Cool weather slowing development and facilitating diseases

The cool weather makes for a great trip to the fair and successful corn pollination, but below normal summer temperatures can adversely affect crops.

“One of the things that concerns me is the foliar disease in corn and soybeans. The cool weather favors the development of some of those foliar diseases,” said Jim Stucke, DuPont Pioneer Account Manager in Mercer Auglaize and Allen counties. “Another problem with the cool temperatures is that we’re not accumulating the growing degree units and it is kind of slowing the development of corn and soybeans both. For the most part the crops look really good. We have checked the pollination in corn and it looks good. Low temperatures have been very favorable for pollination. We have seen very little stress on corn and pollination process proceeded very nicely. On the bad side, we have slower development and less accumulation of growing degree days.”

According to the National Agricultural Statistics Service for the week ending July 27, 69% of the state’s corn was silking compared to the 74% five-year average. Soybean bloom was just below the five-year average at 71%.

Ohio State University plant pathologist Anne Dorrance has been finding foliar diseases showing up around the state.

“During field surveys last week we found the usual culprits, Phytophthora stem rot, some brown spot and frogeye leafspot. Two finds — downy mildew and bacterial pustule — were found in several fields. Both are considered minor diseases as yield reductions have been minor or difficult to document. Reports from other states and some parts of Ohio, Downy mildew may be at higher incidence. I expect that some of this is due to the cool nights, misty weather we have had this summer,” Dorrance said. “Symptoms of downy mildew on the top of the leaves are yellow chlorotic spots, almost round in shape. If the humidity is high, on the underside of the leaf is a necrotic spot (very small) with white ‘fuzz.’  This ‘fuzz’ are the fruiting structures (sporangia) of the oomycete fungus Peronospora manshurica. If in doubt, place the leaf in a plastic bag with a moist (not sopping wet) paper towel and wait about 6 hours.  The sporangia will form and that is the diagnostic feature of this pathogen.

“Symptoms of bacterial pustule are dark brown, necrotic areas surrounded by yellow.  On the underside of the leaf, inside these necrotic areas are raised bumps.  Also on the underside of the leaf we can see watersoaking around the new lesions, very classic for symptoms for a bacterial disease. For bacterial blight, there is still the yellow halo and water soaking but the lesions are flat. To separate the symptoms of soybean rust from bacterial pustule, with high humidity, spores would be present in the pustule, and the necrotic area around the pustule tends to be much smaller.”

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