Tape mount from the underside of a cucumber leaf with downy mildew. Characteristic branched sporangiophores (center) and oval, brown sporangia. Micrograph by Francesca Rotondo.
Downy mildew was confirmed today on cucumbers in Medina County – the field is in the Homerville area and symptoms were just beginning to show. The pathogen that causes cucurbit downy mildew, Pseudoperonospora cubensis, was sporulating well on the underside of leaves (see photo).
This is the second confirmation of downy mildew on cucumbers in Ohio this year – the first was on August 11 in Huron County. Please see my August 11 post in this blog for management recommendations.
We still have not confirmed downy mildew on squash or pumpkins, but we have received quite a few lookalikes, most of which were bacterial spot or angular leaf spot (also a bacterial disease). Bacterial diseases will not be controlled using any of the fungicides recommended for downy mildew, with the exception of copper-based products. However, these are only partially effective against downy mildew and bacterial diseases. If you are not sure about your diagnosis, send samples to the OSU Vegetable Pathology Lab – diagnoses are free for OH growers.