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University of Kentucky College of , Food and Environment PPA-41 Cooperative Extension Service Fundamental Principles of for Agricultural Producers Paul Vincelli, Extension Plant Pathologist

ll produced in Kentucky scopic; however, have the potential to become dis- some produce easedA under certain conditions. structures that of can affect yield and/or quality either alone or of the harvested commodity, which can en mass are vis- impact profitability and increase the risks ible to the un- of farming. aided eye. Fungi A plant is diseased when it is affected and -like by some agent that interferes with its organisms feed on liv- normal development. Some disorders ing plants and/or on dead are caused by noninfectious factors, such organic matter. When they as temperature extremes or nutrient attack living plants, can result. deficiencies. However, this publication These organisms usually produce Figure 1. An illustration of microscopic chains of barrel-shaped , which are produced focuses on diseases caused by infectious spores (Figure 1), which can begin an by a fungus infecting a . organisms. infection when carried to a plant. Spores The spores illustrated here are spread by air Diseased plants exhibit a variety of can be spread from plant to plant by currents. symptoms. These symptoms can include , water, , birds, and equip- stunting, yellowing, wilting, twisting, ment. Adequate moisture and the right reddening, browning, blighting, rot, temperature are required for the spores Some of these organisms have the rot, and other abnormalities. Accu- to begin new infections. Microbes may ability to reproduce very quickly under rate diagnosis is important to effectively be able to penetrate plant host tissues the right environmental conditions. An control a plant disease. Until a disease is directly or through natural openings; infection that began from one properly diagnosed, a grower may waste however, a wound is sometimes needed may produce millions of spores in just a time and money trying to correct a as a point of entry (Figure 2). Plant few days. Others reproduce slowly and problem with an unknown cause. Once a diseases caused by fungi and FLO are may take one or two years to complete a disease is diagnosed, appropriate control common during wet, humid seasons. disease cycle. practices can be selected. Diagnosis is an Survival between growing seasons can Infections of plants by fungi and FLO art as well as a science, and experience is occur in one or more of the following can cause a variety of types of diseases, important. Your local Extension agent ways: in crop residues, weeds, , , including leaf diseases (Figure 3), wilts, can provide information on identification live , or in crops or weeds growing root rots, crown rots, stem rots (Figure and management of crop diseases. south of Kentucky. 4), and other diseases. Infectious Organisms that Cause Diseases Figure 2. A microscopic view of plant infection by spores of various fungi. Some fungi can infect plant through unwounded surfaces (left). Sometimes, fungi can infect best through wounds Plant diseases can be caused by fungi, (right) or natural microscopic pores in plants. fungus-like organisms (FLO), , , , or parasitic plants. Fungi and Fungus-like Organisms Fungi and fungus-like organisms (wa- ter molds) are the most common infec- tious organisms causing plant disease. Many of these organisms are micro-

Cooperative Extension Service | Agriculture and Natural Resources | Family and Consumer Sciences | 4-H Youth Development | Community and Economic Development a. b. Figure 3. Gray , caused by the fungus Cercospora zeae-maydis (a), affects corn foliage. Yield loss can result when the upper are killed during grain fill. Individual lesions on leaves are rectangular (b).

a. b. c. Figure 4. nicotianae is a fungus-like organism (a) that causes black shank of tobacco (b). Plants with rotted and stems (c) suffer from lack of water and nutrients, and they may eventually die.

Bacteria Bacteria often survive between grow- Bacteria are single-celled microscopic ing seasons in crop residue, in crop seeds organisms that can attack living plants or cuttings, or in weeds. and cause plant diseases. Bacteria can may also survive in their host. be carried from plant to plant in droplets Under warm, wet conditions, bacteria of water by wind, rainsplash, insects, and reproduce extremely quickly (Figure equipment (Figure 5). Phytoplasmas, 5). Under conditions ideal for disease which are very small bacteria without development, a single bacterial cell can cell walls, are mainly carried from plant give rise to 17 million daughter cells in to plant by insects with piercing/sucking 24 hours. mouthparts, such as . Diseases of leaves, growing , and fruit are the most common types of bacterial diseases in Kentucky (Figure 6). Figure 5. A microscopic view of leaf infection by bacteria (illustrated as white rods in black droplets of moisture). Bacteria multiply and spread in moisture droplets.

2 Viruses Viruses are the smallest parasites causing plant disease. Because of their extremely small size, they can be seen only with an electron . Struc- turally, viruses are nothing more than a shell surrounding a small amount of genetic material (Figure 7a). Most viruses are spread by insect vec- tors (a vector is a living organism that transmits a ). Conditions that Figure 6. Bacteria streaming from a plant favor large numbers of insect vectors lesion, as seen under often lead to severe outbreaks. Vi- the microscope (a). ruses may also be transmitted from plant Symptoms of bacterial spot on pepper leaves to plant by other means, such as human and (b), a disease hands. Most viruses survive between that may limit the success of pepper pro- growing seasons in live plant tissues a. b. (weeds, crop seeds, cuttings, or ). duction in Kentucky. Viral infections of plants cause stunt- ing, color changes in leaves (Figure 7b), and growth distortions. Nematodes Nematodes are microscopic worms that live in the soil. Parasitic nematodes feed on plant roots (Figures 8 and 9). Low numbers of nematodes are not damaging to crops, but feeding by large numbers of nematodes on roots interferes with uptake of water and nutrients. Plant nematodes often require three to four weeks or more between genera- tions. In contrast to some fungi and most bacteria, destructive levels of nematodes a. b. take some time to develop. Once destruc- Figure 7. Tobacco vein mottling virus (TVMV) viewed through an electron microscope, magni- tive levels are reached, however, even fied 54,000 times (a). These virus particles are flexible, rod-shaped crystals of genetic material nematodes that reproduce slowly can surrounded by a protein coat. Tobacco leaves infected by this virus develop vein-banding and cause serious crop damage. Nematodes mottling symptoms (b). are spread by anything that moves soil: boots, machinery, running water, etc. Nematodes usually survive between sea- sons in the soil. Typical root symptoms of damage include lesions or galls (Figure 8b) that result in root dysfunc- tion.

a. b. Figure 8. Scanning electron microscopic view of root knot nematode feeding on a tomato root (a). Root knot nematode galls on roots of a gourd plant (b).

3 a. b. Figure 9. A microscopic view of pear-shaped soybean cyst nematode female filled with eggs (stained red) (a) and cysts appearing as white to brown specks on soybean roots (b).

Parasitic Seed Plants Parasitic plants, such as dodder (Figure 10), require another living plant (host) to meet some or all of their nutritional needs. These parasitic plants produce seeds, which provide a means of over- wintering as well as dispersal. The Disease Triangle A fundamental concept in plant pa- thology is represented by the disease triangle. Each side/angle of the triangle represents a factor required for disease development: a susceptible host, a patho- gen (the agent that causes disease), and an environment favorable for disease to develop (Figure 11). All three factors must be present simultaneously for a plant disease to occur. Figure 10. Dodder parasitizing alfalfa. For example, consider blue mold of tobacco. A may have susceptible In many cases, the susceptible host Figure 11. The disease triangle. Only when all tobacco plants and the weather may be three criteria are present (pathogen, suscep- cool and damp, but blue mold cannot may be the missing factor when the tible host, and favorable environment) will develop if spores of the blue mold patho- grower has selected a variety bred to disease occur. gen have not been carried by air currents have resistance to particular diseases. If into the crop. In this case, one factor in the variety has high levels of resistance, the disease triangle—the pathogen—is no other management practices may be Susceptible Host missing. necessary against that disease. Likewise, spores may land on a field of Disease develops only when all three susceptible plants, but if the weather is sides of the disease triangle are com- no no hot and dry, infection will not occur and plete. Plant diseases are managed by disease disease blue mold will not develop. A different manipulating the disease triangle: factor of the disease triangle—a favorable the plant, the pathogen, and/or the DISEASE Pathogen environment—is missing in this case. crop production environment. no Favorable disease Environment

4 Managing Plant Diseases Plant diseases can be managed through the use of appropriate cultural practices combined with resistant varieties when available. Appropriate use of in an integrated disease management plan may also be an important part of controlling certain diseases. Cultural Practices Several cultural practices involving Contact Systemic the use of and sanitation, Figure 12. Uniform coverage of both upper Figure 13. Contact (protectant) proper planting dates and fertilization, and lower leaf surfaces is necessary for disease remain on plant surfaces. Systemic fungicides and other practices can be an important control using a protectant . penetrate the plant. part of a disease management program. Crop rotation is a fundamental disease Proper fertility management practice. Growing the same is another important cul- Pesticides crop in a field year after year encourages tural practice for disease control. Prop- When used properly, pesticides can the buildup of . For example, erly fertilized plants are generally more be an important tool for disease control. if the black shank pathogen is present, resistant to diseases than under-fertilized For example, proper use of foliar fungi- growing tobacco every year leads to or over-fertilized plants. cides in high-yield wheat can enhance dangerously high spore levels in the Other practices: Numerous other cul- profits. However, pesticides cannot field. However, when a farmer rotates to tural practices can contribute to a disease substitute for good crop management. In a crop other than tobacco, spores of the management plan. Some examples: fact, many diseases cannot be controlled black shank organism begin to starve yy Proper can affect disease economically with pesticides. Pesticides since they can only infect tobacco. Fewer development. Overcrowding of plants are the last line of defense for controlling spores survive each year, until eventually can lead to excess humidity around crop diseases, not the first line of defense. few remain in the soil. leaves, favoring fungal and bacterial Pesticides used to control plant dis- eases are classified as fungicides, bacte- Sanitation: diseases. Many important disease y ricides, and nematicides. management practices fall under the y Timely can also be important, category of sanitation. For example, re- as some diseases develop on the har- Fungicides are used to control diseases moving infected eliminates a vested commodity only as the crop caused by fungi and fungus-like organ- matures. isms. They may be applied to seed, soil, or source of infectious material, which can y help in disease management. Plowing to y Selecting disease-free seed is impor- foliage (Figure 12). Fungicides have either bury pathogen-infested crop residue is tant when the seed is the source of the a protectant or systemic activity. another form of sanitation. Using steam pathogen. yy Protectant (contact) fungicides remain to disinfest greenhouse pots is another Resistant Varieties on plant surfaces (Figure 13) and must example. be applied before infection occurs Using crop varieties with disease re- (Figure 14). Examples of protectant Planting date: Choosing the proper plant- sistance is one of the best disease control fungicides include products contain- ing date can affect disease development. practices. With a highly resistant variety, ing mancozeb, maneb, chlorothalonil, Some diseases are favored when a crop a crop disease can be controlled with ferbam, captan, and thiram. is planted too early, while others are no added cost and no health risks to the yy Systemic fungicides penetrate and are favored by late planting. The best overall farmer, farm workers, or environment. redistributed within the plant (Figure approach is to plant when environmen- For example, in a field infested with 13). Because they penetrate the plant, tal conditions favor growth of the plant cyst nematode, soybean yields can be systemic fungicides sometimes can be but not growth and reproduction of the improved by as much as 30 to 40 percent applied shortly after infection to eradi- pathogen. simply by selecting the proper resistant cate recent infections. This activity is variety. Improving soil structure and drainage: Poor known as eradicant or “kickback.” Ex- Scouting fields and identifying disease soil drainage can create an environment amples of systemic fungicides include outbreaks are important steps in select- favorable to certain diseases. metalaxyl, mefenoxam, triadimefon, ing appropriate disease-resistant varieties Therefore, improving soil structure and and thiophanate methyl. Movement of in future growing seasons. drainage can alleviate some root rot almost all systemic fungicides within diseases. the plant is upward toward plant tops and leaf tips, not downward from leaves

5 to roots. Some fungicides are locally and seedling disease. systemic—that is, they penetrate the Systemic uptake of plant to a limited extent and move a fungicide also can only a limited distance from the point eradicate some inter- of penetration. nal infections of the seed present before Fungicides may be classified as either planting. broad-spectrum or narrow-spectrum. yy Broad-spectrum fungicides are ac- Common seed treat- tive against a wide variety of fungal ment fungicides include pathogens. captan, thiram, and yy Narrow-spectrum fungicides are effec- metalaxyl. tive against just one or a few types of Soil treatment: Pesti- organisms. This distinction is impor- Figure 14. Protectant fungicides and bactericides must be applied cides may be applied before infection occurs. The fungal infections shown here cannot tant for those who might presume that to the soil to control be eradicated using a protectant fungicide. any commercial fungicide will kill all diseases caused by types of fungi. This presumption is fungi or nematodes. not true. The label is the best guide to Fungicides sometimes applied to the Soil-applied disease-control chemicals which diseases can be controlled with soil include metalaxyl, mefenoxam, are classified as either fumigants or non- a commercial product. thiophanate methyl, and PCNB. Some fumigants. soil-directed pesticides can move with Bactericides are pesticides that kill bac- yy Fumigants are applied to the soil as water movement and have the potential teria. Bactericides act as protectants and liquids or granules before planting. to contaminate groundwater or surface must be applied before infection occurs. After application, they turn into gases. water, especially if used improperly. Bactericides cannot eradicate an existing These gases diffuse throughout the soil infection. Most copper fungicides are to form a fumigated zone. Foliage, , and fruit treatment:Fun - also bactericides. Streptomycin is an- Soil conditions must be suitable if gicides and bactericides are sometimes other bactericide labelled for certain uses. fumigants are to be effective. Generally, a applied to foliage (leaves), , and/ soil that is in “seedbed condition” (slightly or fruits to provide protection against Nematicides are used to control diseases moist and friable) with a temperature infection. caused by nematodes. Nematicides are between 50°F and 75°F is suitable. If the When applying a to foliage, usually toxic to warm-blooded animals soil is too wet, the fumigant gas will not the farmer should consider whether and should only be used with extreme diffuse through the soil. On the other it is a protectant or systemic. If it is a caution. Nematicides may be either hand, if the soil is too dry, the gas will protectant, it must be applied over all fumigants or non-fumigants (discussed escape from the soil too quickly. Either susceptible tissues before infection oc- below). way, fungal spores and nematodes will curs (Figure 12). For example, if you are Types of Application not be killed and poor disease control applying mancozeb to control Septoria Pesticides can be applied to seed, soil, results. leaf and glume blotch of wheat, the appli- foliage, flowers, or fruit. Fumigating when the soil is too cold cation must cover the flag leaf and glumes or too warm also results in poor disease before infection occurs. If symptoms Seed treatment: Fungicides are some- control. The fumigant may not diffuse are showing on the flag leaf, it is too late times applied to seed surfaces to protect properly in cold and may escape too because infection has already occurred. against disease. quickly in warm soils. See label directions Even if the fungicide was applied before yy Protectant fungicides surround the for instructions on proper conditions for infection, if coverage is poor, then unpro- newly planted seed and young seedling applying fumigants. Examples of fumi- tected spots on the flag leaves and glumes with a zone of fungicide. This zone acts gants include dichloropropene, chloro- may become infected later. as a toxic barrier to fungal infection and picrin, dazomet, and metam sodium. Systemic fungicides “translocate,” or protects against seed rots and seedling yy Non-fumigants are applied to soil as move within the plant. Thus, thorough disease. Protectant fungicides can also liquids or granules. These may be ap- coverage is less important with system- eradicate infectious spores present plied to the seed furrow, incorporated ics. In fact, sometimes the best use of a on the surface of the seed coat before before planting, or sprayed onto the soil systemic is to not spray the target tissue. planting. and incorporated during crop growth. For example, spraying tobacco leaves yy Systemic fungicides applied to seed Soil-applied pesticides provide roots with the systemic fungicide mefenoxam are absorbed by the developing seed- with a zone of protection against infec- (Ridomil Gold®) for blue mold control ling. This absorption protects against tion. Some soil-applied pesticides are results in somewhat poor coverage. (It infection by fungi causing seed rots systemic. Systemics applied to the soil is also an illegal use of the product.) On are taken up by roots and translocated the other hand, spraying the soil allows to leaves, providing roots and leaves tobacco roots to absorb and translocate with protection against infection. the fungicide. This way, mefenoxam is

6 uniformly distributed throughout the Integrated Disease Sources of Information tobacco plant, and best control of blue Management on Plant Diseases mold is obtained. Foliar fungicides sometimes must be The old saying “Don’t put all your eggs A great deal of information on plant applied several times to control a dis- in one basket” also applies to good disease diseases and their management is avail- ease because fungicide deposits do not control. Integrated disease management able through the University of Kentucky persist indefinitely. Fungicide deposits means applying a combination of tech- Cooperative Extension Service. Several are subject to weathering from rainfall, niques to achieve effective disease con- websites are listed below. Contact your sunshine, etc. Also, if a protectant was trol. Growers should not rely on only one local Extension agent to obtain more applied, leaves that have emerged since method to manage diseases. For example, information on plant diseases. y the last application are unprotected. continuous reliance on chemical meth- y Plant Pathology Extension publi- When should foliar sprays be applied? ods runs the risk of pesticide resistance, cations: http://www2.ca.uky.edu/ The answer depends on the disease not to mention the detrimental effects to agcollege/plantpathology/extension/ triangle. When is your crop susceptible? the environment. Instead, practice crop pubs.html y When are the spores present? When do rotation, use resistant varieties when y Integrated Management pub- environmental conditions favor infec- available, and learn how cultural prac- lications: http://www.uky.edu/Ag/ tion? Understanding the biology of the tices can be used to reduce disease pres- IPM/ipm.htm sure. Often pesticide use can be avoided disease is important for knowing the Acknowledgment best time to spray. Your local Extension by following good cultural practices. agent can provide information on timing Appreciation is expressed to Academic of pesticide applications. Press, Inc., New York, for permission to reproduce figures 2, 5, and 14 from the second edition of Plant Pathology by George Agrios (1978).

Figure Credits Figure 1. Originally published by L.R. and C. Tulasne, 1861 Figure 2. Agrios; see Acknowledgment Figure 3. Cheryl Kaiser (a) and Richard E. Stuckey (b), UK Figure 4. Paul Bachi (a) and Kenny Seebold (b, c), UK Figure 5. Agrios; see Acknowledgment Figure 6. Cheryl Kaiser (a) and Kenny Seebold (b), UK Figure 7. Ulla Jarlfors (a) and William Nesmith (b), UK Figure 8. USDA-ARS (a) and Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org (b) Figure 9. Jonathan D. Eisenback, VPI, Bugwood.org (a) and Paul Bachi, UK (b) Figure 10. Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org Figure 11. Paul Vincelli, UK Figure 12. Elizabeth Bush, VPI, Bugwood.org (arrows added for clarity) Figure 13. Cheryl Kaiser, UK Figure 14. Agrios; see Acknowledgment

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Educational programs of Kentucky Cooperative Extension serve all people regardless of , color, age, sex, religion, disability, or national origin. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Nancy M. Cox, Director of Cooperative Extension Pro- grams, University of Kentucky College of Agriculture, Food and Environment, Lexington, and Kentucky State University, Frankfort. Copyright © 2015 for materials developed by University of Kentucky Cooperative Extension. This publication may be reproduced in portions or its entirety for educational or nonprofit purposes only. Permitted users shall give credit to the author(s) and include this copyright notice. Publications are also available on the World Wide Web at www.ca.uky.edu. Issued 3-2015