Effects of Drought on Field Crop Disease
By Laura Sweets

Drought may affect the development and severity of field crop diseases in several ways. Some diseases are favored by hot, dry conditions. During the growing season, drought may lead to an increase in charcoal rot on soybean, corn and sorghum or an increase in Aspergillus flavus and aflatoxin in field corn.

Other field crop diseases require periods of free moisture on the plant surfaces for infection to occur and disease to develop. For example, many foliage diseases of field crops are favored by frequent light rains, high humidity and overcast weather. During a drought season, the incidence and severity of these foliage diseases may be quite low. Residues left from that season's crop will contain very little pathogen inoculum to carry over to the next season.

Charcoal rot is a good example of a disease that is favored by drought conditions. The fungus that causes charcoal rot, Macrophomina phaseolina, is a widely distributed soil fungus. Charcoal rot does not occur at low temperatures. Macrophomina phaseolina begins to grow at temperatures between 82 to 95 F. Pathogen growth may occur early in the season with infection of soybean seedlings occurring two to three weeks after planting, if conditions of high soil temperatures and low soil moisture exist.

On older soybean plants, symptoms are more likely to occur if temperatures are in the 82 to 95 F range during periods of hot, dry weather, or when unfavorable environmental conditions stress the plants. On corn and sorghum, charcoal rot is primarily a stalk rot disease. The fungus infects senescing stalks during conditions of drought.

We have been receiving samples of soybean plants that are somewhat stunted, slightly off-color and even wilting. Some of these plants have lesions or cankers close to the soil line. Rhizoctonia, Macrophomina (the charcoal rot pathogen), Phomopsis, Fusarium and other fungi can be isolated from these cankered areas. Dry conditions may be predisposing these plants to invasion by these various fungi. Rain should certainly help these plants; continued hot, dry conditions may lead to more extensive yellowing of the foliage and even premature death of affected plants.

Aflatoxin is another problem typically associated with drought years. Aflatoxin is a naturally occurring mycotoxin produced by the fungus Aspergillus flavus. Aspergillus flavus can invade corn in the field. Temperatures ranging from 80 to 100 F and a relative humidity of 85 percent are optimum for Aspergillus flavus growth and aflatoxin production. Periods of drought and heat stress during the growing season, especially during pollination and as kernels mature, favor Aspergillus flavus infection.

Aspergillus flavus can survive for extended periods on plant residues in the soil, and it also produces survival structures called sclerotia that allow it to survive for even longer periods of time in the soil. Hot, dry conditions during and after pollination will lead to a greater likelihood of aflatoxin problems. Additional information on Aspergillus flavus and aflatoxin is available at http://aes.missouri.edu/delta/croppest/alfacorn.stm.

There are many field crop diseases that are less common and less severe during periods of drought conditions. Most of the corn foliage diseases are favored by wet conditions, frequent rains, high humidity and overcast weather. One reason for this is the fungi that cause many of the corn foliage diseases need free moisture on the leaf surface to germinate and penetrate into the corn leaf tissue.

Gray leaf spot has not been as widespread or severe in most of the state this season. There may be localized areas that have received more normal rainfalls and have problems with gray leaf spot; but, overall, it is not a serious problem. Likewise, levels of soybean foliage diseases such as Septoria brown spot, frogeye leaf spot and bacterial blight have been fairly low this year.

August is usually the time of year when symptoms of sudden death syndrome (SDS) are evident in Missouri soybean fields. SDS is more severe when wet conditions occur early in the season resulting in high levels of root infection and then cool, wet conditions occur as the crop is flowering and setting pods. So far this season, symptoms of SDS on bean plants have been rare, and, if hot, dry conditions continue, SDS may not be very evident this season.

Development of most of the common field crop diseases in Missouri depends on the weather conditions during the given growing season. A drought in one year may lead to higher than normal levels of diseases such as charcoal rot and Aspergillus flavus and may lead to lower than normal levels of diseases such as gray leaf spot of corn and sudden death syndrome of soybean during that year. But the incidence and severity of these diseases the following season will still depend to a large extent on the weather patterns that develop during that coming season.

And, it is still important to plant varieties and hybrids with resistance to field crop diseases that commonly occur in Missouri, even if those diseases were not prevalent in 2005.

Site maintained by people at AgEBB
agebb@missouri.edu

University of Missouri Extension does not discriminate on the basis of race, color, national origin, sex, religion, age, disability or status as a Vietnam-era veteran in employment or programs.