This is turning out to be another unusual growing season, with a cool and wet spring followed by a hot and rainy summer. The cooler than normal weather of June that continued into the first week of July suddenly changed when a heat wave accompanied by rainy days arrived.
The hot and wet final two weeks of July 2010 reminded Iowa State University Extension plant pathologist X.B. Yang of the subtropical weather of the Gulf Coast of Louisiana. That's where he did his graduate study and where foliar diseases are a major issue in crop protection each year.
"Such a unique growing season as we are having in 2010 leads to the occurrence of different diseases in corn and soybeans, testing the disease management skills of the producer," says Yang. "In corn, gray leaf spot risk is higher than last year and common corn rust spores arrived in Iowa two weeks earlier than normal. Both diseases can be found in almost any corn field with varying severity from south to north in Iowa and from field to field this year. Soybean brown spot has been widespread since early this season and other diseases are likely to show up when fields are scouted this summer."
Sudden death syndrome is likely to be widespread this year
Sudden death syndrome or SDS is likely to be widespread in Iowa the rest of the 2010 growing season, says Yang. Several factors during these critical weeks suggest favorable conditions for occurrence of this disease. The disease showed up as early as mid-July in southern Iowa, according to ISU Extension Field Agronomist Mark Carlton. SDS also can be found in fields in central Iowa.
With high disease pressure, some SDS tolerant soybean varieties may not hold up in some fields, says Yang. "This disease if found in your fields will be an indication of the soybean variety's tolerance to SDS. Good recordkeeping of the disease location will help you develop a variety selection strategy for the next soybean planting."
Soybean white mold
Soybean white mold was widespread in Iowa last year and has been a concern for many growers this season although the warmer weather this year has been generally less favorable to the disease. The exception was a period early in the flowering season which was favorable for white mold infection. "Some fields will have the disease in 2010," says Yang. "We have found white mold infected plants in northern Iowa this summer." For management options, he suggests you read his ICM article on 2010 white mold management
Downy mildew, a later season disease in Iowa, likes cool temperatures and rain, says Yang. The lesions are found in the upper plant because the fungal spores are airborne. Infected soybean leaves have regular shaped small lesions. The lesions are pale or light yellow in color on the upper surface of the leaves. On the underside of the infected leaves, the lesions are grey in color with turf like mycelium which can be seen with the bare eye.
"This year the disease showed up earlier due to frequent rains and will be most likely found in northern Iowa or in fields along the rivers," he says. "The disease may buildup later in the season in regions where temperatures are cool. The disease mainly affects seed quality and it can be controlled by fungicide spray."
Many outbreaks of brown spot disease being reported in Iowa
"This year brown spot disease pressure is unusually high due to weather conditions with many reports of outbreaks," says Yang. "The disease had caused defoliation in fields before R2 stage of soybean plants. Because the weather conditions have been good for this disease since spring, there are a lot of inoculums in soybean fields and the risk of brown spot occurrence remains high for many soybean fields. The level of damage will vary from field to field, depending on the current level of disease. A fungicide spray is recommended for fields when the disease level is high."
Other foliar diseases, such as Cercospora leaf spot and frogeye leaf spot, will likely show up during the rest of the 2010 growing season. "Farmers should treat foliar diseases as a group," advises Yang. "Use of foliar-applied fungicides is the most effective way to control foliar diseases, if the applicators can get into the wet soybean fields."