Plant Diseases PDF

PPA-46 Plant Diseases By Kimberly Leonberger, plant pathology Extension associate; Kelly Jackson and Robbie Smith, Extension horticulture agents; and Nicole Gauthier, Extension plant pathologist....

PPA-46 Plant Diseases By Kimberly Leonberger, plant pathology Extension associate; Kelly Jackson and Robbie Smith, Extension horticulture agents; and Nicole Gauthier, Extension plant pathologist. pathogen is still present in production systems today, but it is In this chapter: managed by resistant varieties, sanitation, and fungicides. Near complete loss of the American chestnut was caused by a fungal disease that nearly wiped out forests in the eastern History and Significance 01 United States. The trees once grew to majestic heights within Pathogens 01 their native forest habitat, as well as in urban plantings. They Conditions for Disease Development 02 provided high-quality hardwood for building construction and Fungi 02 nuts as a food source for people and wildlife. However, in the late 1800s, chestnut blight was accidentally introduced to the Oomycetes (Water Molds) 07 United States through imported Chinese chestnut trees. While Bacteria 09 Chinese chestnuts are tolerant to the blight, American chestnut Viruses 11 trees are not, and in less than 40 years, approximately 30 million Plant Parasitic Nematodes 13 acres of chestnut trees died. Chestnut blight remains a problem, Phytoplasmas 15 and researchers are still seeking options for management. Disease management changed drastically when the French Parasitic Seed Plants 15 wine industry gave way to the first fungicide, Bordeaux mixture. Table of Common Symptoms 16 France’s grape production had long been devastated by pow- Plant Disease Diagnostics 18 dery mildew and downy mildew. Then in 1882, the discovery of a copper sulfate and lime mixture helped manage these Plant Disease Management 19 vineyard diseases. Bordeaux mixture is still used in a modified Resources 20 form to manage powdery mildew, downy mildew, and other fungal diseases on numerous types of plants. These examples represent situations in which plant diseases A nyone who has ever planted a garden knows not only the have reached historical proportions. However, the amount of rewards of beautiful flowers, fruit, and/or vegetables, but damage that plant diseases cause varies depending upon fac- also the disappointment when plants become diseased or dam- tors such as environment, host health and susceptibility, and aged. Many factors cause plants to exhibit poor vigor, changes in pathogen biology. There are many options for managing disease appearance, or even death. Both abiotic (non-living) and biotic development and spread. The effectiveness of management (living) factors can negatively impact plant heath. Disorders techniques begins with proper identification of the disease that result from non-living factors (such as nutrient deficien- and/or causal organism. cies, too much or too little watering, temperature stress, and chemical damage) are discussed in subsequent chapters. This chapter focuses on those living organisms that cause disease: Pathogens fungi, water molds, bacteria, viruses, nematodes, phytoplasmas, A plant disease is any physiological or structural abnormal- and parasitic plants. ity that is caused by a living organism. Organisms that cause disease are referred to as pathogens, and affected plants are referred to as hosts. Many organisms rely on other species for History and Significance sources of nutrients or as a means of survival, but they are not Plant loss to homeowners may result in frustration and always harmful to the host. For example, saprophytic organisms minor monetary cost. However, on a global scale, plant diseases obtain nutrients from dead organic material and are a vital cause an estimated $38 billion1 in annual losses. History also part of many ecosystems. Plant pathogens, on the other hand, provides some perspective on the impacts of plant disease. utilize hosts for nutrients and/or reproduction at the hosts’ One of the most notable historical impacts of plant disease expense. Disease-causing organisms include fungi, oomycetes was caused by late blight of potato. This disease was a major (fungus-like organisms called water molds), bacteria, viruses, contributing factor in the Irish potato famine of 1845. During nematodes, phytoplasmas, and parasitic seed plants. this time, approximately one million people perished from Once a pathogen infects a host, symptoms often develop. starvation; a million and a half more are believed to have left Symptoms are the outward changes in the physical appearance Ireland and immigrated to the United States. The late blight of plants. Symptoms take time to develop, and thus, disease CHAPTER 05 Plant Diseases development may be delayed for several days, weeks, months, or Environment plays an important role in disease develop- even years after initial infection occurs. Examples of symptoms ment. Pathogens generally require specific environmental include wilt, leaf spots, cankers, rots, and decline. conditions for infection and spread. Most plant pathogens Physical evidence of pathogens (called signs) may also be require high humidity and moderate temperatures. Other observed on diseased tissue. Examples of signs include fungal pathogens, such as bacteria and water molds, require surface fruiting bodies, bacterial ooze, nematode cysts, and fungal water for spread. In some disease cycles, environmental condi- mycelia. Both symptoms and signs are utilized in making dis- tions influence the development of symptoms. For example, ease diagnoses. extreme temperatures or drought can cause plant stress; this loss of vigor can increase host susceptibility to both infection Conditions for Disease Development and disease development. Other environmental factors affect- ing disease can include those resulting from planting and main- Disease development is dependent upon three conditions: tenance practices. For example, high-density plantings can have a susceptible host plant, a favorable environment, and a viable higher relative humidity, while overhead watering increases leaf pathogen. All three of these factors must be present for disease surface moisture needed by pathogens to infect plant leaves. to occur. Figure 5.1 presents this concept as a “disease triangle.” Pathogens must be present and viable in order to infect Each side of the triangle represents one of these factors: host plants and cause disease. Removal of infected plant parts and plant, environment, or pathogen. When all three sides of the other remnants of pathogens makes them unavailable for infec- triangle are complete, disease occurs. If one of the conditions tion. Fungicides also reduce amounts of inoculum (infective is not present (one side of the triangle is missing), then disease propagules) available for infection. Many pathogens, however, does not occur. By altering the susceptibility of host plants, the have developed specialized structures that ensure survival dur- surrounding environment, and/or the viability of pathogens, ing adverse conditions. For example, several water molds and the disease triangle can be broken and disease development fungi are capable of surviving in soil for many years until con- prevented. ditions are favorable for infection. Pathogens may also survive Host plant genetic makeup determines its susceptibility to winter temperatures and other harsh conditions in infected disease. This susceptibility depends upon various physical and plant tissue. If a susceptible host and favorable environment biochemical factors within the plant. A plant’s stature, growth are not available, some pathogens can assume a dormant state habit, cuticle thickness (a protective outer layer on plant tis- for many years. sues), and shape of stomata (small openings that allow water, oxygen, and carbon dioxide in and out of plant tissues) are a few physical factors that influence disease development. Plants Fungi may also produce biochemical compounds that limit or prevent Fungi are the most abundant group of plant pathogens. There colonization or infection. Growth stage and ability to deter are thousands of fungi capable of causing plant diseases. These pathogens can also impact plant susceptibility to disease. For multicellular organisms are typically microscopic. The body of example, young leaves are often more susceptible to infection a fungus is composed of filament-like threads called hyphae. than mature leaves. Masses of hyphae are called mycelia. When large enough, these masses can be seen without the aid of a microscope. Powdery mildew is one example of a disease in which fungal mycelia are visible. Fungi reproduce via spores, which can be produced sexually or asexually. Spores vary in color, shape, size, and function, and this variation can often be used by diagnosticians as a means to identify pathogens. Some fungi produce spores within sexual fruiting structures (ascocarps, pustules, mushrooms) or asexual fruiting structures (pycnidia, acervuli). While many of these fruiting structures are small and may require the aid of a micro- scope, some (such as mushrooms or rust pustules) can be seen with the naked eye. Other types of fungi produce exposed or unprotected spores that are not enclosed in structures. These spore types are more sensitive to environmental conditions than enclosed spores. Once a fungal spore makes contact with a plant surface, it germinates, much like a seed, as long as conditions are favor- able for the pathogen. Hyphae emerge from spores and are Figure 5.1. Disease Triangle—Plant disease results when there is a capable of infecting plants via natural plant openings (such susceptible host, viable pathogen, and favorable environment. 5-2  Plant Diseases CHAPTER 05 as stomata), through wounds, or by direct penetration of the system or by splashing spores that result in multiple infection plant epidermis. Upon infection, fungi utilize nutrients from sites on multiple plant tissues. their hosts. Many fungi produce additional spores as they grow, Common symptoms caused by fungi include leaf spots, which aids in the spread of the pathogen. wilts, blights, cankers, fruit rots, and dieback. For additional Spores or mycelial fragments (these infective units are information on symptoms that can result from fungal infections, referred to as propagules) can be spread via wind, water, soil, refer to Table 5.1. Fungi cause a wide range of diseases includ- insects, animals, and humans. Fungi spread not only from plant ing Septoria leaf spot, powdery mildew, cedar-apple rust, and to plant, but also within a single plant. Fungi may spread through Armillaria root rot. Life cycles of these diseases are presented a single plant as a systemic infection by utilizing the vascular in Figures 5.2, 5.3, 5.4, and 5.5 respectively. 2a 2b 2c 2d Figure 5.2. Septoria leaf spot of tomato is a disease familiar to many gardeners. Spores survive winter on infected plant debris (2a) and weed hosts. In spring, when temperatures are favorable, rain splashes spores (2b) onto new plant growth where infection occurs. Small circular lesions develop on leaves (2c), petioles, and stems. This pathogen is not known to infect fruit. Symptoms first develop on lower plant parts where humidity is high and the first splashed spores land. As the season progresses the fungus produces additional spores, which are splashed by water further up plants. Severely infected plants may exhibit necrosis and defoliation (2d). 5-3 CHAPTER 05 Plant Diseases 3a 3b 3c 3d Figure 5.3. Powdery mildew is a common disease that is known to infect numerous landscape and garden plants. Spores of this fungal patho- gen survive winter on infected plant debris or woody tissue (3a). In spring, when temperatures are favorable, rain splashes spores (3b) from leaf litter onto new plant growth where infection occurs. Throughout the season the fungus produces additional spores (3c) causing subsequent infections. The result is a white powdery plant (3d) that may exhibit early defoliation. 5-4  Plant Diseases CHAPTER 05 4a 4b 4c 4d Figure 5.4. Cedar-apple rust is caused by a unique group of fungi known as rusts. Many rust pathogens require multiple hosts to complete their life cycles. In the case of cedar-apple rust, the pathogen overwinters as galls on juniper (4a). Once spring rains begin, galls produce slimy “horns” comprised of spores (4b). These spores infect leaves and fruit of apple (and related hosts) (4c). In late summer, apple lesions produce another spore type that infects juniper (4d). 5-5 CHAPTER 05 Plant Diseases 5a 5b 5c 5d Figure 5.5. Recently transplanted and older stressed trees are susceptible to Armillaria root rot (also called shoestring rot). These fungal patho- gens survive in dead wood and roots, as well as soil. Infection occurs in the roots or base of the tree via rhizomorphs (cord-like strands of fungal hyphae) (5a). If bark is removed from infected trees, a white mycelial growth may be observed (5b). During rainy weather, pale brown mushrooms may develop at bases of infected trees or along dead or dying surface roots (5c). Severely diseased plants exhibit decline or even death (5d). 5-6  Plant Diseases CHAPTER 05 Oomycetes (Water Molds) are microscopic, but their examination is essential for proper species identification. As the name implies, water is essential for survival, repro- Water molds infect in the same ways as true fungi by entering duction, infection, and spread of oomycetes (commonly called through natural plant openings or by direct penetration into water molds). Water molds were once considered true fungi, plant tissues. Once infection occurs, water mold pathogens con- but they are now classified as fungus-like organisms. Water tinue to grow and produce additional spores for new infections. molds and fungi are similar in appearance, as the body is com- Infective propagules are spread via water, soil, and infected posed of hyphae that mass together to form mycelia. Downy plants and weeds, as well as by wind and wind-driven rain. mildew is an example of a disease with visible oomycete mycelia. Survival structures produced by water mold pathogens have Reproduction by water molds may be via sexual or asexual the ability to persist in water and soil for several years. spores. Asexually produced spores (zoospores) have the ability Common symptoms caused by water molds include leaf to move in water using tail-like structures (flagella) that propel spots, blights, cankers, root rots, wilt, dampingoff, and dieback. them, a trait not associated with true fungi. Zoospores develop For additional information on symptoms that can result from within capsules (sporangia) under specific environmental con- water mold infections, see Table 5.1. Two common diseases ditions. Sporulation can occur numerous times per growing caused by water molds are downy mildew and late blight. season, as long as water is available. In contrast, sexual spores Life cycles of these diseases are presented in Figures 5.6 and of water molds are typically produced prior to dormancy in 5.7, respectively. response to environmental stress. They serve as a means for survival under adverse conditions. Spores of water molds 6a 6b 6c 6d Figure 5.6. Downy mildew is a common disease that is known to infect numerous landscape and garden plants, including cucumber. Most water mold pathogens are host-specific. Spores (6a) of the cucurbit downy mildew pathogen are windblown north each year from overwintering sites in the southern United States Soon after infection, yellow, angular leaf spots develop (6b). Spores are produced on the undersides of leaves (6c); spores are spread to new tissue via water/rain. Defoliation may occur on severely infected plants (6d). The cucurbit downy mildew pathogen cannot survive winter in Kentucky because it requires living plant tissue, and it is not known to produce overwintering spores outside of labora- tory conditions. Other downy mildew pathogens can overwinter on leaf debris in Kentucky. 5-7 CHAPTER 05 Plant Diseases 7a 7b 7c 7d Figure 5.7. Late blight is a disease of solanaceous crops, primarily potato and tomato. The pathogen most commonly overwinters in the south- ern United States in infected plant or weed tissue. In spring, overwintering spores are windblown north from these southern sites. Once infection occurs, water-soaked, grey-brown lesions may develop on leaves (7a), petioles, stems, and/or fruit (7b). Defoliation follows (7c). Potato tubers (7d) may become infected via systemic infections or by spores washed into soil. 5-8  Plant Diseases CHAPTER 05 Bacteria for infection. Once inside plants, bacteria begin to reproduce immediately. Some types of bacteria produce toxins or enzymes Bacteria are microscopic organisms typically composed of that degrade plant tissue, and the tissue is then utilized as a food single cells. About 200 types of bacteria are known to cause source. Some bacteria can colonize vascular systems of plants, plant diseases. Due to their small size, a high-magnification which results in restriction of water movement. microscope is required to observe bacteria. Occasionally, when Bacteria spread by water/splashing rain, wind, or insects and a large number of cells are present, plants may be observed then move across plant tissues in surface water to reach wounds “oozing” bacteria and other organic byproducts. or natural openings. Some can survive for five or more years in Bacteria are capable of rapid reproduction through a process soil, as well as in plant debris and cankers. known as binary fission. In this process, one cell divides to Common symptoms caused by bacteria include leaf spots, become two, then two divide to become four cells, and so on. blights, cankers, galls, wilt, dieback, and soft rots. For addi- Within a few hours, one bacterial cell can become thousands, tional information on symptoms that can result from bacterial and under ideal conditions, populations can double in as little infections, refer to Table 5.1. Two common diseases caused by as 20 minutes. bacteria are bacterial wilt and fire blight. Life cycles of these Unlike fungi and water molds, bacteria are not able to pen- diseases are presented in Figures 5.8 and 5.9, respectively. etrate plant tissue directly. They must infect via wounds or natural plant openings such as stomata. Free water is required 8a 8b 8c 8d Figure 5.8. Bacterial wilt is a disease of cucumber, melons, and pumpkin. Bacterial cells overwinter in striped and spotted cucumber beetles, which transmit the pathogen during the growing season (8a). When insects feed on plant tissue, bacteria in their feces are deposited near feeding wounds. Infected plants wilt (8b) when large numbers of bacterial cells block water movement. Feeding beetles obtain bacterial cells from infected plants and move them to new portions of plants or to new plants. Bacteria continue to reproduce and move throughout plants, eventually resulting in plant death (8c). Bacterial ooze can be observed by cutting vines, holding the two cut ends together for 5 seconds, and then slowly pulling them apart again (8d). 5-9 CHAPTER 05 Plant Diseases 9a 9b 9c 9d Figure 5.9. Certain apple cultivars and other pome fruit are susceptible to a bacterial disease called fire blight. The pathogen overwinters in can- kers and other diseased or dying wood (9a). In spring, bacterial cells are transported from cankers to blossoms by wind and rain or by pollinat- ing insects. This first phase of disease is called the blossom blight phase (9b). Blossom and spur infections move through woody tissue to form cankers (9c). Rain, wind, and insects later carry bacterial cells from infected blossoms to actively growing shoots, causing shoot blight (9d). As weather cools and active plant growth stops, bacteria become dormant until warm rainy weather returns (often the following spring). Over many seasons, bacteria can spread throughout trees, resulting in branch or trunk girdling and eventual plant death. 5-10  Plant Diseases CHAPTER 05 Viruses plants often survive for many years before they die, as rapid plant death would be detrimental to a pathogen that depends Viruses are extremely small particles that require magnifica- upon its host for replication. tion of 100,000 times or more for observation. This can only Insect vectors are a common means for virus spread. be achieved with specialized equipment, such as an electron Numerous insects are capable of acquiring virus particles while microscope. Diseases caused by viruses are often named for the feeding on infected plants. These particles are transferred to first host plant for which symptoms were reported and/or the new plants or plant parts during subsequent feedings. Some most common symptom. An example is tobacco mosaic virus; virus particles can enter an insect’s gut, where the virus may this virus was first reported as a mosaic symptom on tobacco. persist and replicate throughout the insect’s life. Other types of However, this disease is known to infect more than 100 plant particles are only carried for short periods inside probing insect species, including many vegetable and ornamental plants. mouthparts. Insects are capable of transmitting viruses mul- Once viruses enter host cells, they “hijack” plants and tiple times throughout the season. Viruses can also spread by “instruct” cells to produce more virus particles. As plant cells infected seed or pollen, nematodes, humans, animals, or tools. are converted from their normal function and processes (such A few viruses with very stable structures can remain viable in as cell division or chlorophyll production), changes in plant dormant plant tissue and on nonliving materials, which can growth and development may be observed. serve as inoculum for future infections. Plant viruses do not move in and out of plant tissue as Common symptoms caused by viruses include mottling, readily as fungal and bacterial pathogens. They require vec- mosaic, leaf distortion, stunting, poor fruit set, and chlorosis. tors (such as insects or humans) to carry them from one plant For additional information on symptoms that can result from to another. After entry into plant cells, reproduction begins. virus infections, see Table 5.1. Two common viral diseases are Viruses spread throughout plant hosts, infecting all plant parts rose rosette and tomato spotted wilt. Life cycles of these diseases (systemic infection). Viruses are dependent upon live hosts are presented in Figures 5.10 and 5.11, respectively. for replication, thus disease progresses slowly. Virus-infected 10a 10b 10c 10d Figure 5.10. Most cultivated roses are susceptible to rose rosette virus. This virus is vectored by a microscopic rose leaf curl mite (10a). These mites move from plant to plant in wind currents, via other insects, or by crawling to adjacent plants. Symptoms, which may develop months or years after infection, can include short internodes (called witches’ broom), strapping and reddening of leaf and stem tissue (10b), and excessive thorn development (10c). Infected plants that remain in the landscape (10d) serve as inoculum and sources of vectoring mites. When removing infected plants, dig out all roots and destroy all plant parts. 5-11 CHAPTER 05 Plant Diseases 11a 11b 11c 11d Figure 5.11. Tomato spotted wilt virus (TSWV) is known to infect thousands of different plant species. Virus particles are vectored by thrips, which acquire the virus while feeding during the larval stage (11a). TSWV can replicate within insect hosts and persist throughout an insect’s life. Once infected, plants such as tomatoes may exhibit symptoms of wilting and ringspots on leaves (11b), lesions on stems (11c), and mottling or ringspots on fruit (11d). Multiple generations of thrips occur each season, and infected weedy hosts contribute to virus spread. 5-12  Plant Diseases CHAPTER 05 Plant Parasitic Nematodes or feed in multiple locations. Long-distance spread is achieved via movement of infested soil, floodwater, or plant material; Plant parasitic nematodes are microscopic roundworms nematodes are only capable of moving very short distances that primarily infect roots, but a few occur in foliar portions of on their own. plants. While there are many species of nematodes, only a few Common symptoms caused by nematode feeding include are known to parasitize plants. All plant parasitic nematodes chlorosis, root galls, damaged or stubby roots, stunting, die- have needle-like mouthparts (stylets) that are used to pierce back, and reduced yields. Some heavily infested herbaceous plant tissues and extract cell contents. Nematodes reproduce plants develop symptoms similar to nutrient deficiencies as via eggs that result from either the mating of a male and a female a result of root loss. For additional information on symptoms or by the female alone. that can result from nematode infestations, refer to Table 5.1. Symptom development occurs as a result of extracted cel- Two common plant parasitic nematodes include soybean cyst lular contents or other plant damage. Nematodes may remain nematode and foliar nematode. Life cycles of these organisms on the exterior of roots during feeding (stubby-root nematode) are presented in Figures 5.12 and 5.13, respectively. or penetrate plant tissues completely to feed while inside plants (dagger nematode). Nematodes may select a single feeding site 12a 12b 12c 12d Figure 5.12. Soybean cyst nematode juvenile nematodes (12a) infect plants by penetrating root tissue. Males feed for a short time before mov- ing on to another feeding site. Females remain at the same feeding site and expand to encyst egg masses. These lemon-shaped females are white or yellow, but they become brown as they mature and die (12b). The dead female bodies are known as cysts; each cyst contains several hundred eggs. Cysts protect eggs from adverse conditions, such as heat and drought. When conditions are favorable for nematode development, cysts burst open and release eggs (12c). Soybean plants may not exhibit any aboveground symptoms other than poor vigor and/or yield loss (12d). 5-13 CHAPTER 05 Plant Diseases 13a 13b 13c 13d Figure 5.13. Foliar nematodes can infect numerous plant species, including anemone. Adult nematodes (13a) survive winter in plant crowns or leaf debris. In spring, foliar nematodes move upward via splashing water. Once adults reach leaves (13b), they enter through stomata and begin feeding. Eggs (13c) are produced within leaves, where they hatch, molt, and begin to feed. Infected leaf cells are varying shades of green that turn brown and necrotic (13d) as cell contents are consumed. 5-14  Plant Diseases CHAPTER 05 Phytoplasmas symptom. The most common disease caused by a phytoplasma is aster yellows. The symptoms of this disease are depicted Phytoplasmas are extremely small, bacteria-like plant patho- in Figure 5.14. gens. While they are similar to bacteria, phytoplasmas differ in their inability to survive without a host, their smaller size, and their lack of cell walls. Phytoplasmas rely on insect vec- Parasitic Seed Plants tors, such as leafhoppers, for transmission into hosts. During Parasitic seed plants share many common characteristics feeding, leafhoppers acquire phytoplasmas from infected host with true plants, such as their ability to reproduce and spread phloem (nutrient-conducting vascular system) and introduce by seeds. Like other plants, seeds are disseminated via wind, them into healthy tissue. Once in the phloem, phytoplasmas water, and animals. However, parasitic plants lack the ability to are capable of reproducing and spreading throughout plants, produce all of the nutrients or water they require, so they rely which results in systemic infections. Phytoplasmas persist on host plants. In some cases, parasitic plants have developed within their insect vectors, making this the main method of modified structures, such as haustoria, that allow them to spread for these pathogens. penetrate host plants and obtain nutrients and water. Common Common symptoms caused by phytoplasmas are virescence symptoms caused by parasitic plants are reduced vigor and (development of chlorophyll in tissues where it is normally dieback. The most common parasitic plant is mistletoe, shown absent) and abnormal growth such as a “witches’ broom” in Figure 5.15. Figure 5.14. Aster yellows, which is caused by a phytoplasma, is Figure 5.15. Mistletoe often parasitizes oaks and other forest trees. a common disease of purple cone flower. Infected plants exhibit It produces large clumps of foliage on host plant branches. Mistletoe symptoms such as stunting, abnormal growth, sterile flowers, and does not produce roots and is reliant upon hosts for water and virescence in flowers. nutrients. In Kentucky, infected host plants do not suffer major dam- age or stress. In locations where other mistletoe species are present, extensive damage to the host may occur. 5-15 CHAPTER 05 Plant Diseases Table 5.1. The most common symptoms associated with each pathogen group. Plant Pathogen Groups Symptom Description Water Parasitic Fungi Bacteria Viruses Nematodes Phytoplasmas molds plants Rapid discoloration, Blight wilting and death of X X X plant tissue Blotch or large spot Blotch on leaves, shoots, X X or fruit Leaves or needles Bronzing develop a bronze X X color Dead region on bark of twigs, stems, or trunks, Canker X X X often discolored and either raised or sunken An abnormal Chlorosis yellowing of plant X X X X X parts Decay of seeds in soil or young Dampingoff X X seedlings shortly after emergence Gradual, often uniform, decline of Decline X X X X X X plant health or death of plant tissue Progressive death of shoots, branches, Dieback X X X X X or roots, generally starting at tips Irregularly shaped Distortion X X X X X plant parts Decline of a shoot or branch, while nearby Flagging X X X branches remain healthy Abnormal, localized Gall swelling on leaf, X X X stem, or root tissue Production of a Gummosis sticky gum that is X X X exuded by the plant Lesion on a leaf, Leaf spot which may vary in X X X X color, shape, and size 5-16  Plant Diseases CHAPTER 05 Table 5.1. The most common symptoms associated with each pathogen group (continued). Plant Pathogen Groups Symptom Description Water Parasitic Fungi Bacteria Viruses Nematodes Phytoplasmas molds plants Nonuniform foliage coloration, normally an intermingling Mosaic X of green color variations and yellowish patches Irregular pattern of Mottle X light and dark areas Hard, dried, diseased Mummy X fruit Necrosis Death of plant tissue X X X A lesion with a Ring spot dark outer ring and X lighter center Decomposition and Rot X X X destruction of tissue Wrinkled Rugose appearance to plant X X tissue Yellowish-brown or reddish-brown scar Russet X tissue on a fruit’s surface Crust-like disease Scab X X lesion Browning and Scorch necrosis of leaf X X X margins Lesions where Shot-hole centers have fallen X X out Reduced growth of a plant, where plant Stunting X X X X X or plant parts are smaller than normal Death of tissue at Tip blight X X the tip of a shoot Leaf veins become Vein clearing X yellow or clear Wet, dark, or greasy lesions, usually Watersoaking X X X sunken and/or translucent Drooping of leaves Wilt X X X X or other plant parts Abnormal brush-like Witches’ broom X X X shoot development 5-17 CHAPTER 05 Plant Diseases Plant Disease Diagnostics bedding plants, fruits, vegetables, and turfgrasses. However, multiple pathogen groups can cause similar symptoms (Table Plant disease diagnostics begins with the observation of 5.1). This is further complicated when abiotic factors induce both the symptomatic plant(s) and surrounding environment. disease-like symptoms. For example, fungi and bacteria can Sometimes the cause of plant problems is evident: wildlife cause leaf spots that are similar to those caused by herbicides damage, insect feeding, or mechanical injury. However, a wide and ozone. Plant disease diagnoses, therefore, cannot be based range of abiotic and biotic factors can cause disease or disease- upon symptoms alone. Identification of signs (mycelial growth, like symptoms. Basic plant diagnostics and differentiation fruiting structures, and bacterial cells) with a hand lens or between biotic and abiotic problems are discussed in Chapter microscope is required to confirm diagnoses. Often, a trained 6, Diagnosing Plant Problems. Plant disease diagnostics are diagnostician can assist with this task. covered in this section. There are several steps to disease diagnostics, including Examine the Site evaluation of vital site information and examination of dis- eased tissue. Since the majority of plant pathogens cannot be Examine the area around plants for additional clues as to seen without the aid of a microscope, it may not be possible to the cause of plant problems. This is particularly important confirm disease agents by visual assessment alone. In order to with abiotic problems. Information regarding soil makeup and diagnose plant problems, it may be necessary to submit samples disturbances, as well as soil compaction and drainage patterns, of symptomatic plants to a university or commercial laboratory can assist in differentiating abiotic maladies from diseases. for further analysis. Additionally, examination of surrounding plants can provide Even though it may not always be possible to diagnose plant vital information regarding disease spread. Patterns of injury diseases by symptoms alone, the following strategies can be or symptoms can give clues as to whether variety- or species- used to determine whether an infectious agent is the cause of specific infections are possible, or whether a more expansive disease symptoms. In addition, detailed site and plant informa- problem exists. tion can lead to a more complete diagnosis. Ask Questions Know the Host Plant(s) Ask questions and collect additional information that is criti- A single plant species can have numerous cultivars or variet- cal in making diagnoses. More information can lead to a more ies, with many different colors, shapes, patterns, textures, and accurate diagnoses. Consider these site specifics: sizes. Occasionally, diversity or variation in plant appearance Planting date and practices can be mistaken for disease. The difference between normal Irrigation practices growth and appearance and abnormal appearance can indicate Fertilization whether a problem is present. Furthermore, disease symptoms Symptom development may differ with plant species or cultivar. Some species and/or Weather conditions cultivars may be more susceptible or tolerant to disease than Site disturbances others. Host information can be critical for diagnosis. Traffic or pedestrians Pet and livestock habits Herbicide applications (dates applied in relation to when Document Plant Part(s) Affected symptoms were observed) In some cases, symptoms develop on plant parts that are different from infected tissue. For example, dieback and wilting Diagnosis seem to indicate problems in the plant canopy. However, a root rot pathogen or bacterial colonization of vascular tissue may After collection of site information and analysis of the cause similar symptoms. It is important to examine all above- symptomatic plant(s), it may be possible to diagnose some and belowground portions of plants to determine potential plant problems at this stage. However, if the cause cannot be infection site or symptom origin. Digging or cutting into plant determined, it may be necessary to submit symptomatic plant tissue may sometimes be necessary to fully understand the samples to a diagnostic laboratory. Contact a county Extension extent of parts affected. agent regarding appropriate steps for selecting and submitting samples to the University of Kentucky Plant Disease Diagnostic Laboratories. The University of Kentucky Plant Pathology Check for Symptoms and Signs fact sheet, Submitting Plant Specimens for Disease Diagnosis The presence of symptoms often indicates some sort of (PPFSGEN-09), provides helpful information for collecting plant problem. A wide range of symptoms can be expressed appropriate samples. by diseased plants. Appendix A (separate document) pres- ents common diseases and symptoms of woody ornamentals, 5-18  Plant Diseases CHAPTER 05 Plant Disease Management stems, branches, roots, and fruit) should be removed since many pathogens overwinter in plant debris. Throughout the Disease management begins with accurate diagnoses. growing season, including senescence and dormancy, all plant Diseases can only be managed once they are correctly identified. debris should be gathered and destroyed by burning, burying, The best management practice is to avoid disease altogether. or putting in the trash. Composting diseased plant mate- While disease prevention is not always possible, there are often rial is not advised, as most home compost bins do not reach recommended management practices that can be used to limit temperatures high enough to kill plant pathogens. Sanitation disease spread and subsequent infections. practices also assist in increasing the effectiveness of other management techniques. Principles of Plant Disease Management Healthy, stress-free plants are less likely to become diseased. Maintain plant vigor and avoid stress with proper site selection There are five principles to disease management that focus and nutritional balance; this significantly decreases the likeli- on prevention and limit of disease spread. These techniques hood of infections. Select plants with resistance or tolerance are most effective in combination. to common diseases, when available. Never save seeds from Exclusion diseased fruit or vegetables. Definition: Prevent pathogen introductions to areas where Fungicides can be effective in protecting plants from infec- they do not currently exist. tion or limiting spread. Although the term “fungicide” is often Common Practices: Quarantine, inspection, and certified used as a broad term, products are usually pathogen-specific. disease-free plant material. True fungicides are used to manage fungal pathogens; other products are specific to water mold pathogens. Bactericides/ Avoidance antibiotics are used to manage bacterial pathogens. All of these Definition: Inhibit establishment of pathogens that exist in products are suppressive, but only a few have curative effects. other areas. Fungicide applications will not reverse disease symptoms or Common Practices: Use certified disease-free plants or seed, save plants from death if disease is severe. Always follow label inspect plants before purchase or installation, reduce plant directions when applying fungicides. For up-to-date informa- stresses, rotate crops, and avoid wounding. tion regarding fungicide use and recommendations, consult a county Extension agent. Resistance Definition: Select plants with increased tolerance to pathogen(s). Summary Common Practices: Select seeds or plants with resistance to Plant diseases can be caused by fungi, water molds, bac- common pathogens. Consult a county Extension agent or teria, viruses, nematodes, phytoplasmas, or parasitic plants. reliable source for information on resistant cultivars. Understanding the biology and symptoms common to these plant pathogens will aid in identification and management of Protection disease problems. Once a disease is properly diagnosed, man- Definition: Implement steps to protect plants from infections. agement options can be deployed to mitigate disease impact. Common Practices: Modify environment to prevent patho- gen infections, remove alternate hosts, apply physical barriers, use biological or fungicide treatments. Appendix Common diseases and symptoms of woody ornamentals, Eradication bedding plants, fruits, vegetables, turfgrass, and grains can be Definition: Limit pathogen spread once a plant is infected. found in the Appendix A document at: Common Practices: Remove infected plant portions, remove https://plantpathology.ca.uky.edu/files/mg_ch6_appendix.pdf (rogue) entire herbaceous plants including roots, practice sanitation, use fungicide applications to minimize inoculum (fungicides do not cure disease or eradicate pathogens). Important Plant Disease Management Practices Once plant disease is confirmed, sanitation is the most important management practice in the garden. However, homeowners and growers should implement sanitation prac- tices throughout the growing season to prevent and limit plant disease development. Infected plants and plant parts (leaves, 5-19 CHAPTER 05 Plant Diseases Resources Leiden University Mohammad Mirnezhad – Fig. 5.11A University of Kentucky Department of Plant Pathology Publications Swiss Federal Research Station for Agroecology and https://plantpathology.ca.uky.edu/extension/publications Agriculture Agroscope FAL Reckenholz, Bugwood.org – Fig. 5.12A Submitting Plant Specimens for Disease Diagnosis http://plantpathology.ca.uky.edu/files/ppfs-gen-09.pdf University of Delaware American Phytopathological Society Introductory Materials Tracy Wootten, Bugwood.org – Fig. 5.10A https://www.apsnet.org/edcenter/resources/Pages/ Introductory.aspx University of Georgia American Phytopathological Society Glossary University of Georgia Plant Pathology, Bugwood.org – Fig. 5.3A https://www.apsnet.org/edcenter/resources/illglossary/Pages/ University of Hamburg default.aspx Ulrich Zunke, Bugwood.org – Fig. 5.13C 1The Future World Food Situation and the Role of Plant Diseases, Dr. Per Pinstrup-Andersen https://www.apsnet. University of Kentucky org/edcenter/apsnetfeatures/Pages/WorldFoodSituation. Paul Bachi – Fig. 5.2A-B, 5.4D, 5.6A, 5.11B, 5.12B aspx Ric Bessin – Fig. 5.8A Nicole Ward Gauthier – Fig. 5.3B-C, 5.4A, 5.9A-D, 5.10B-C John Hartman – Fig. 5.3D, 5.8C, 5.15 Acknowledgements Cheryl Kaiser – Fig. 5.5C, 5.12C Brenda Kennedy – Fig. 5.12D The authors would like to acknowledge and thank the fol- Kimberly Leonberger – Fig. 5.4C lowing University of Kentucky contributors: Amanda Sears William Nesmith – Fig. 5.8B and Andy Rideout, Extension horticulture agents, for feedback Steve Osborne – Fig. 5.8D regarding drafts of this chapter; and Dr. Emily Pfeufer, Extension Kenneth Seebold – Fig. 5.2C-D, 5.6B-D, 5.7A-C, 5.8D plant pathologist; Julie Beale and Brenda Kennedy, plant disease diagnosticians; and Cheryl Kaiser, Extension staff associate, for University of Maine Bruce Watt, Bugwood.org – Fig. 5.13A editorial and composition contributions. University of Tennessee Extension Photo Credits Alan Windham – Fig. 5.10D, 5.13D California Polytechnic State University at San Luis Obispo USDA Agricultural Research Service Gerald Holmes, Bugwood.org – Fig. 5.11C-D Scott Bauer, Bugwood.org – Fig. 5.7D Colorado State University USDA Forest Service Whitney Cranshaw, Bugwood.org – Fig. 5.14 Joseph OBrien, Bugwood.org – Fig. 5.5A William Jacobi, Bugwood.org – Fig. 5.5B Virginia Polytechnic Institute and State University Cornell University Elizabeth Bush, Bugwood.org – Fig. 5.4B Karen Snover-Clift, Bugwood.org – Fig. 5.13B Florida Department of Agriculture and Consumer Services Florida Division of Plant Industry, Bugwood.org – Fig. 5.5D Revised 01-2024