Row Crop Diseases in Drought Years

PP-1371, July 2008

Sam Markell, Extension Plant Pathologist, Department of Plant Pathology, NDSU
Mohamed Khan, Extension Sugar Beet Specialist, Department of Plant Pathology, NDSU and University of Minnesota
Gary Secor, Plant Pathologist, Department of Plant Pathology, NDSU
Tom Gulya, Research Plant Pathologist, Sunflower Unit, U.S. Department of Agriculture–Agricultural Research Service
Art Lamey, Professor Emeritus, Department of Plant Pathology, NDSU

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Drought impacts many diseases of row crops. In some cases, drought conditions will impact disease by making the environment 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).

Diseases less severe in droughts

Root Diseases. Many different species of fungi cause root rots in North Dakota. Oomycete organisms (commonly called water molds, or near-fungi) such as Phytophthora, 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 sunflower, 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 sunflowers. 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 unlikely. One notable white mold exception is sunflower wilt, which is caused when the white mold pathogen infects sunflower plants through the roots. This method of infection is less likely to be influenced by drought.

Diseases more severe in droughts

Root 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 significant. 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 pathogens, 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 pathogens 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, causing 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 environment 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 florets (Phomopsis and Phoma of sunflower, white mold) are inhibited by drought conditions. However, stem diseases that first infect roots are largely unaffected. Charcoal rot (Macrophomina) is a stem disease of corn, soybeans, sunflowers, 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.

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PP-1371, July 2008