Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu
We are reaching the end of the season and past the point that new foliar lesions should be detracting from corn yields. However, I did want to update that a new corn disease has made an appearance across many fields in Sussex County over the past week. I have been observing lesions in the upper canopy that resemble young grey leaf spot lesions, but when the leaf is flipped over and held up to the light, a gray center, prominent margin, and yellow halo can be observed (Figure 1). Under the microscope we observed crescent shaped spores characteristic of the pathogen that causes Curvularia leaf spot (Figure 2). The range of Curvularia leaf spot in the US has been expanding in recent years. While there have not been any reports of extreme yield loss from Curvularia in the US, the impact of this disease is not yet fully understood. We will work with this pathogen in the lab to molecularly verify species and conduct greenhouse inoculation screenings this winter. Curvularia leaf spot starts as small tan lesions. These lesions have a brown/maroon border often surrounded by a yellow halo that is particularly noticeable with back lighting (Figure 3). Symptoms can be observed at any stage, but I did not start observing this disease until R4/R5 in the fields I have scouted. We will continue to keep an eye on this disease for the 2021 growing season and care should be taken to not confuse these lesions with young grey leaf spot. Data from other states has shown fungicides to lack efficacy for Curvularia leaf spot and there are not currently any fungicides that list Curvularia on their label. Excessive dew and sporadic rainfall favor the development of this disease.
Figure 1. Corn leaves with grey leaf spot and Curvularia leaf spot. Left image top side of leaf, right image underside backlit with sun.
Figure 2: Spores of Curvularia leaf spot pathogen
Figure 3. Long box shaped Grey leaf spot lesion and Curvularia leaf spot lesions with prominent margins and yellow halos located beside each other on a corn leaf magnified under a dissecting microscope.