Wet Spring and Summer Mean Sudden Death Syndrome is Thriving in Soybean Crops

Shine a light on issues that matter—support local news with your gift today. Sudden death syndrome, a fungal disease affecting soybeans, has no cure in the field and can lead to significant yield losses. The disease is present in Iowa bean fields every year, but one researcher said conditions have not been this optimal for the disease since 2018. Daren Mueller, a plant pathologist with Iowa State University Extension and Outreach, said if a good year for sudden death syndrome, commonly abbreviated to SDS, could be defined, "this would be the year." Mueller said SDS thrives when a cool, wet spring is followed by heavy summer rain events — and these circumstances have been present across most of the state. The fungus can live in the soil for several years as it feeds on dead plant material, even through a corn rotation. When it's present, spring planting that is followed by a wet spring creates conditions for the fungus to get into the roots and create root rot. Heavy summer rains during the plant's reproductive season make it easy for the toxin that is emitted by the fungus to travel up the plant and into the leaves. Here in the foliar stage, the toxin causes the leaves to be photosensitive, turn yellow, then brown and sometimes drop off. Precipitation accumulation in Iowa since April 1, 2025. (Map courtesy of Iowa Environmental Mesonet) Most producers aren't aware the disease is affecting their plants until it reaches the foliar stage, but Mueller said that doesn't matter as there is "absolutely nothing" a farmer can do to stop the disease. Mueller said the best defense against SDS is a seed treatment, but once the seed is in the field there are no sprays or other treatments that can save the plants. "The day after planting, it's too late to do anything," Mueller said. "Even if you don't have symptoms … once the seed's been put in the ground, all of your options for that year are done." The severity of the disease can be increased when other diseases or pests like soybean cyst nematodes are also attacking the plants. Mueller said the best thing that farmers can do when they notice SDS is to take note. Write down what type of seed they used, try to obtain an aerial image of the field to determine how much was affected, and talk to neighbors and seed companies to see what they experienced this year. Soybean plant showing foliar symptoms of sudden death syndrome. (Photo courtesy of the Crop Protection Network) Mueller said it's hard for seed companies to accurately rate their varieties for resistance to the disease when it has not been widely present in several years. "We've had three or four or five years in a row of pretty poor SDS, which is good for farmers, but it's also not good for the people trying to rate and put an accurate rating on the varieties," Mueller said. While much of the state has received the rain conditions that are conducive for SDS, Mueller said he has not been to "all the corners of the state" to observe the effects of the disease. He said most of his calls and identified cases he's fielded have come from farms in the southeast, southwest and northwest portions of the state. SDS can also look like brown stem rot or red crown rot, the latter of which has not yet been identified in Iowa, according to ISU's Integrated Crop Management report. The best way to distinguish SDS from brown stem rot, is to look at the internal stems on soybeans. SDS stems will be white, while brown stem rot infected plants will have brown pith in the stems, according to the Crop Protection Network. Agronomists with the Iowa Soybean Association noted cases of SDS in all regions except southwest Iowa in the association's latest "Walking Rows" article. Southern corn rust, another fungal disease, has also been widely reported in corn acres across the state. Unlock a world of knowledge! Sign up for free DecorahNews.com newsletters and receive local news, local events and everything that is happening in and around Decorah.