetables such as asparagus and rhubarb.. Plants often characterized as weeds such as common milkweed and morning glory are also perennials.

Perennials are classified in various ways:

The structure and appearance of plants' flowers, leaves, fruit and seed play a large part in how we think of them and also provide useful information about their classification. For example, the flowers of a daisy indicate a probable relationship with other plants that have similar flowers. The majority of grasses are easily recognized by their long leaf blades.

More than 500,000 different kinds of plants and plantlike organisms exist in the world. Of these, the flowering plants classified as angiosperms are the most abundant and familiar to us. Gymnosperms are the other main group of seed-bearing plants. There are also more primitive plants such as mosses and ferns that reproduce by spores.

AngiospermsAngiosperms have seeds encased in closed ovaries that become plants' familiar fruits, pods, grains or capsules. They represent virtually all crop plants and those we think of as flowers.

The angiosperms are further classified into two groups according to the number of seed leaves, called cotyledons, that emerge from a germinating seed:

GymnospermsGymnosperms are plants that develop exposed or naked seeds. These include the coniferous plants such as fir, pine and spruce. Ginkgo and the tropical cycads are also gymnosperms.

Modern plant taxonomy is based on a system developed by the Swedish physician and botanist Carl von Linn, who later changed his name to Carlos Linneaus. His classification is based on the flowers and reproductive parts of a plant. Because these are the parts of a plant least influenced by environmental changes, this system has been found to be the best.

Grouping plants with similar botanical structure helps us to understand how they are related to one another. Close relatives often have similar pest problems. Botanical similarities may also show, for example, how long certain plants can be expected to live and why they react as they do to certain conditions. In addition, their botanical, Latin or "proper" names help to avoid confusion when the same or similar common names exist for different plants.

Each plant is assigned two names. The genus or generic name can be likened to a person's last name, as in "Doe." The specific epithet or species name is that person's given name, "Jane" or "John." This combination of two names is the plant's botanical, scientific or Latin name.

For example, the botanical name for sugar maple is Acer saccharum (pronounced AY-ser sa-KAH-rum). The genus name Acer is a classical (Latin) name. The genus name for the Indian bean tree Catalpa is a Native American name. Other botanical names provide descriptions of the flower: for example, Antirrhinum (snapdragon) is from the Greek anti, which means "like," and rhinos meaning nose or snout. One familiar genus is Narcissus (daffodil) named for the mythological character who was turned into this flower when he drowned attempting to reach the person he saw reflected in a pool of water.

Specific epithets may have similar descriptive value, such as rubra for red and major for large or larger. In the sugar maple example, the word saccharum is from the Latin for sugar cane, and it is similar to words we know that mean sweet. Some species commemorate a botanist or plant explorer. The late 18th century Swedish naturalist Carl Peter Thunberg introduced many Asian plants. He is remembered in plant names, including the species Berberis thunbergii, the Japanese barberry, and a genus of the warm-climate, climbing blackeyed-susan, Thunbergia.

Words in many complete Latin names include botanical variety, subspecies and cultivar. These build upon the basic binomial naming system to further separate individuals that differ from one another in, for instance, flower color or growth habit. They are not so different as to require new specific names.

Botanical classification of four plants

Every living organism plant, animal, insect and so forth can be classified into the following categories or taxa:

For plants, the kingdom is Plantae and division is Tracheophyta. Class is usually either Angiospermae or Gymnospermae, the angiosperms and gymnosperms that make up most of our cultivated plants. At the subclass and order level, further groupings of similar plants are named.

FamilyA family of plants shares similar characteristics. For example, the spring-flowering magnolia trees, whose deciduous forms are best known in the north, and the evergreen southern forms are in the same family, not surprisingly called Magnoliaceae. Different magnolia specimens can be "keyed out" using a botanical key. The combination of characteristics that identify this family are enclosed ovules, flowers that are not catkins, flowers with calyces, clear and separate "distinct" carpels (reproductive portion of flower), overlapping or imbricate sepals, and alternate, simple leaves.

In another example, peas belong to a large family of legumes called Fabaceae (formerly named Leguminosae). The edible pea flower is shaped much like the flower of a tree in the same family commonly referred to as redbud (Cercis canadensis). All legumes have similar flowers and fruiting structures even though they may be vastly different in form. Other legumes include alfalfa, beans, clover, honeylocust, Kentucky coffee tree, Siberian pea shrub and wisteria.

What do roses have in common with apple trees? They are members of the same family, Rosaceae. Their fruits are pomes. Plants in this family share susceptibility to the same diseases. For example, pears and roses are susceptible to fire blight, and both are subject to mildew during humid weather. Other plants in the Rosaceae family include cotoneaster, spirea, juneberry, quince and mountain ash.

GenusWhen groups of similar plants are categorized into families, the next lower level of classification is the genus. Plants in the same genus often share similar fruits, flowers, roots, stems, buds and leaves. The genus name is always capitalized and italicized or underlined. Examples:

SpeciesSpecific definition comes with the species name, or specific epithet. At this level, marked features that are carried from generation to generation distinguish the group. Specific names are not capitalized, but they are italicized or underlined. Examples:

Variety (botanical), subspecies, formSometimes the specific name is followed by a botanical variety, subspecific name or form that denotes a fairly consistent, naturally occurring variation within the species. This second specific name is preceded by the abbreviation var., ssp., or forma (f.).

Examples

Cultivar (short for cultivated variety)A cultivar is a group of plants that is clearly distinguished by certain characteristics that may be morphological (structural), physiological (functional), cytological (cellular) or chemical. The differences do not have to be visual for a variation to gain cultivar status perhaps it is simply more hardy or disease resistant. When a plant is reproduced asexually (by cloning), it retains these distinguishing characteristics.

Cultivar names are always capitalized within single quotes or preceded by the abbreviation cv. In the nursery industry, the cultivar name is recognized as a plant's official name.

Examples

Along with cultivar designation, recent new cultivars may have other assigned names that are often trademarked (Golden NuggetAA dwarf Japanese barberry, Berberis thunbergii 'Monlers')

More plant identification termsSeveral more terms may be used to define particular plants or plant groups:

Dichotomous plant keys are used to identify plants through a series of choices between pairs of alternatives. Each pair refers to a specific plant characteristic such as arrangement of leaves on the stem, type of leaf margin or type of fruit. By selecting the option that accurately describes the plant, you will be led to the next choices until you determine the genus or species.

If a result is ambiguous, final verification can be made by comparison with a known example of that species. In their detailed comparisons, plant taxonomists often use preserved specimens stored in an herbarium.

Reference books for specific types of plants, such as ferns, wildflowers or shrubs, frequently contain their own specialized plant keys. Try to use keys that employ botanical rather than common names. Common names can be confusing for several reasons: one plant may have several common names; the common name for a plant often differs from one region to another; and the same common name can also apply to more than one plant. Botanical names, by contrast, are unique and relatively permanent.

Several major plant keys are available, including the following:

Figure 1Plant cell.

The plant cell is the basic organizational unit of plants (Figure 1). Each living plant cell contains a nucleus that controls all of the chemical activities in the cell. Within the nucleus, division of the DNA provides the way for the cell to pass on heritable information from one generation of cells to the next.

Cytoplasm is the other main part of the living plant cell. It is composed of many cell structures (organelles), water, pigments, sugar and various minerals. The cytoplasm is bound by a plasma membrane that regulates the flow of water and nutrients into and out of the cell.

The plant's cell wall is one of the fundamental differences between plant and animal cells. The somewhat rigid cell wall is made up of a number of chemical compounds, primarily the carbohydrate cellulose.

The second major difference between plant and animal cells is that many plant cells contain the green pigment chlorophyll. Chlorophyll is contained in chloroplasts, where photosynthesis, the food manufacturing process, takes place. A chloroplast is a type of organelle known as a plastid. There are also plastids that contain pigments other than chlorophyll.

Plant cells can have specialized functions, and there are many cell types. Plant cells are largely made up of water held within the vacuole, which exerts a pressure against the rigid cell wall. This pressure, called turgor pressure, gives the plant shape and structure. When insufficient water is available in the plant to maintain this pressure, the plant begins to droop or wilt.

Individual cells work together to form the whole plant. Tissues are organized groups of cells that are similar in appearance and function. An organ is a group of tissues that accomplishes a common function. Plants have two organ systems: roots and shoots. Shoots, in turn, have two main organs: leaves and stems. These organs are made up of various tissues that are called meristematic, which may be dermal or vascular.

Meristematic tissues are sites of cellular activity and division. This is where all of the cell division takes place. Meristematic tissues give rise to the other tissue systems and are named for their location. Animal tissues do not have these specific sites of cell division rather, all animal cells can divide to create new tissues.

An apical meristem is located at the apex, or tip, of a shoot or root. The lateral meristems exist in the stems and roots of many plants. They help the plant grow in thickness or diameter. The vascular cambium is a lateral meristem that forms new xylem (water-conducting) cells on the inside and new phloem (food-conducting) cells on the outside. Active cambium cells are exposed when the outer skin or bark is peeled away from a dicot stem (monocots usually have no cambium).

Dermal tissuesThere are two types of dermal tissues epidermis and periderm.

Vascular tissuesVascular tissues make up the water- and food-conducting system of a plant. They consist of the xylem and phloem.

Every plant has a unique form and structure and is made up of several distinct organs. All of these influence a plant's overall health and appearance. Gardeners need to consider all parts of the plant and the effects of the environment on these structures, which include roots, stems, buds, leaves, flowers, seeds and seedlings, and fruits.

Healthy roots are vital to the well-being and the continued development of most cultivated plants. Roots' structure and growth habits have pronounced effects on the size and vigor of a plant, its ability to adapt to various soil types, and its responses to cultural practices and irrigation. In addition, many plants spread through buds that develop on vigorous roots, and portions of root can be used for vegetative reproduction or propagation. Examples are phlox and lilac (Syringa). Roots that store carbohydrates are often used as food for us and for animals. Carrots, beets, sweet potatoes and turnips are examples.

Types of rootsOne or more primary roots originate at the lower end of a seedling or cutting. From here, the root system develops, which is usually characteristic of the plant. Specific soil conditions can cause modifications in roots, however. For example, the taproot of a carrot growing in stony soil will be stunted and branched.

Figure 2Longitudinal section of root.

Root structureA root has no nodes and never directly bears leaves or flowers (Figure 2). Lengthwise, it has four main parts:

More on rootsThe quantity and distribution of plant roots are important because these two factors have a major influence on the root's ability to absorb moisture and nutrients. The depth and spread of the roots depend on the plant's inherent growth characteristics and on the texture and structure of the soil. Roots will penetrate more deeply into a loose, well-drained soil, where there is adequate soil oxygen, than into a dense, poorly drained soil. A solidly compacted layer in the soil, sometimes called a hardpan, will restrict or terminate root growth.

During early development, a seedling plant absorbs nutrients and moisture from the soil within a few inches of the location of the seed from which the plant grew. As plants become well established, the root system develops laterally and usually extends to several times the spread of the branches. The greatest concentration of fibrous roots occurs in the top 12 inches of soil, but significant numbers of laterals may grow downward from these roots to provide an effective absorption system several feet or more underground.

Stems are generally the bulkiest and most obvious part of the plant. They support the leaves, buds, flowers and fruit. Water, nutrients, the products of photosynthesis, and gases pass up and down stems, to and from the roots. In certain plants, stems function as storage organs for food manufactured through photosynthesis. They may spread out and root, making new plants. Portions of stem, often called cuttings or slips, are used in vegetative reproduction or propagation. Examples are ivy, blackberry and willow (Salix). We commonly use stems as food examples include asparagus, kohlrabi, broccoli, cauliflower, rhubarb and potatoes.

Figure 3Cross section of woody plant stem. Figure 4Cross section of woody plant stem.

Structure of stemsBark is the external covering of the stem of woody plants. Internally, the stem's three major parts are the xylem, phloem and cambium (Figure 3). The xylem tissue consists of tube-like cells that conduct water and dissolved minerals and gases in the stem, while the phloem tissue conducts food products. Xylem forms the inner rings to become sapwood and heartwood of woody stems. The cambium is dicotyledonous meristematic tissue with cells that divide and enlarge to force the stem to expand outward. New xylem is formed on the inner side of the cambium and new phloem on the outside. The cambium is a thin, actively growing layer that is vulnerable to girdling by wires, weed trimmers and even a tree's own roots.

Herbaceous plants have stems that differ in internal arrangement when compared with woody plants. Although monocots and dicots both contain xylem and phloem, their vascular systems are arranged differently (Figure 4). In the stem of a monocot, the xylem and phloem are paired into bundles that are dispersed throughout the stem. In herbaceous dicots, those vascular bundles are arranged in a circle in the stem.

Figure 5Typical woody stem.

External features of stemsStems grow either above- or belowground. They may be long with large distances between leaves and buds, or they may be compressed with almost no distance between leaves and buds. The location on the stem where a leaf or bud occurs is called a node (Figure 5). It is sometimes difficult to distinguish between stems and roots, but one sure way is to look for nodes. Stems have nodes; roots do not.

The internodes are the regions between nodes. The length of an internode depends on many factors. One of these is genetic oaks usually have shorter internodes than sycamores. Environment is also a great influence. For example, decreasing fertility will decrease internode length. Early-season growth, which is often the most vigorous, usually results in the greatest internode length. Too little light will cause stems to elongate, resulting in long, spindly growth. Paradoxically, plants that are growing vigorously tend to have longer internodes than weak plants. Internode length will also be affected by competition from surrounding stems or fruits. If the plant's energy (available water and food) is divided between three or four stems, or if fruits (seeds) are also developing on the stem, less energy is available for any one shoot, and internode length is shortened.

Look at the varying internode lengths in a full season's growth of a deciduous tree, such as an oak or an apple. An interesting exercise for a gardener is to look at a stem and then try to identify the conditions that may have affected growth.

Types of stemsTypical stems are the trunks and branches of shrubs and trees, and the stalks of nonwoody plants. Modified stems can be found both aboveground and belowground.

Parts of aboveground modified stems.

Parts of belowground modified stems.

A bud is an undeveloped shoot from which leaves or flower parts grow. The buds of deciduous trees and shrubs typically are protected by leathery bud scales or, in the case of some evergreens, a resinous covering. Some buds are termed "naked" because they have no covering. Herbaceous plants have naked buds in which the outer leaves are green and somewhat succulent.

Buds may require exposure to a certain number of days below a critical temperature before they will resume growth in the spring. This time period varies for different plants. During rest, dormant buds can withstand low temperatures, but after the rest period, buds become more susceptible to weather conditions and can be damaged easily by cold temperatures or frost.

A leaf bud is composed of a short stem with embryonic leaves and develops into leafy shoots. Leaf buds are often less plump than flower buds. Flower buds are made up of a short stem with embryonic flower parts.

Buds are classified by their location on the stem. Terminal or apical buds are located at the apex or tip of the stem. Lateral or axillary buds are found on the sides of the stem, usually in the leaf axil, the point of leaf attachment to the stem. Adventitious buds arise at other sites, including the internode of the stem, at the edge of a leaf blade, from callus tissue at the cut end of a stem or root, or laterally from the roots of plants.

The principal function of leaves is to absorb sunlight for the manufacture of plant sugars. This process is called photosynthesis. The typical leaf has a flattened surface to present a large area that efficiently absorbs light energy. In most cases, the leaf is supported by a stemlike appendage called a petiole. The base of the petiole is attached to the stem at the node. The angle formed between the petiole and the stem is called the leaf axil. A bud or cluster of buds is usually located in the axil.

Figure 6Cross section of dicot leaf.

Structure of leavesThe leaf blade is composed of several layers (Figure 6). On the top and bottom is a layer of small, tough epidermal cells. The primary function of the epidermis is to protect leaf tissues. The arrangement of the cells in the epidermis determines the texture of the leaf surface. Hairs that are present on some leaves are extensions of epidermal cells.

The thickness of the cuticle (the layer of cutin produced by epidermal cells) is a direct response to sunlight. The stronger the light, the thicker the cuticle. For this reason, plants grown in the shade should be moved into full sunlight gradually over a period of a few weeks to allow the cutin layer to build and to protect the leaves from rapid water loss and sunscald.

Cutin repels water and can shed pesticides if spreader/sticker agents or soaps are not used. This is the reason many pesticide manufacturers include some sort of spray additive to adhere to or penetrate the cutin layer.

On the surface of leaves are the stomata. Some plants have stomata on both surfaces; others have them only on the lower surface. Formed from epidermal guard cells that are capable of opening and closing, the stomata regulate the passage of water vapor, oxygen and carbon dioxide into and out of the leaf. The opening and closing of guard cells is determined by the environment. Conditions that cause large water losses from plants (high temperature, low humidity) stimulate closing, while mild weather conditions leave guard cells open. Guard cells close in the absence of light.

The middle layer of a leaf is known as the mesophyll. This is the location of the chloroplasts that contain the green pigment chlorophyll. Photosynthesis takes place here. In monocot plant leaves, the mesophyll consists of cells and air spaces. In dicot leaves, it is divided into a dense upper layer called the palisade and a lower, spongy layer of cells with air spaces.

Leaf types

Figure 7ASimple leaf type.

Figure 7BCompound leaf type.

Figure 8Leaf venation.

Leaf venation Venation describes the patterns in which the veins are distributed in the blade (Figure 8).

Figure 9Leaf shapes, simple, left, compound, above right, and conifers, below right.

Leaf shapesThe shape of the leaf blade and the type of leaf margin are important characteristics that help identify plants (Figure 9). Leaf blades vary a great deal. They may be simple (apple, oak) or compound (divided into several smaller leaflike segments, as in honeylocust). The smaller segments are called leaflets and are attached to a stalk (rachis) with a petiolule. Leaflets can also be arranged palmately (horse chestnut) or pinnately (ash). Pinnately compound leaves are said to be odd pinnate (ash) when ending in one leaflet and even pinnate when ending in two leaflets (locust). This terminology is important in identifying plants by their leaves.

Leaf modificationsLeaves have adapted to survive a wide range of environmental conditions. For instance, leaves exposed to strong sunlight are often smaller and have thicker cuticles than leaves of the same plant growing in shade. The reduced surface area and thicker cuticle reduces water loss. Leaves that develop in shade have a larger surface area to absorb light.

Chloroplasts respond to light by exposing as much pigment as possible in low light situations and by exposing less pigment in bright conditions. We see the results: dark green foliage on shade-grown plants and paler green foliage when the same plant grows in a sunnier location. Examples of this include hosta, Norfolk Island pine and weeping fig.

Leaves on plants that grow in dry environments will often be thick or narrow with few intercellular (air) spaces in the mesophyll, while guard cells are sunken below the level of the regular epidermis to minimize water loss. In some desert plants, such as cacti, the foliage leaves may be modified into thorns and photosynthesis occurs in chloroplast-containing cells in the stem.

Plants that grow underwater have just a few widely spaced mesophyll cells and big intercellular spaces for holding gases that are harder to acquire underwater.

Many conifers have leaves adapted for windy or low-moisture conditions. Needlelike leaves on pines have little wind resistance, and the flattened or scalelike leaves of junipers are waxy and well protected from the hot sun.

Distinct leaf modifications that occur on plants

Figure 10Leaf arrangements.

Leaf arrangement and attachmentLeaves at the nodes may grow in pairs opposite one another (maple) or alternate (birch) from side to side along the stem (Figure 10).

They also may be whorled, with three or more leaves arising from a node, such as hydrangea. Subopposite leaves are slightly offset from one another; these are relatively rare. An example is the katsura tree, Cercidiphyllum japonicum.

Although there are many different kinds of flowers, they are similar in their organization. The function of flowers is sexual reproduction. Thus, flowers often form the showiest part of the plant. Their color and fragrance attract pollinators such as insects or birds to assure the continuance of the species. Flowers that are neither showy nor scented rely on other methods for pollination wind, for example. Yet all have the same basic structures.

Figure 11Parts of a typical flower.

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