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Ohio State University Extension


Downy Mildew and Bacterial Pustule on Soybean

Downy Mildew on Soybeans

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.


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. 

Again, for both of these pathogens, downy mildew and bacterial diseases in general, there are no products that have demonstrated efficacy in field conditions.  Enjoy the scouting there is lots to find this year.

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.