CHAPTER 2
Understanding Wheat Diseases and Disorders Wheat diseases reduce grain yield and quality every year. Some diseases are common and may not result in consistent yield losses, while other diseases occur less frequently and may cause severe damage in wheat. The type, level, and magnitude of disease will change yearly and regionally and will be significantly influenced by weather conditions and the varieties of wheat grown. Wheat disease management begins with proper disease identification. This guide is intended to be a practical resource to aid farmers, Extension educators, and agribusiness professionals in
identifying wheat diseases. It also outlines general management options for specific diseases; however, certain situations may require different management strategies than those outlined here. It is essential to determine the economic feasibility and sustainability of any course of action and to consider how it fits with the existing goals of the farm operation. Extension personnel in each state and province can assist farmers with developing situation-specific disease management plans to help reach the goal of producing an economically viable and healthy crop (Fig. 2.1).
FIG. 2.1.  Healthy wheat field.
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IMPACT OF DISEASES ON WHEAT Each year, wheat diseases reduce yields in the United States and Canada. Estimates of losses to disease vary, but the best estimates in the United States and Canada for 2018 put losses caused by diseases alone at more than 53 million bushels. On individual farms, yield losses can be higher and sometimes total loss of the crop can occur. Disease impact varies from year to year depending on weather conditions, crop production practices, variety selection, susceptibility to disease, and geography, among other factors. Disease prevalence and economic importance fluctuate and change over time. For example, Fusarium head blight is currently one of the most important diseases of wheat but was only of minor importance during the mid-1900s (Fig. 2.2). Pathogens continually evolve, and as they change, the risk and importance of certain diseases change as well. A new race of stem rust pathogen (Ug99) threatens wheat production in Africa and the Middle East, and climate adaptations of stripe (yellow) rust, which historically impacted wheat in the U.S. Pacific Northwest, have expanded the impact of stripe rust on wheat nationally. Varieties also change as breeders select for various characteristics, which may inadvertently impact a given crop’s susceptibility to a specific disease. Similarly, changes in weather
FIG. 2.2. Fusarium head blight.
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patterns and management practices affect disease development. In many areas of the United States and Canada, the incidences of certain wheat diseases have either increased or are expected to increase because of the adoption of different production practices, such as no-till or minimum tillage, utilization of summer fallow, changes in rotation practices, and growing crops affected by similar pathogens (e.g., finger/ foxtail millet, corn, and wheat) that increase the potential for a pathogen to persist in a field. When greater emphasis is placed on selecting varieties with high yield potential than on selecting varieties with strong disease resistance, the risk to the farmer increases (Fig. 2.3). Factors beyond the farmer’s control—such as increases in humidity and the frequency of rainfall events—can also increase the likelihood of disease. Losses from disease are not always tied directly to yield loss caused by plant injury or disease symptoms. Economic loss can occur if a quarantined pathogen is detected in a wheat seed lot designated for export that is subsequently rejected. Profitability can also be reduced through lost time and higher fuel costs that occur when harvesting lodged grain. Grain buyers can discount and/or reject wheat because of reduced quality resulting from the presence of a disease such as ergot or Fusarium head blight infected grain, which often has high levels of mycotoxins. Mycotoxin levels can increase in
FIG. 2.3. Varieties susceptible to soil-borne wheat mosaic virus (right) can have significant yield loss. PART ONE Introduction
lodged grain, in which exposure to the pathogen and humidity increase. In addition, grain prices influence the potential for economic loss. A loss of 2 bushels per acre has less economic impact when wheat is worth $3 per bushel than when it is worth $10 per bushel. Similarly, economic losses that occur in diseased organic wheat production may be greater than in conventional wheat production because of the potentially higher value of organic wheat.
INFECTIOUS DISEASES What Is Plant Disease? The term plant disease can be defined as an abnormal change in the physical form or function of a plant caused by a living organism. Disease-causing agents include bacteria, fungi, oomycetes, viruses, and nematodes. Living organisms, such as bacteria and fungi, cause diseases, while nonliving agents, such as drought and nutrient deficiencies, cause disorders. Plant disorders are sometimes confused with infectious diseases, so it is critical to distinguish between diseases and disorders. Changes in plant tissue color, shape, and function in response to disease are called symptoms. These visible effects include leaf spots, leaf streaks, yellowing of leaves, premature defoliation, plant stunting, wilting, formation of stem cankers, and many other abnormal appearances (Fig. 2.4).
FIG. 2.4. Purpling on leaves is a possible symptom of barley yellow dwarf.
Plant pathogens cause symptoms to develop in several ways. They may release toxins within the plant, destroy and feed on plant tissue, disrupt photosynthesis, restrict the plant’s ability to obtain or transport water and nutrients, or cause other issues in the host plant. Each pathogen has evolved a unique niche or mode of causing infection, so that no two wheat diseases produce the same symptoms. Other important diagnostic features of plant diseases are signs. Signs are physical components of plant pathogens, including fungal fruiting bodies (such as pycnidia, sclerotia, and mushrooms), spores (Fig. 2.5), mycelium, and bacterial ooze. Some pathogens produce unique or characteristic signs and symptoms of disease that aid in identification. Other pathogens produce symptoms that are subtle and difficult to distinguish from those of other diseases and disorders. Moreover, symptoms on plants can change as the season progresses and can be influenced by crop genetics, environmental conditions, and even host response to the pathogen. Some symptoms are found on only one type of plant tissue, such as the roots or stems, while others are observed throughout the plant. Identifying a disease with nondescript symptoms often requires performing additional steps, including examining plants in the laboratory or looking for pathogen growth on or in plant tissue.
FIG. 2.5. Spores of the fungus that causes stem rust are a sign of the pathogen.
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The Plant Disease Triangle Plant disease can occur only if these three elements exist simultaneously (Fig. 2.6): 1. a susceptible host 2. a disease-causing pathogen 3. an environment suitable for disease development A susceptible host is a plant that has been infected by and is providing a food source for a disease-causing pathogen. The invading organism will disrupt development and/or reproduction in a susceptible wheat plant. Pathogens are living, disease-causing agents. They may enter a plant in a variety of ways, such as through wounds and natural openings. Some pathogens secrete enzymes that enable them to penetrate the plant cell walls directly. Insect and nematode feeding on plants can transmit some pathogens, including viruses, or provide an opening for invading pathogens. Disease will not develop without a favorable environment, and the environmental conditions that favor different pathogens vary. Even highly susceptible wheat varieties exposed to pathogens will not get infected or exhibit symptoms when temperature and moisture levels are not conducive for disease development. This fact helps explain why certain diseases are problematic one year but not another or in one specific location but not across an entire region. Almost all pathogens are sensitive to temperature and moisture levels in the air or the soil or on plant
FIG. 2.6. Plant disease triangle.
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surfaces. Temperature and moisture dictate when many pathogens can infect plants, move through soil or on leaves, or produce reproductive structures in plant tissue. Each pathogen has a specific temperature or moisture range at which it is most active, and that range varies widely across pathogens. The wheat plant’s susceptibility to a diseasecausing pathogen also can be influenced by environmental conditions. For example, when growth or germination slows because of cool and/ or wet soil, young wheat plants are more vulnerable to certain seedling diseases (Fig. 2.7). In contrast, seeding winter wheat into warm, very dry soil may result in dry seed decay caused by invading fungi such as Penicillium and Aspergillus species (Fig. 2.8).
FIG. 2.7. Wet soil conditions can lead to seedling diseases such as Pythium root rot.
FIG. 2.8. Wheat seed affected by dry seed decay. PART ONE Introduction
Lack of moisture also can stress plants, making them more susceptible to disease. Other environmental factors involved in disease development are soil pH, soil texture and compaction, relative humidity level, available nutrients, and light level. Temperature and moisture also can influence vectors of certain pathogens, such as plant viruses. Vectors include insects, plant-parasitic nematodes, fungal-like organisms, mites, and parasitic plants. Hot, dry conditions can increase the presence of some important vectors, such as wheat curl mites and aphids, while wet conditions might favor the presence of a parasitic protist such as Polymyxa graminis that may transmit viruses. Understanding how weather affects vectors can facilitate accurate plant disease diagnoses. The use of management tools can influence disease development by effectively removing or diminishing one or more components of the disease triangle. For instance, planting disease-resistant varieties reduces the impact of disease on the host. Removing crop residue, rotating crops, applying pesticides, and managing vectors are means of diminishing the pathogen. Draining low areas of a field and practicing effective irrigation targeted to crop usage are examples of changing environmental conditions to help reduce disease. Cultural practices such as tillage, residue management, seedbed preparation, planting date, and planting depth all affect disease development. Plant disease can only occur when all three elements of the plant disease triangle are present. Epidemics develop when all three elements occur simultaneously at optimal levels over large regions.
cannot self-produce nutrition the way plants do, because they lack chlorophyll (the compound that allows plants to carry out photosynthesis). Most fungal organisms produce hyphae (singular: hypha): threadlike fungal strands that extract nutrients and water from a host. Hyphae can also produce toxins, enzymes, and other chemicals, substances that influence disease progression and symptom expression in an infected host. A mass or network of hyphae is called mycelium (plural: mycelia). Mycelium often is not visible, because it develops inside host tissue. Sometimes, mycelium can be seen on host surface tissue, like the fluffy, white mycelial growth of the fungus that causes powdery mildew on wheat (Fig. 2.9). Spores are the most common way for fungi to reproduce and spread. Spores germinate and can initiate new infections. Fungal spores are typically very small and cannot be seen in detail without a microscope. Water and wind are the primary methods for moving spores from plant to plant and to new geographic areas. Spores of fungi that cause diseases such as stem rust and stripe rust can travel long distances on wind currents. Some fungi produce specialized structures known as fruiting
Major Groups of Plant Pathogens and Mycotoxins Fungi Fungi are common plant pathogens that cause a variety of diseases, from root rots to leaf and head blight diseases. Not all fungi cause crop disease; many are beneficial. Certain fungi decompose organic matter (dead plants and animals) and can be harnessed to produce medicine and food. Fungi can obtain energy from dead or decaying plant and animal tissue or from a living host. Fungi
FIG. 2.9.  Mycelium of the fungus that causes powdery mildew.
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bodies, which contain spores (Fig. 2.10). Fungi may survive long periods in soil or on dead plant tissue as specialized, toughened spores or fruiting bodies. Identification of fungi is sometimes accomplished by looking at spores and fruiting bodies under magnification. Symptoms of fungal diseases are very diverse and include leaf lesions or discoloration, decaying roots, seedling blight, plant wilting, and discolored seeds. Signs of fungal pathogens include mycelium on the surface of a leaf or stem; fruiting bodies embedded in leaf, spike, or stem tissue; and hardened sclerotia in a mycelial mat on an infected spike.
and infection of plants. Symptoms of oomycete diseases include leaf lesions and discoloration, stem lesions, root decay, seedling blight, wilting, damping off, and seed discoloration. The plant diseases caused by oomycetes are among the most devastating and economically important and have played an important role in history. For example, the Irish potato famine of the mid-1800s was caused by late blight, and the causal pathogen is the oomycete Phytophthora infestans. In wheat, a common disease caused by an oomycete is Pythium root rot.
Oomycetes
Bacteria also cause some of the most important plant diseases, such as bacterial streak of wheat. Bacteria reproduce by splitting in half and can increase in population very quickly. Under ideal conditions, a single bacterium splitting once every half hour will grow to more than 250 trillion bacteria in a day’s time. Colonies of bacteria form when many bacterial cells clump together. Colony characteristics—such as color, shape, and size when grown in culture— can help to identify bacteria. Large clumps of bacterial cells can produce stringy ooze and bacterial streaming when an infected plant part is cut and placed in water. Single bacterial cells are too small to see without a microscope. Plant-pathogenic bacteria do not make their own food; they must obtain it from living or dead plant tissue. Nearly all plant-pathogenic bacteria are poor competitors in soil, and their populations
Oomycetes resemble fungi in appearance, and like fungi, they are composed of mycelia. However, oomycetes have several characteristics that differentiate them from fungi. The cell walls of oomycetes are composed of cellulose (the same material as plant cell walls), whereas the cell walls of fungi are composed of chitin. Oomycetes produce spores that have tail-like structures called flagella, which enable them to swim in water (Fig. 2.11). Consequently, saturated soil conditions favor oomycete spore production, pathogen movement,
FIG. 2.10. Fungal fruiting bodies of the fungus that causes Septoria tritici blotch.
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Bacteria
FIG. 2.11. Oomycete oospores and zoospores.
PART ONE Introduction
decline rapidly without a host. Bacteria enter plants through natural plant openings (such as leaf pores) and wounds caused by hail, insect feeding, and windblown soil. Spread from plant to plant occurs primarily by wind-driven rain and mechanical movement. The symptoms caused by bacteria include water soaking, head discoloration, and leaf lesions (Fig. 2.12). A sign of a bacterial pathogen is bacterial streaming (described earlier in this section) and bacterial exudate (ooze) on moist leaf surfaces that when dried becomes scale-like, reflecting sunlight.
Viruses Viruses are another type of plant pathogen. They are extremely small and can be seen only with an electron microscope (Fig. 2.13).
A virus is usually composed of a protein coat surrounding a piece of genetic material (DNA or RNA). It multiplies by programming infected plant cells to make more virus particles, instead of the cell carrying out normal plant functions. This phenomenon explains why virus-infected plants often exhibit abnormal growth, such as stunting and unusual leaf colors and patterns (mosaic). Viruses are generally able to survive only within living host cells and require direct transport from infected to healthy plants. Most plant viruses are spread by insects (vectors), which transmit these pathogens as they feed on healthy plants. People, nematodes, tools, and machinery can also spread viruses. Sometimes, leaf-to-leaf contact is enough to spread a virus from an infected plant to a healthy plant. Symptoms of virus diseases include mosaic and mottle patterns on foliage, tissue distortion, and stunted growth.
Nematodes
FIG. 2.12. Lesions of bacterial leaf streak on wheat.
FIG. 2.13. Virus particles.
Nematodes are nonsegmented roundworms that live in soil and water. They are much smaller than earthworms, and typically magnification is required to see them. Some non-plant-pathogenic nematodes can be seen with the naked eye. Nematodes feed on plants, animals, fungi, organic matter, and even other nematodes. One acre of a cultivated agricultural field contains hundreds of millions of nematodes. Plant-parasitic nematodes puncture plant cells with a specialized feeding structure called a stylet. The stylet—which is hardened, needle shaped, and hollow—is used to inject substances into and remove food from plant cells. Plant-parasitic nematodes may invade the plant and feed within plant tissue, or they may feed on the plant from its exterior. Most plantparasitic nematodes feed on plant roots, interfering with the uptake of water and nutrients. Nematodes often interact with other nematodes and pathogens to injure plants or increase plants’ susceptibility to disease. The presence of nematodes may increase the likelihood that some diseases will develop, mainly root rots. Nematode feeding will also increase the susceptibility to other plant stresses, such as dry soil conditions. A nematode begins its life cycle as an egg; masses of eggs are laid by female nematodes. Juveniles
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emerge from the eggs and they molt (shed their body walls) several times before becoming adults. Symptoms of damage caused by nematodes occur both above and below the ground. Nematodeinfected root systems may appear rotted or stunted or have portions destroyed; some nematodes cause root proliferation (gall formation and abnormal branching). Aboveground symptoms include wilting, stunting, yellowing, and loss of plant vigor; these symptoms can be confused with those caused by other diseases and disorders. Signs include visible nematode bodies—for example, the cyst formed by the cereal cyst nematode (Fig. 2.14).
Mycotoxins Mycotoxins are toxic byproducts produced by some of the fungi that cause head diseases, such as ergot and Fusarium head blight. These toxins are produced during fungal infection and are not living organisms. Mycotoxins can cause a range of health issues when animals and humans consume contaminated plant material. Mycotoxins are extremely stable in grain and plants, and heat, freezing, and chemicals will not typically degrade these compounds. Mycotoxins are found at higher levels in broken grain particles
known as fines, and it is possible to screen or clean grain to remove these smaller particles containing higher levels of mycotoxins. Grain separation techniques based on kernel size and/or weight also may remove some kernels containing mycotoxins. Coring bins to monitor for mycotoxins will determine if grain should be stirred or aerated to reduce the risk of fungal growth and toxin accumulation. None of these practices removes mycotoxins directly from grain but rather removes grain affected by mycotoxins. Sample collection and preparation are extremely important in getting an accurate test for mycotoxins in grain. Several samples should be taken from grain and then combined into one larger sample and submitted for analysis. Sampling several different times from a moving stream of grain while the grain is being loaded or unloaded is the preferred method for sample collection. Farmers should avoid putting potentially contaminated grain into bins, if possible, and they should exercise caution and be aware of potential dangers if they must enter a bin. The U.S. Department of Agriculture Mycotoxin Handbook has specific recommendations and methods for sampling to ensure that the sample to be tested represents the grain population accurately (www.ams.usda.gov/sites/default/files/ media/MycotoxinHB.pdf). The Canadian Grain Commission grain sampling procedures also are available online (www.grainscanada.gc.ca). Mycotoxins can be assessed using several different chemical and immunocapture technologies. Samples should be sent to a professional laboratory for analysis.
The Disease Cycle The disease cycle is the series of events that occurs as a plant pathogen infects and causes disease on a wheat plant (Fig. 2.15). This concept is similar to describing the life cycle of an insect, which passes through various stages before reaching maturity. For many plant diseases, the disease cycle includes these stages:
FIG. 2.14.  Cereal cyst nematodes on wheat roots.
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1. initial production of inoculum 2. dispersal of inoculum to a suitable host 3. penetration of host
PART ONE  Introduction
4. infection of host 5. colonization of host tissue and incubation 6. development of symptoms 7. initiation and completion of secondary disease cycles (if any) 8. survival of the pathogen (when not in a living host)
Inocula (singular: inoculum) are the forms of pathogens capable of spreading and facilitating host infection. Fungal spores, bacterial cells, and virus particles are all forms of inocula. Primary inoculum is dispersed initially to a host, and secondary inoculum is dispersed from one host to another or from one plant part to another.
FIG. 2.15.  Disease cycle of stripe rust.
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