By Lauren Quinn

Pests and pathogens have kept Illinois farmers busy this season, with reports of Japanese beetles, gray leaf spot, diplodia leaf streak and southern rust in fields across the state.

“Late July saw a slight uptick in the number of foliar disease reports in corn, likely as many people are actively scouting prior to making a fungicide application decision,” says Nathan Kleczewski, field crops pathologist in the Department of Crop Sciences at the University of Illinois.

Gray leaf spot is present to varying degrees in most cornfields. While not a big surprise to most Illinois corn growers, Kleczewski notes farmers should be on the lookout for diplodia leaf streak and southern rust.  

“Diplodia leaf streak can easily be misdiagnosed as gray leaf spot, northern corn leaf blight, or other foliar diseases and disorders,” he says.
“Characteristic foliar symptoms include oblong, irregular or blocky lesions with green-yellow edges.

“Often, ‘targets’ can be seen in the lesions, where the initial infections occurred. Older lesions contain black pycnidia, which resemble tiny pinheads or dots. You will not see these in gray leaf spot or northern corn leaf blight lesions.”

Southern rust rolls up toward Illinois every season from the Southeast. So far, it has only been confirmed in low frequencies in two Illinois counties — Bond and Franklin — but Kleczewski recommends farmers keep an eye on the southern rust iPiPe map for daily updates, citing the devastating effects of southern rust on the Illinois corn crop in 2016 for motivation.

Bug beat
Meanwhile, both corn and soybean growers are assessing damage from Japanese beetles.

“It is proving to be a big year for Japanese beetles in Illinois, and while populations should be starting to decline in much of the state, there is still a lot of feeding going on,” says Nick Seiter, U of I field crops entomologist.

While damage looks bad, Seiter reminds farmers to measure economic threshold before spraying. 

For corn, the economic threshold for damage is three or more beetles per ear, with silks clipped to half an inch or less while pollen shed is less than 50% complete.

For soybeans, the economic threshold sits at 30% defoliation before bloom, and 20% after bloom if the target insect is still present in the field and actively feeding. Seiter says defoliation is easy to over-estimate.

Seiter says initial control can be achieved using a pyrethroid or carbamate insecticide, but cautions that the residual activity of the chemicals is short-lived.

“I have heard anecdotal reports of fields being sprayed multiple times for Japanese beetles — a frustrating situation given that the economic threshold was likely never reached,” he says.

“Scouting fields thoroughly and using economic thresholds to guide treatment decisions helps to ensure that these inputs are used only in situations where they are likely to provide a positive return on investment.”

Quinn is a writer for the University of Illinois College of Agricultural, Consumer and Environmental Sciences.