Fungi: The common rust situation on field corn has slowed down somewhat, and currently the only other leaf blight that we have been seeing with any regularity is gray leaf spot. The disease situation was well summarized in an e-mail update to me from Kevin Black of Cargill. Kevin reports "in spite of ideal conditions for rust infection in the last two weeks, I'm not seeing much new lesion or pustule development. I can see why they were recommending fungicide application for some hybrids, but I'm not sure that applications were made early enough. It appears that maturation of leaf tissue had as much to do with reduced rust infection as did any fungicide application. Gray leaf spot is present, but not too aggressive at this point and very little above the ear." Kevin goes on to note that "we are running into some nitrogen deficiency symptoms in corn, especially in areas that experienced heavy rainfall this season." This is a very important point in terms of leaf blight development. We have seen in past years fungal leaf disease virtually stop in its tracks when nutrient deficiency symptoms start to become severe.
Viruses: Some virus symptoms are showing up in the corn in southern Illinois. Dennis Epplin, crop systems educator with the Mt. Vernon Extension Center has been working with a field that was symptomatic of virus infection. Tests for maize dwarf mosaic virus (MDMV) came back negative, and now they are looking into whether wheat streak mosaic virus (WSMV) or other viruses may be present. This is a logical progression for testing, given the abundance of inoculum from the severe WSMV epidemic on the wheat crop this season and knowing that corn can be a green bridge for the virus.--Suzanne Bissonnette