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8-2011 Diseases Jay W. Pscheidt Oregon State University

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PPA-46 Plant Diseases Kentucky Master Gardener Manual Chapter 6

By Jay W. Pscheidt, extension plant pathologist, Oregon State University. Edited by Lindsey du Toit, plant diagnostician, Washington State University-Puyallup, and Warren Copes, pathologist, Washington State University. Adapted for Kentucky by John Hartman, extension plant pathologist, University of Kentucky.

very gardener has put in with hopes for wonderful In this chapter: flowers, , or vegetables, only to have those hopes dashed as the plants get sick and die. Plants that die are considered Pathogens...... 83 Ediseased. Many things can cause plants to become diseased, includ- How to Discourage Plant Diseases in Your ing living agents, other factors (nonliving), or a combination of the Garden...... 84 two. This chapter focuses only on living agents—fungi, bacteria, viruses, nematodes, and parasitic plants. Nonliving factors, such as Conditions Necessary for Pathogenic Diseases...... 85 nutrient deficiencies, lack of water, temperature stress, and these problems in combination as they relate to specific types of plants, Disease Cycle...... 86 are discussed elsewhere. Disease Diagnosis...... 86 History provides some perspective on the tremendous impact of plant diseases. Perhaps foremost is late blight of potato, a fungal Symptoms...... 88 disease that caused the Irish potato famine in 1845. Approximately Plant Disease Control...... 89 2 million people either starved or left Ireland, many for the United Disease Terminology...... 92 States. The fungal diseases powdery mildew and downy mildew devastated the French wine industry until a Bordeaux mixture was Fungicides for Home Gardeners...... 92 found to control them. For More Information...... 94 In the United States, the fungal disease chestnut blight was introduced accidentally into New York City in the late 1800s on imported Chinese chestnut trees. The Chinese trees were resistant to the blight, but American chestnut trees were not. In less than 40 years, 30 million acres of chestnut trees died. Chestnut blight remains a problem in the eastern United States. Dutch elm disease, which infects and kills elm trees throughout the nation, was also introduced accidentally. These examples caused a lot of damage—that’s why they are well known. However, the amount of damage that plant diseases cause varies depending on conditions and on the cycle of each disease- causing agent. Diagnosing the disease correctly and understanding techniques for growing healthy plants will help mimimize impact. Pathogens Many plant diseases are caused by pathogens, which are difficult to see or recognize without magnification. Fungi, bacteria, viruses, nematodes, and even plants can be pathogenic on garden plants. Pathogens generally obtain nutrients, water, and everything they need to reproduce from their host. Such a relationship is called parasitic. Fungal and viral pathogens cause many plant diseases; bacterial and nematode pathogens cause a few. Some pathogens can infect several kinds of plants; others require a specific type of host. Pathogens such as fungi and bacteria differ in their ability to survive, spread, and reproduce. Different varieties (strains or races) of a pathogen may differ in how much disease they cause.

Agriculture and Natural Resources • Family and Consumer Sciences • 4-H Youth Development • Community and Economic Development EXTENSION 84 • Plant Diseases Chapter 6

Fungi Bacteria Fungi make up the largest group of Bacteria are single-celled organisms pathogens, and they come in a wide variety. In that are much smaller and less complex general, they are multicellular organisms with than plant cells. Many are about the size a threadlike body. These filamentlike threads, of a plant chloroplast. Their cell walls are which are called hypha, have cell walls. When covered with a slimy material known as many threads mass together, they form what a “matrix,” and they reproduce by split- is called a mycelium, a mass of interwoven ting in two. Bacteria can build up to such threads. Further growth of a mycelium may high numbers in a plant that they ooze out produce fruiting bodies, in which sexual or the plant’s tissues and may attract insects asexual spores are formed. The mycelium, that spread the bacteria to healthy plants. fruiting bodies, and spores are used to identify Bacteria can survive in unfavorable condi- and diagnose fungal problems. tions in plant debris or even in seeds. Some fungi can survive and grow with- Bacteria cause plant diseases by forming out a living host. Others die if they are not toxins or by producing enzymes that break closely associated with a host. down plants’ cell walls. Crown gall bacteria Fungi cause plant diseases in one of the actually genetically engineer their host plant following ways: to make galls and amino acids, thus giving • making toxins that kill plant cells the bacteria a place to live (the galls) and the • growing within a plant’s vascular system chemicals they need to grow and reproduce and plugging it up (the amino acids). • rotting roots (which cuts off the ’ supply of water and nutrients) • sending rootlike structures into plant cells.

How to Discourage Plant Throw away any rotted or poorly • Control insect pests. Some growing plants or plant parts. insects spread certain viral and Diseases in Your Garden • Maintain correct soil fertility bacterial diseases. • Select a suitable location for and pH through careful use of • Destroy and remove diseased planting. Some plants like sunny fertilizers, lime, soil amend- plants as soon as you see them. locations; others prefer more ments, compost, etc. Plants that They may be a source of patho- shade. Avoid extremely wet and/ are too weak or too vigorous as gens that can spread to other or cold soils because they favor a result of improper fertilization plants. many root disordersIf you must are more subject to diseases. • Spade under or compost plant in these soils, select plants • Do not overcrowd your plants. crop refuse as soon as pos- that tolerate them. High humidity beneath the sible after harvesting your • Practice annual rotation if plant canopy may favor certain garden. This practice not only possible (changing the plant- fungal diseases. Thin your plants recycles organic matter to the ing location of specific plants) to permit free air movement. soil, it reduces the possibility to help prevent the buildup of • Water properly. Try to main- of disease-causing organisms disease-causing organisms. tain an even water supply and overwintering in your garden. • Select disease-resistant varieties avoid dry-wet fluctuations. Drip Add diseased plant parts to your if they are available and adapted systems and soaker hoses are compost pile only if you are to your growing conditions. preferred to overhead sprin- “hot” composting. • Use only disease-free seed and klers. To encourage quick drying • Be alert to diseases that transplants. Many disease-caus- of foliage, water in the morning occur annually on certain plants. ing organisms are carried to gar- instead of evening, especially if Take care of them early, before dens on seeds and transplants. you use an overhead system. damage becomes so severe that Buy from a reputable source. the plant is lost. Chapter 6 Plant Diseases • 85

Viruses Parasitic Plants Virus particles are composed of a few Some plants are parasitic to other plants. strands of molecules and are even smaller Dodder, for example, is a parasitic plant than bacteria. Even with a high-magnifi- that produces flowers and seeds, but has no cation microscope, they can be seen only chlorophyll, so it cannot manufacture its when massed together in a plant cell. own food. Its threadlike yellow body twines Electron microscopes reveal them to have around its host and its rootlike branches many shapes, including long strands, short (haustoria) penetrate the host plant and rods, or multi-sided balls. withdraw food and water. Some parasitic Viruses use a host plant’s cell organelles plants, such as mistletoe, manufacture chlo- (cell organs) to produce more viruses. The rophyll but have no real roots and depend result can be strange plant colors, forms, or on their host (oaks for mistletoe) for water structures. Some viral infections, however, and nutrients. Seeds of parasitic plants are don’t result in any visible plant problems. spread by birds or contaminated soil, or Touching virus-infected plant material they may be shot out of plant structures like and then touching healthy plants can trans- little bombs. mit some viruses. For example, a smoker Moss, lichens, and many other plants can transmit tobacco mosaic virus from found in trees are not parasitic, however. a cigarette to a plant. Many other viruses They use trees only as a platform. are transmitted by insects such as aphids, scales, or whiteflies. Fungi, mites, nema- todes, and even parasitic plants also can Conditions Necessary for transmit viruses. Viruses also may infect a Pathogenic Diseases host plant’s seeds and thus be transmitted to the next generation. In order for a pathogenic (biotic) plant disease to occur, three conditions must be met: Nematodes • The host plant must be susceptible. Nematodes are multi-cellular round- • An active, living pathogen must be worms. Because they are clear and live in present. the soil, they are impossible to see with- • The environment must be suitable or out magnification. Look at a U.S. dime. favorable for disease development. Nematodes may be no larger than the letter I in the word DIME. All three factors must occur simultane- All plant pathogenic nematodes have ously. If one factor or more is absent or a mouthpart called a stylet, which is like a unfavorable, disease does not occur. The lance or hypodermic needle that the nema- relationship among a plant, a pathogen, and tode uses to puncture plant cells and feed the environment is shown in Figure 1. The from them. more the circles overlap, the more severe Some nematodes move from root to will be the disease. Some texts refer to this root (lesion nematodes), while others set up concept as the “disease triangle.” one feeding site in a single root (root-knot A host plant’s genetic makeup deter- nematodes). Feeding may cause root lesions mines its susceptibility to disease. This or galls that restrict water and nutrient flow susceptibility depends on various physical to the host plant. A few foliar nematodes and biochemical factors within the plant. attack aboveground plant parts. If soil or Stature, growth habit, thickness of the infected plant parts are moved, nematode cuticle (a protective part of the plant’s outer diseases can be transmitted. layer) and the shape the plant’s stomata (the small openings that allow water, oxygen 86 • Plant Diseases Chapter 6

Figure 15.1. Disease is Apple scab is an example of a fungal a result of simultaneous interactions between the disease with multiple annual cycles (Figure environment, host, and 2). The fungus survives the winter on fallen pathogen. infected leaves (Figure 2a), where it pro- Environment duces small fruiting bodies that contain sex- ual spores. In the spring, starting about the time apple trees begin to grow, these spores are squirted out of the dead leaves (Figure 2b), especially during periods of frequent Disease or long rainstorms and warm weather. The Host Pathogen spores drift onto healthy leaves or young , where they germinate and enter plant tissue, much as a seed germinates and sends roots into the soil (Figure 2c). Once inside the plant, the fungus gets all of its food from the plant’s tissues. In a few days or weeks, a scab lesion and carbon dioxide in and out of the plants’ forms (Figure 2d1). Within this lesion, leaves) are a few physical factors that another type of spore is produced—an influence disease development. A plant’s asexual spore (Figure 2e). When these developmental stage and stressful grow- spores are released (Figure 2f), they drift to ing conditions can also increase a plant’s healthy tissue (Figure 2g), where they cause susceptibility to diseases. more scab lesions (Figures 2d1 and d2) and Environmental factors such as tempera- produce more spores (Figure 2e). This cycle ture, light, or moisture can accentuate dis- can be repeated many times during wet eases. For example, cool, moist conditions springs. are ideal for many fungal pathogens. Summer generally brings unfavorable hot, dry conditions. The fungus survives on leaves until they fall in the autumn. Then Disease Cycle it invades the leaves and once again makes The sequence of events from a patho- spore-bearing fruiting bodies. gen’s survival to plant disease development and back to pathogen survival is called the Disease Diagnosis disease cycle, or the pathogen’s life history. By understanding of the disease cycle— There is no single set of questions or learning the chain of events that contribute technique for diagnosing plant diseases. to a disease—you can find the weakest links Experience and practice are the best teach- and take measures to break the cycle. ers. Plant problems are most easily diag- Most pathogens must survive a period nosed through personal, on-site inspection. of adverse conditions, usually winter, when You may notice subtle influences of the site, they do not actively cause disease. The environment, or management practices that host plant is infected or continues to be make identification easier. infected by the pathogen’s overwintered Diagnosis is more difficult when only disease-transmitting substance, inoculum, part of a plant is seen, which may or may in the spring. Some diseases, such as many not indicate the real problem. The worst canker diseases, have only one cycle during diagnostic method is by phone, which the year. Others, such as powdery mildew, easily can lead to misunderstanding and continually produce more inoculum, thus inaccuracy. repeating the cycle many times during a single growing season. Chapter 6 Plant Diseases • 87

Figure 15.2. Disease cycle of apple scab: Overwintering fungi (a) produce sexual spores (b) that penetrate and infect trees in the spring (c). Soon scab lesions appear (d1), within which asexual spores (e) are produced. These spores are released (f) and drift onto healthy tissue (g), where they cause more scab lesions (d1 and d2) and produce more spores (e). The cycle then begins again.

Many beginners attempt to match a • What are the soil conditions? plant’s problem with a picture. Although • What are the soil coverings? this method is useful for simple and com- • How is the soil cared for? mon problems, it usually is inefficient and • What is the plant’s overall condition? inaccurate for local plant diseases, complex situations, or hard-to-solve problems. Symptoms and signs also are used to Another technique is to ask a series diagnose a plant’s condition. Symptoms of questions to build a case history of the are a plant’s physical disease characteris- problem. tics. Examples are wilt, galls, cankers, rots, You should ask: necrosis, chlorosis, and general decline. • What kind of plant is it? Signs are physical evidence of a pathogen. • How old is it? Examples include fungal fruiting bod- • How big is it? ies (such as mushrooms or vase-shaped • Where is the property located? bodies called pycnidia), mycelia (masses • Where is the plant on the property? of filamentlike threads), bacterial slime, or • What has the plant’s weather been like nematodes. recently? 88 • Plant Diseases Chapter 6

Chapter 7, “Diagnosing Plant Problems,” Yellowing, stunting, or wilting of aboveg- will help you organize your diagnostic round parts is often caused by root injury. approach. This systematic method involves Internal vascular discoloration in stems is defining the real problem and distinguish- caused by many fungi and some bacterial ing between living and nonliving causes wilts. of plant damage. Steps include looking for damage patterns, tracking the development Storage Organs of damage over time, and building a case history. With these steps, it usually is easy to Discolored or dead areas that penetrate narrow the possibilities and turn to special- deep into storage organs such as tubers, ists or appropriate reference materials, such bulbs, corms, etc. is caused by many fungi as textbooks or herbarium samples, for and some bacteria. Dry rots often are precise identification. caused by fungi, which also may produce Once you determine that a real prob- mycelia or spores. Soft rots usually are lem exists and is caused by a living disease associated with bacteria such as Erwinia organism, decide what type of organism spp. and can be accompanied by strong, may be causing the damage (fungi, viruses, repulsive odors. Often, bacterial soft rots bacteria, or nematodes); The symptoms follow rots caused by fungi, making diag- described below deal only with pathogens, nosis difficult. Other internal problems (for though some insect problems mimic dis- example, ring rot of potato) can be caused eases, but the symptoms described below by bacteria or by several viruses. deal only with pathogens. Scurfy, dead surface tissue may be caused You can eliminate disease possibilities if by a variety of funguslike organisms (for you know which diseases a plant is suscep- example, powdery scab of potato). Galling tible or resistant to. of storage organs can be caused by fungi or nematodes. Symptoms Seedlings Symptoms of plant disease can be found Seedlings sometimes fail to emerge or in and/or on the plant’s roots; storage fall over and die. This condition usually is organs; seedlings;leaves; stem, branch, and referred to as damping-off. Fungi such as trunk; flowers; and fruits. Rhizoctonia, Pythium, and Fusarium are common causes; they affect seedlings just at Roots or below the soil line. Dead areas on stems or on seed leaves External root symptoms include galls, (the first true leaves, called cotyledons) are discoloration, or death of roots or parts of usually caused by fungi but occasionally by roots. bacteria. Spots caused by fungi may con- Crown gall, caused by the bacterium tain hyphae or fruiting bodies, while those Agrobacterium tumefaciens, is a common caused by bacteria may look water-soaked cause of galls on many plants. Galls are also around their margins. caused by some fungal diseases, such as Rusty-red, brown, or black spots or club root of cabbage. Large or small irregu- stripes indicate rust or smut fungi. White, lar galls can be caused by root-knot nema- moldy growth can be caused by powdery todes (Meloidogyne spp.). mildew or downy mildew fungi. Small, discolored, dead areas may be Yellowing (either mottled or patterned) caused by a wide variety of fungi or root- can be caused by several viruses. Ring spots, lesion nematodes (Pratylenchus spp.). Death blotches, and streaks of yellow or green also of feeder roots or the entire root system is may occur with viral infections. indicative of many fungi. Swelling or galling of stems or leaves can be caused by nematodes such as Ditylenchus. Chapter 6 Plant Diseases • 89

Leaves Flowers Discoloration (yellow or shades of green) Abnormal color changes and/or distor- that is localized or in a distinct pattern usu- tions can be caused by several viruses. ally indicates a virus. General or uniform Partial or complete death of flower parts yellowing may indicate a root rot. can be caused by fungi or bacteria. Fungi Just as with stems or seed leaves, dead usually produce characteristic spores, while (necrotic) areas on leaves can be caused by bacterial infections often look water- fungi or bacteria, with those caused by fungi soaked. Individual flowers into masses of potentially containing hyphae or fruiting black spores by smut diseaes; an example is bodies and those caused by bacteria often corn smut. having water-soaked margins. Small, rusty-red, brown, or black spots or Fruits stripes may be caused by rust or smut fungi. Leaf distortion (for example, elonga- A wide variety of decays, rots, and tion or dwarfing) can be caused by several superficial spotting or russetting are caused viruses. Leaf galls, such as peach leaf curl by fungi. Important symptoms include the and azalea leaf gall, usually are caused by specific color of rotted tissue, tissue firm- fungi. Moldy, white leaves indicate powdery ness, and signs such as spores or fruiting or downy mildew. structures. Viruses can cause discoloration Wilting indicates lack of water, which and malformation can be caused by viruses may be due to vascular wilt fungus, root and discrete spots on fruit in the field or soft rot, nematodes, or bacteria. Remember to rots in storage can be caused by bacteria. examine the entire plant to find possible reasons for the symptoms. Plant Disease Control The five basic principles of plant disease Stem, Branch, and Trunk control are exclusion, avoidance, eradica- The leaf disease symptoms described tion, protection, and resistance. Although above also apply to most stem disorders of therapy is another basic principle, it gener- annuals and herbaceous perennials. ally is not practiced by home gardeners. Cankers (complete or partial death of woody stems or branches) can be caused Exclusion by a wide variety of fungi or several bac- teria. Cutting into the wood with a knife This method includes quarantines, may reveal a sharp border between healthy inspections, and certification. These tech- and infected tissue. Some bacterial cankers niques prevent movement of diseased plant excrete a sticky ooze in the spring. material into a particular country, state, or Wood rots and decays are caused mainly geographic area where the disease doesn’t by fungi. They may be accompanied by large exist. The most visible use of this method is conks or bracketlike fruiting structures. in California. At its state border, everyone Galls can be produced by the crown gall must stop at an agricultural inspection sta- bacterium. A few galls or swellings, such as tion in a screening for fresh fruit, vegetables, white pine blister rust, are caused by fungi. and plants being brought in from areas Witches’ brooms (excessive branching) where certain diseases or insects are known can be caused by several fungi or by mistle- to exist. toes, a group of parasitic plants. 90 • Plant Diseases Chapter 6

Quarantines are regulations forbidding Eradication sale or shipment of plants or plant parts. Many quarantines of plant materials com- Rotation, sanitation, elimination of ing into the United States as well as into alternate hosts, chemical applications, and individual states are in place. For example, heat treatment are eradication methods to plants harboring sudden oak death or plum reduce or eliminate diseases when a plant pox virus are quarantined to prevent impor- is infected or an area is infested with a tation of these diseases into Kentucky. As a pathogen. result, you can’t obtain certain varieties or Crop rotation is common in both com- types of plants from nurseries outside the mercial agriculture and home gardens and Kentucky region. involves planting different crops in a given location each year. This practice can reduce soil populations of fungi and nematodes. Avoidance To practice rotation, you need to know If a disease does occur in your area, you the pathogen and which plants are its hosts. may be able to avoid its development on Rotation works only if you plant non-hosts. your plants. Choosing the right site and For example, melons and cucumbers are time for planting, storing produce properly, hosts of many of the same diseases, so they and avoiding wounds to plants are a few of are not an effective rotation. Cabbage, how- these techniques. ever, is a host of different diseases, so it is a Planting certified, virus-free stock is a good crop to rotate with melons. See Table good way to avoid viral diseases. 1 for groups of related plants so you can You can avoid root rots by not planting rotate plants from different families. in heavy, poorly drained soils. Also, to avoid Sanitation—removing plant debris—is some damping-off diseases common to important where pathogens may overwin- vegetables, you can delay planting until soils ter. Rake leaves, remove rotted fruit, pick are dry and warm. up old vines, and prune out dead wood or Avoid wounding plants when pruning or canes. Dispose of the debris by burning, using equipment such as mowers or weed burying, or hot composting. trimmers, because wounds can be entry Rusts are a group of fungi that complete points for pathogens or can weaken a plant their life cycle on two or more hosts, only so it cannot defend itself—for example, the one of which may be a crop. The other host crown gall bacterium needs an injury to is known as an alternate host. Eliminating begin the infection process. alternate hosts may reduce pressure from Finally, to keep plants healthy and better these diseases, since the fungus can’t com- able to ward off infection, use good horti- plete its life cycle without the alternate host. cultural practices, such as proper fertility, Infections or infestations can be elimi- pruning, watering, and training. nated by certain chemicals. Some fungi- cides have “kickback” activity, which means they can stop some fungal infections if applied within a few days after the infection has started. Certain fungi and nematodes can be reduced by fumigating or steam steril- izing the soil. Heat treatment usually is used to eliminate viruses from propagation material. Chapter 6 Plant Diseases • 91

Table 1. Plant families for rotations. Common name of popular flowers and Plant family Some common genera vegetables Amaranthacea Celosia spp. Cockscomb Apiaceae Apium spp., Coriandrum spp., Daucus spp., Foeniculum Carrot, celery, cilantro, fennel, parsley, and parsnip spp., and Pastinaca spp. Apocynaceae Catharanthus spp. Madagascar periwinkle Asteraceae Ageratum spp., Brachycome spp., Calendula spp., African daisy, calliopsis, China aster, chrysanthe- Callistephus spp., Centaurea spp., Coreopsis spp., Cosmos mum, cineraria, cornflower (bachelor’s button), spp., Dimorphotheca spp., Gaillardia spp., Gomphrena cosmos, dandelion, floss flower, globe amaranth, spp., Helianthus spp., Helichrysum spp., Lactuca spp., Jerusalem artichoke, lettuce, marigold, pot marigold, Senecio spp., Tagetes spp., Taraxacum spp., and Zinnia spp. sunflower, strawflower, Swan River daisy, and zinnia Balsaminaceae Impatiens Impatiens Boraginaceae Myosotis spp. Forget-me-not Brassicaceae Brassica spp., Lobularia spp., and Matthiola spp. Broccoli, Brussels sprout, cabbage, candytuft, cau- liflower, daikon, flowering kale, horseradish, kale, kohlrabi, mustard green, radish, rutabaga, stock, sweet alyssum, turnip, and watercress Campanulaceae Campanula spp. and Lobelia spp. Bellflower, cardinal flower, and lobelia Caryophyllaceae Dianthus spp. and Gypsophila spp. Dianthus and baby’s breath Chenopodiaceae Beta spp. and Spinacia spp. Beet, spinach, and Swiss chard Convolvulaceae Convolvulus spp. and Ipomoea spp. Morningglory and sweet potato Cucurbitaceae Citrullus spp., Cucumis spp., and Cucurbita spp. Cucumber, gourd, honeydew melon, muskmelon, pumpkin, squash, watermelon, and zucchini Dioscoreaceae Dioscorea spp. Yam Dipsacaceae Scabiosa spp. Pincushion flower Fabaceae Glycine spp., Lathyrus spp., Phaseolus spp., Pisum spp., and Lima bean, mung bean, pea, snap bean, soybean, Vigna spp. and sweet pea Lamiaceae Salvia spp. Sage Liliaceae Allium spp. and Asparagus spp. Asparagus, chive, garlic, leek, onion, and shallot Malvaceae Abelmoschus spp. and Lavatera spp. Mallow and okra Papaveraceae Eschscholzia spp. and Papaver spp. California poppy and poppy Plumbaginaceae Limonium spp. Sea lavender and statice Poaceae Zea spp. Corn Polygonaceae Rheum spp. Rhubarb Primulaceae Primula spp. Primrose Ranunculaceae Consolida spp. Annual delphinium and larkspur Rosaceae Fragaria spp. Strawberry Scrophulariaceae Antirrhinum spp., Diascia spp., Linaria spp., Mimulus spp., Monkey flower, snapdragon, toadflax, and twinspur and Nemesia spp. Browallia spp., Capsicum spp., Lycopersicon spp., Nicotiana Amethyst flower, butterfly flower, eggplant, nico- spp., Salpiglossis spp., Schizanthus spp., and Solanum spp. tiana, painted tongue, pepper, petunia, potato, and tomato Tropaeolaceae Tropaeolum spp. Nasturtium Verbenaceae Verbena spp. Verbena Violaceae Viola spp. Pansy and violet

Protection in a preparation of the bacterium before planting. Protection means treating a healthy plant Chemical protection is one of the before it becomes diseased. Both biological most widely used means of control. Many and chemical means are available. One of fungicides are on the market, but few are the most successful examples of biological readily available to home gardeners. (See protection is use of a bacterium to protect “Fungicides for Home Gardeners” later in against crown gall bacterial disease. This this chapter.) process involves dipping a plant’s roots 92 • Plant Diseases Chapter 6

Resistance Fungicides for Home Gardeners Resistance, immunity, tolerance, and suscep- Many fungicides are registered for use tibility are terms that describe a plant’s genetic on plants, but only a few are readily avail- makeup and its reaction to plant pathogens. able to gardeners. Although many others Resistance and its opposite, susceptibility, are lev- are neither highly toxic nor restricted in els or degrees of a plant’s reaction to a pathogen. use, they may be difficult to obtain in small Different cultivars of a plant can react differently quantities. to the same pathogen. Some fungicides, such as sulfur and If a plant never becomes diseased, the copper products, are allowed under organic term “immune” can be used. The word “tol- growing guidelines. Others, such as captan, erant” describes a plant that may become triforine, and chlorothalonil, are not consid- diseased yet grow much like a healthy plant. ered organic. Planting resistant cultivars is the easiest The label is the law. No matter what any- means of disease control. You can find lists one tells you, always follow the label direc- of resistant plants in many texts and seed tions. To do otherwise is illegal. However, catalogs. keep in mind the following:

Disease Host—A plant afflicted with a Powdery mildew—A group Pathogens disease. of fungal pathogens with Bacterium—A single-celled, spore production that causes microscopic organism having Terminology Hypha—A single filament of a white-to-gray powdery myce- fungus. a cell wall but no chlorophyll; General terms lia on the host’s exterior. it reproduces by cell division. Disease—Abnormal and harm- Immunity—A relationship Quarantine—A regulation Fungus—A plant organism ful physiological condition between a plant and patho- forbidding sale or shipment with no chlorophyll that brought about by living gen in which the plant does of plants or plant parts in reproduces via spores and (biotic) agents, such as fungi, not become diseased. an area, usually to prevent usually has thready growth. bacteria, nematodes, or Incubation—A period of invasion by disease, insects, Examples are molds, yeasts, viruses, or by non-living (abi- development during which a nematodes, or weeds. and mushrooms. otic) factors, such as nutrient pathogen changes to a form Resistance—Qualities in a host deficiencies or water stress. that can penetrate or infect a Nematode—A microscopic plant that make it slow a roundworm, usually living in Dodder—A parasitic seed new host plant. Some fungi, pathogen’s activities. the soil; many feed on plant plant lacking chlorophyll; for instance, grow a structure roots and can be disease its thread-like yellow body called a penetration peg that Sanitation—The process of pathogens. twines around its host, from can grow through a plant’s removing sources of plant which it withdraws food and cell walls. pathogens from a growing Parasitic seed plant—A higher area (for example, by cleaning water. Infection—The condition plant that lives parasiti- up plant debris and sterilizing cally on other seed plants. Downy mildew—A group of reached when a pathogen tools and growing media). Mistletoe is an example. fungal pathogens with spore has invaded plant tissue production that looks like and established a parasitic Spore—(1) The reproductive Pathogen—A disease-produc- downy growth on the under- relationship between itself body of fungi and other ing organism. side of leaves. and its host. lower plants, containing one or more cells. (2) A bacterial Phytoplasma—A microscopic, Exclusion—A process by which Inoculation—The introduc- cell modified to survive in an bacteria-like organism that certain plants are not allowed tion of a pathogen to a host adverse environment. lacks a cell wall. in an area in order to protect plant’s tissue. Stylet—The lance-like or Virus—An infectious agent against pests and diseases Inoculum—The disease- from other areas. hypodermic-needle-like too small to see with a high- transmitting substance in a mouthpart of a nematode, magnification microscope; it Haustoria—Modified hyphal pathogen that infect plants. used to puncture and feed multiplies only in living cells. branches that grow into a from plant cells. plant cell to absorb food and water. Tolerant—A crop plant that will produce a normal yield even if diseased. Chapter 6 Plant Diseases • 93

You cannot use more product than chlorothalonil (Daconil), and sulfurs must indicated by the label, but if experts recom- be present before fungi arrive. Although mend using less, that is acceptable. benomyl and triforine are locally systemic You should never use a product on a (they move inside the plant), they must be plant not listed on the label, but if an expert applied before or soon after infection for recommends using the product for a disease maximum benefit. None of these fungicides that is not listed on the label, that is accept- can revive heavily diseased plants. able as long as the plant is on the label. Some fungicides work better when a You need to understand a pathogen’s spreader-sticker is mixed with the solu- disease cycle and host susceptibility to get tion to help the fungicide stay on the plant good control using fungicides. Proper tim- longer or spread better over the leaf surface. ing, coverage, and fungicide selection also Most wettable powder formulations benefit are important. from the addition of a spreader-sticker. Many fungicides work by protect- Liquid formulations usually already contain ing healthy plant tissues. Captan, copper, such compounds.

Symptoms Dwarfing—The underdevelop- Necrosis—Death of plant Signs ment of any plant organ. tissue. (external or internal physical (physical evidence of a disease- disease characteristics expressed Enation—Epidermal out- Phyllody—A change from nor- causing agent) by a host plant) growths on leaves or stems. mal flower structure to leafy Bacterial slime—A gooey or Blight—Rapid, extensive dis- structure. Epinasty—An abnormal dried mass of bacterial coloration, wilting, and death downward-curving growth or Rot—Decomposition and cells that oozes out of plant of plant tissue. movement of a leaf, leaf part, destruction of tissue. tissues. Blotch—A blot or spot (usually or stem. Rugose—Wrinkled. Conk—A fungal fruiting superficial and irregular in Etiolation—Development of structure (for example, shelf shape and size) on leaves, yellow, long, spindly growth Russet—Yellowish-brown or or bracket fungi) formed on shoots, or fruit. as a result of insufficient light. reddish-brown scar tissue on rotting woody plants. a fruit’s surface. Canker—A dead place on the Fasciation—A distortion of a Cyst—The swollen, egg- bark and cortex of twigs, plant that results in thin, flat- Scab—A crust-like disease containing female body of stems, or trunks, often dis- tened, and sometimes curved lesion. certain nematodes; it can colored and either raised or shoots. be seen on the outside of sunken. Water-soaked—Lesions that infected roots. Flagging—Loss of turgor and appear wet and dark and Chlorosis—An abnormal drooping of plant parts, are usually sunken and/or Fruiting body—A fungal struc- yellowish-white or gray color usually following a water translucent. ture that contains or bears of plant parts resulting from shortage. spores. incomplete destruction of Wilt—(1) Lack of freshness chlorophyll. Gall—An abnormal, localized and turgor and drooping of Mycelia—Masses of fungal swelling on leaf, stem, or root leaves from lack of water. (2) threads (hyphae) that com- Damping-off—Decay of seeds tissue. A vascular disease that inter- pose the vegetative body of in the soil or young seed- rupts a plant’s normal uptake a fungus. lings shortly after they Mosaic—Non-uniform foliage and distribution of water. emerge; usually caused by coloration with a more or Rhizoctonia, Pythium, or less distinct intermingling of Witches’ broom—Abnormal Fusarium fungi. normal green and light green brush-like development of or yellowish patches. many weak shoots. Dieback—Progressive death of shoots, branches, or roots, Mottle—An irregular pattern generally starting at the tips. of light and dark areas. 94 • Plant Diseases Chapter 6

For More Information PPA-10D Plant Disease Management Guide for Forage Legumes UK Extension publications PPA-13 Bean Diseases ID-21 Disease and Insect Control Program PPA-14 Blackleg Disease of Potato for Home Grown Fruit in Kentucky PPA-15 Bacterial Spot of Pepper and ID-59 Aflatoxin in Corn Tomato ID-84 Iron Deficiency of Landscape Plants PPA-16 Twig, Branch, and Stem Diseases of ID-85 Needle Cast Diseases of Conifers Pine ID-87 Guide for Control of Annual and PPA-17 Anthracnose Diseases of Shade Perennial Flower and Ground Cover Trees; PDF Diseases in the Landscape PPA-18 Verticillium Diseases of Shade ID-88 Woody Plant Disease Control Guide Trees for Kentucky PPA-19 Tomato Wilt Problems ID-104 Managing Diseases of Alfalfa PPA-20 Root Knot Nematode ID-105 Disease Management in the Home PPA-23 Rust Diseases of Apple Lawn PPA-24 Apple Scab; PDF ID-110 Soybean Cyst Nematode: A PPA-25 Leaf Rust of Wheat Potential Problem for Nurseries PPA-26 Corn Stalk Rots ID-112 Brown Patch Disease PPA-27 Black Rot of Grapes ID-121 Fumonisin, Vomitoxin, and Other PPA-29 Diseases of Grain Sorghum Mycotoxins in Corn Produced by PPA-30 Sampling for the Tall Fescue Fusarium Fungi Endophyte in Pasture or Hay Stands ID-122 Patch DIseases in Kentucky PPA-31 Gray Mold of Strawberries Bluegrass Lawns PPA-32 Anthracnose of Bramble Fruits ID-130 Managing Spring Dead Spot of PPA-33 Stewart’s Wilt of Corn Bermuda Grass PPA-34 Fire Blight IP-9 Food Safety Pesticide Residues in PPA-35 Gray Leaf Spot of Corn Grains, Vegetables, Fruits and Nuts PPA-37 Soybean Sudden Death Syndrome PPA-1 Chemical Control of Turfgrass in Kentucky Diseases PPA-38 Head Scab of Small Grains in PPA-9 Submitting Plant Specimens for Kentucky Disease Diagnosis PPA-39 Septoria Diseases of Wheat PPA-10A Plant Disease Management Guide PPA-40 Virus Diseases of Corn for Corn and Sorghum PPA-41 Fundamental Principles of Plant PPA-10B Plant Disease Management Guide Pathology for Agricultural Producers for Soybeans PPA-42 Soybean Cyst Nematode: Soybean PPA-10C Plant Disease Management Guide Thief and Public Enemy Number One for Small Grains PPA-43 Ear Rot of Corn Caused by Stenocarpella Maydis

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