5. Diseases and Disorders Outline I. Objectives II. Introduction III. Healthy and Unhealthy Plants IV. Abiotic Disorders of Plants A. Moisture B. Plant Nutrition C. Light, Temperature, Wind and Weather V. Plant Pathogens A. Fungi B. Bacteria C. Nematodes D. Viruses E. Parasitic plants VI. Disease Development VII. Spread and Survival of Pathogens VIII. The Diagnostic Process IX. Principles of Plant Disease Management a. Cultural Management b. Chemical Management X. Case Study—Think IPM: Cucumbers in Distress XI. Frequently Asked Questions XII. Further Reading XIII. Chapter Text Hyperlinks XIV. For More Information A. NC State and NC State Extension publications B. Internet Resources XV. Contributors I. Objectives This chapter teaches people to: 1. Identify certain plant diseases and disorders using a step-by-step process. 2. Recognize when a laboratory diagnosis for a plant problem is warranted. 3. Describe and explain the differences between the major categories of plant diseases. 4. For each plant disease, explain the implications for plant health. 5. Explain how the host plant and environmental conditions affect disease development. 6. Recommend preventive strategies and management techniques for the most common plant diseases in North Carolina. 7. Distinguish between plant damage caused by diseases, insects, and environmental conditions. II. Introduction The term plant disease refers to an impairment in the structure or function of a plant that results in observable symptoms. In this chapter the focus will be on infectious diseases—those that result from an attack by a fungus, bacterium, nematode, virus, or another organism. Other disorders can be caused by abiotic (environmental and cultural) factors, such as compacted soil, excess water, nutrient deficiencies, chemical injury, or air pollution. Many of these factors produce symptoms similar to those caused by infectious agents. Some detective work is often necessary to figure out what is wrong with a particular plant. This chapter provides an introduction to the causes of plant diseases, their diagnosis, and the methods used to prevent and control them. For information on particular diseases, refer to the chapters on specific types of plants. III. Healthy and Unhealthy Plants To recognize a plant problem, you must first know what the healthy plant looks like. Although this may seem obvious, some plants have characteristics or habits at certain stages of growth that can be mistaken for symptoms of disease. For instance, patterns of light and dark colors on foliage can sometimes indicate disease, but color patterns can also be normal variegation in certain varieties of ornamentals (Figure 5–1). A deodar cedar might appear to be suffering from a nutrient deficiency because it has yellow-green leaves, but this coloring is normal for this cultivar (Figure 5–2). The spore-bearing sori on the underside of a fern frond might be mistaken for insects (Figure 5–3). The female cones on the tips of oriental arborvitae foliage look somewhat like galls (Figure 5–4). Leaves on some evergreen trees (such as some hollies and magnolias) drop in the spring as new leaves expand; homeowners new to North Carolina might become concerned if they expect leaves to shed only in the fall. None of these examples are diseases. Instead, all are normal plant structures and responses. In order to talk about diseases and disorders, we need a set of terms to describe plant abnormalities and pathogen structures. These are detailed in Tables 5–1a, 5–1b and 5–2. Table 5–1a. Common symptoms of plant diseases. Symptoms are not unique to a particular disease as a specific symptom can be caused by a variety of pathogens. Chlorosis (adj. chlorotic)(Figure 5–5): Necrosis (adj. necrotic)(Figure 5–6): Yellowing of a normally green plant part. If it Death of plant tissue. A necrotic leaf blotch is is more pronounced between veins it is called pictured. Note that any well-defined dead “interveinal chlorosis.” area of a leaf, stem, or root can be called a “necrotic lesion.” Example of necrotic (dead) leaf spots with Shot-hole (Figure 5–8): Clean-edged, chlorotic (yellow) halos (Figure 5–7). round to oval holes in leaves where necrotic spots have fallen out . Wilting (Figure 5–9): Loss of turgor in all or Scorch (Figure 5–10): Necrosis and part of a shoot. desiccation of leaf tissue, starting at the margins. Mosaic or mottle (Figure 5–11): Ringspot (Figure 5–12): Chlorotic or Patchwork of colors, usually light-green, necrotic rings or arcs surrounding healthy yellow, or dark-green, against the normal tissue. green background color of the leaf. Water-soaking (Figure 5–13): A dark, Flecking or stippling (Figure 5–14): “wet” appearance to a spot, best seen by Numerous very small chlorotic or necrotic holding the leaf up to a light source. points. Blight (Figure 5–15): Extensive and rapid Dieback (Figure 5–16): Death of a branch death of plant tissue. from the tip down. Canker (Figure 5–17): Sharply-defined dead Example of a canker visible only under the area on a woody plant part. bark (Figure 5–18). Root rot (Figure 5–19): Decay of roots. The Fruit rot (Figure 5–20): Decay of fruit. May exterior portion is easily pulled off the central be firm or soft. core of vascular tissue. Mummy (Figure 5–21): A dried, shriveled Damping off (Figure 5–22): Death of fruit. seedlings, before or after emergence from the soil. Malformation (Figure 5–23): Any deviation Leaf galls (Figure 5–24): Swellings on leafy from the normal shape of a plant organ. tissue. Stem galls (Figure 5–25): Swellings, Root galls (Figure 5–26): Swellings on root usually woody, on stems. tissue. Gummosis (Figure 5–27): Exudation of Witches'-broom (Figure 5–28): Abnormal sticky sap. In conifers, this is known as proliferation of shoots on one area of a stem. “resinosis.” Vascular discoloration (Figure 5–29): Darkening of the plant’s conductive tissue. Visible after cutting along or into the stem. Table 5–1b. Stand symptoms in turfgrasses. Specific terms are used to describe the overall patterns of death or discoloration in turf, independent of what the symptoms may be on individual leaves, stolons, and other parts. These are known as stand symptoms. Several of the most important are illustrated below. All photos are courtesy NC State University. Spots (Figure 5–30): Each area of Patches (Figure 5–31): Irregularly affected turf is less than 4” in diameter. shaped areas greater than 4” in diameter. Rings (Figure 5–33): Surrounded by Circle (Figure 5–32): Perfectly circular healthy turf to the inside and outside of the areas greater than 4” in diameter. affected area. Irregular (Figure 5–34): No visible pattern. Table 5–2. Common signs of plant pathogens. Here we see the causal organism itself, usually a fungus. Mycelium (Figure 5–35): The visible vegetative Gray mold (Figure 5–36): Fungal body of a fungus, made up of threads called surface growth, gray in color. hyphae. (sing., hypha). Powdery mildew (Figure 5–37): White Downy mildew (Figure 5–38): Group of surface growth on living leaves, stems, flowers, diseases characterized by white, bluish, or fruit. With time this can turn gray or develop or gray sporulation on the underside of minute black flecks within. leaves. Sclerotium (pl. sclerotia) (Figure 5–39): Rust (Figure 5–40): One of a group of Hardened brown or black fungal survival important fungal diseases with multiple structure. May be round or irregular. stages, at least one of which typically produces dry yellow to orange spores. Rust (see above) (Figure 5–41). Example of Rust (see above) (Figure 5–42). the aecia of quince rust on ornamental pear fruit. Example of the gelatinous telia of cedar- Note the tubular white membranes that cover the apple rust. spore-producing pustules in this species. Fruiting body (Figure 5–43): Any spore- Canker (Figure 5–44): Fruiting bodies producing structure of a fungus. Many are small that develop in a canker, below the bark, and dark. Some grow on the surface of leaves or and break through the surface. stems. Mushroom (Figure 5–45): A soft, stalked Conk (Figure 5–46): A tough, often shelf- fungal fruiting body. One genus —Armillaria—is like fungal fruiting body emerging from a plant pathogen. Most are mycorrhizal or are standing, fallen, or buried wood. Sign of secondary decomposers. wood decay in progress. Dodder (Figure 5–47): A parasitic plant with Slime mold (Figure 5–48): A group of yellow to orange stringlike stems and soil-, litter-, or bark-dwelling organisms, inconspicuous flowers. usually unnoticed until the conspicuous spore-producing phase develops. Figure 5–1. A variegated loquat Figure 5–2. A sunburst honeylocust (Eriobotrya japonica) that could be has naturally yellow-green leaves. mistaken for a disease. Attribution: Suzanne Cadwell, Flickr Attribution: Megan Hansen, Flickr CC CC BY-NC - 4.0 BY-SA - 4.0 Figure 5–4. Unripe female cones of oriental arborvitae (Platycladus Figure 5–3. Spores on the underside orientalis). of fern leaves can sometimes be confused with insects. Attribution: Daniel Fuchs, Wikimedia commons CC BY-SA - 4.0 Attribution: Honey Bee, Flickr CC BY - 2.0 Figure 5–5. Interveinal chlorosis of periwinkle (Vinca minor). IV. Abiotic Disorders of Plants When the cause of a problem is an environmental condition, cultural practice, or chemical exposure, the causal agent is abiotic (nonliving). Disorders caused by abiotic factors are not contagious, but such disorders can severely damage plants. In these cases, it is more appropriate to use the term injury or disorder rather than disease. Most environmental problems are caused by deficiencies or excesses of factors that support life (including soil moisture, light, and temperature). Simple actions such as soil testing, finding out if a plant prefers sun or shade, loosening the edges of the root ball when planting, watering during dry weather, mulching, and knowing when to lime, fertilize, and prune are major factors in preventing many plant problems.