Khan, Mohamed

Markell, Samuel G.

Secor, Gary A.

NDSU Libraries Institutional Repository

Plant Disease ManagementNDSU Extension ServiceNorth Dakota State UniversityFargo, North Dakota 58105July 2008PP-1371Row Crop Diseases in Drought YearsD rought impacts many diseases of row crops. In some cases,  drought conditions will impact disease by making the environ-ment more (or less) favorable for infection, disease development and/or disease spread. In other cases, drought may not impact the pathogen at all, but may exacerbate the damage caused by disease in drought-stressed plants. Although the effects of drought on disease are variable, certain diseases of some crops tend to be consistently more or less severe in drought conditions (Table 1). Sam MarkellExtension Plant Pathologist, Department of Plant Pathology, NDSUMohamed KhanExtension Sugar Beet Specialist, Department of Plant Pathology, NDSU and University of MinnesotaGary SecorPlant Pathologist,Department of Plant Pathology, NDSUTom GulyaResearch Plant Pathologist, Sunfl ower Unit, U.S. Department of Agriculture–Agricultural Research Service Art LameyProfessor Emeritus, Department of Plant Pathology, NDSUTable 1. General effects of drought on different diseases. *Disease DamageDisease Crop Greater No effect LessOomycete root rots (Phytophthora,  Many   XPythium, Aphanomyces) Fungal root rots causing wilt  Many X  X(Verticillium, Fusarium species) Downy mildew Sunfl owers   XWhite mold (Sclerotinia) Many broadleaf crops   XSclerotinia wilt  Sunfl owers  X(root infection only)Charcoal rot Soybeans, sunfl owers, X edible beans, corn, others   Leaf spot pathogens All   XPowdery mildew Most crops X  XRusts Most crops X  XLate blight Potato, tomato   XRhizoctonia stem canker Potato  X Early dying (Verticillium) Potato X  Black scurf (tuber) Potato   XAscochyta blight Pulse crops (peas,   X lentils, chickpeas)     *These are general effects only; exceptions occur in all categories. In categories with more than one box checked, drought may cause different effects under different conditions. See text for detail.Diseases less severe in droughts Root Diseases. Many different species of fungi cause root rots in North Dakota. Oomycete organisms (common-ly called water molds, or near-fungi) such as Phytophtho-ra, Pythium, Aphanomyces and Plasmopara, need wet soils to cause disease. Some common diseases caused by Oomycete pathogens that are usually less severe in drought conditions include downy mildew of sunfl ower, Phytophthora root rot of soybeans, Aphanomyces root rot of sugar beets and damping off of many plants caused by Pythium species. Leaf Diseases. Many foliar pathogens are able to infect plants only when leaves are wet. In drought situations, a lack of free moisture may occur on the leaves, which reduces the pathogens’ ability to infect plants. Fungal or bacterial pathogens causing leaf spots and some rusts tend to fall into this category. Additionally, many foliar pathogens produce spores that are dispersed only by rain splash. Pathogens that need rain to spread are unlikely to cause epidemics in drought years. Many fungal and bacterial pathogens causing leaf spots fall into this category, but rusts do not.  Stem Diseases. Some stem diseases may be less likely to occur in drought years. White mold (caused by Sclerotinia sclerotiorum) infects many broadleaf crops grown in North Dakota, including canola, edible beans, soybeans and sunfl owers. The soil must be wet for the white mold pathogen to germinate and produce airborne ascospores, and an extended wet period is needed for those ascospores to infect plants. When soils are dry and limited wet periods occur in the canopy, white mold is un-likely. One notable white mold exception is sunfl ower wilt, which is caused when the white mold pathogen infects sunfl ower plants through the roots. This method of infec-tion is less likely to be infl uenced by drought. Diseases more severe in droughtsRoot Diseases. Most root rot pathogens need some soil moisture to cause infection. However, once disease has been established in a drought-stressed plant, the impact of the root rots may be signifi cant. Some fungal species (Fusarium and Verticillium) that cause wilts reduce the plant’s ability to transport water. The pathogen plugs the xylem tissue that transports water through the plant. When root rots caused by these pathogens occur in drought-stressed plants, the damage may be much greater than might occur in plants in a nondrought situation. These wilts are most severe when enough moisture is available to cause infection in the early part of the growing season, but the rest of the season experiences a drought. Leaf Diseases. Droughts occur because of a lack of rainfall, but that does not necessarily translate into a lack of humidity. Dew is likely to form if the air is humid and night temperatures fall below the dew point. Dew on leaves creates a favorable environment for some patho-gens, and in drought-stressed plants, damage from some leaf diseases may be extreme. Dew on leaves often provides enough moisture for rust pathogens to infect plants. While many leaf spot patho-gens cause dead spots on leaves, rust pathogens produce an open wound in living plant tissue. This open wound allows moisture to escape readily from the plant, caus-ing a drought-stressed plant to lose water more quickly. Additionally, rusts do not need rain to disperse spores, so if adequate dew is available for infection, rust epidemics may occur. In some drought situations, rusts can become a major concern. Powdery mildew occasionally is found on most crops in North Dakota. High humidity provides a favorable envi-ronment for the infection and development of powdery mildew. Powdery mildew is likely to occur in drought years when the humidity is high.  Stem Diseases. Many stem diseases that begin as infections on leaves or fl orets (Phomopsis and Phoma of sunfl ower, white mold) are inhibited by drought condi-tions. However, stem diseases that fi rst infect roots are largely unaffected. Charcoal rot (Macrophomina) is a stem disease of corn, soybeans, sunfl owers, edible beans and other crops grown in North Dakota. The disease begins when microsclerotia (the pathogen’s survival structure) germinate in the soil and infect the plant’s roots. Once infection progresses into the stem, the disease partially degrades the pith. This limits the plant’s ability to transport water. Charcoal rot is favored by high temperatures and water-stressed plants, so the disease is more likely to be a problem in drought years.County commissions, North Dakota State University and U.S. Department of Ag-riculture cooperating. 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Row Crop Diseases in Drought Years

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