Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Until this week conditions have not been favorable for significant levels of disease development in corn and soybean. Most full season soybean is between R1 and R2, and low levels of Septoria brown spot and Downy mildew are present at many sites. Double crop beans are just getting out of the ground and are perhaps at V5 is some of the early fields. In corn, Grey leaf spot is now present in many fields, although levels have been very low in the majority of fields I have examined. Potential irrigation burn was observed in one field, and some smut has been found at a couple of sites. Things to keep an eye out for in the next couple of weeks are Sudden Death Syndrome in full season soybeans and Anthracnose top dieback in corn. Many full season beans were planted into cool, wet soils, which favor infection by the SDS pathogen. As the beans mature the fungus will produce a toxin, which results in the characteristic interveinal necrosis often associated with SDS (Figure 1). Blue fungal growth may be seen on the base or the roots of infected plants. SDS may be associated with Soybean cyst nematode, so full season beans planted into fields with a history of SCN may have a greater potential to show disease symptoms.
Figure 1. A leaf with onset of SDS symptoms. Several other disorders and diseases can cause similar symptoms.
With the amount of anthracnose we have seen this year there may be a greater potential to see anthracnose top dieback in some corn fields. Typically the top dieback phase starts two or three weeks after pollination. Top dieback caused by the anthracnose pathogen is characterized by yellowed, purple, or dead/dying flag leaves on plants scattered throughout the field. When the leaf sheaths are peeled back at the top of the affected plants, shiny black lesions can be observed on the outside of the stalk. Under appropriate conditions a salmon-colored gel can be seen on the stalk. This gel contains spores of the fungus. If the stalk of the top is split, the pith will often appear discolored and rotted in the upper internodes. Factsheets for SDS and Anthracnose can be viewed on the University of Delaware Field Crops Factsheet site (http://extension.udel.edu/factsheet/), hard copies will be available at your county Extension office soon.
A final note- it is too early to see symptoms of red root rot and stalk rots in corn. These diseases occur much later in the season. Remember, many things can cause red brace roots.