Two unusual diseases, one in corn and one in soybeans, have been diagnosed by Iowa State University Extension plant pathologists in the past two weeks in Iowa. Extension plant pathologist Alison Robertson has recently written in the ISU Integrated Crop Management newsletter about cornstalk breakage caused by Physoderma maydis, a disease which she explains later in this article (see below). And her colleague, ISU Extension plant pathologist Daren Mueller, wrote an ICM news article last week about Tobacco Streak Virus causing necrotic spots on soybeans (see below).
"I have heard of several reports across Iowa of cornstalk breakage due to Physoderma maydis," says Robertson. "We don't really understand what the circumstances were that led to purple lesions girdling the lower nodes of corn plants rather than the typical brown leaf spot symptoms we usually see. There does not seem to be any correlation with planting date or hybrid."
Regarding Tobacco Streak Virus causing necrotic spots on soybean plants, Robertson says, "Most of the reports and samples we've seen in the ISU Plant Disease and Insect Clinic have been from central and northwest Iowa, but we think this disease may be more widespread. TSV is transmitted by thrips. This insect also vectors soybean vein necrosis virus that was recently reported in Iowa."
Now is the time to start scouting your cornfields for ear rots
"Although many parts of Iowa have experienced drought conditions this year, we do not think there is a risk of aflatoxin in grain since most of the growing season was predominantly cool," says Robertson. "We may be at risk for fumonisins (associated with Fusarium ear rot) or DON/vomitoxin (Gibberrella ear rot), however. In our field scouting we haven't seen a whole lot of ear rot. I'd be interested to hear from farmers about what they are finding in their fields."
Tobacco Streak Virus confirmed on soybean plants in some Iowa fields
Tobacco streak virus is being confirmed in Iowa fields, says ISU plant pathologist Daren Mueller. He suggests farmers and crop consultants learn how to identify the symptoms and where to send the plants for diagnosis. Over the past several weeks, ISU specialists have received several samples in the Plant and Insect Diagnostic Clinic at Ames with irregular blotches and necrotic lesions on the pods.