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Corn+Soybean Digest

Wet and Dry Diseases Pose Challenges for Soybean Farmers

Excessive rain and extremely dry weather in varying parts of Illinois are responsible for two quite different diseases showing up in soybean fields across the state.

Symptoms of charcoal rot are being spotted in dry areas of southern Illinois, and sudden death syndrome (SDS) has been found in wet areas across the state, reports University of Illinois Extension Plant Pathologist Carl Bradley.

"When we find both 'wet' and 'dry' diseases in the same year, it proves how different the spectrum of disease is under varying environmental conditions," Bradley says.

Typically charcoal rot and SDS show up in different years. But this year, with the varying rainfall across the state, these two diseases are popping up.

Charcoal rot can be identified by individual plants or patches of wilted and dead plants, Bradley says. In affected plants, gray to black "specks" are apparent on the lower stem when the epidermis is shaved off with a knife.

"These 'specks' are the survival structures of the soilborne fungus Macrophomina phaseolina, also known as microsclerotia," he says. "Charcoal rot is making its debut in southern Illinois for this growing season. It may also be starting up in other areas of the state experiencing hot and dry weather."

Management of charcoal rot requires an integrated approach. Although no soybean varieties have complete resistance to charcoal rot, varieties can differ in their levels of susceptibility.

Macrophomina has a wide host range, which includes corn, sorghum and sunflower, so crop rotation alone may not provide complete management, Bradley warns. Practices that reduce drought stress may help, such as avoiding high seeding rates and using conservation tillage practices that conserve soil moisture.

However, foliar and seed treatment fungicides do not provide protection against charcoal rot or SDS.

"Unfortunately, there are no silver bullets available to control SDS completely either," he says. "Variety choice is a management tactic that can be used to limit the severity and impact of SDS. Although no soybean varieties have complete resistance to SDS, differences in susceptibility do exist. Many seed companies provide SDS resistance ratings for their soybean varieties."

Planting date is another management tactic to consider. Early planting may predispose soybean plants to infection by the SDS fungus, Bradley says. Plant fields with no history of SDS first and those with a history of SDS last.

Soils with compaction or drainage problems may lead to bigger SDS problems, too. Management practices to alleviate soil compaction and drainage problems in a field may help limit SDS losses.

"Watch for initial symptoms of SDS to appear as light-yellow flecking on the leaves," Bradley says. "The yellow areas enlarge to cause interveinal chlorosis, or yellow leaves with the veins remaining green. This eventually turns into interveinal necrosis, or dead leaves with the veins remaining green."

Although SDS symptoms appear on the foliage of the plants, the Fusarium fungus that causes SDS actually infects the soybean roots early in the growing season, Bradley says. The foliar symptoms are caused by a toxin that the fungus produces, and the toxin then moves upward in the vascular system of the plant.

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