Cucurbit downy mildew has been moving up the east coast, with some westward movement in the South, but there have been no reports of downy mildew on cucurbits in Ohio, its surrounding states, or Ontario. Downy mildew pathogen (Pseudoperonospora cubensis) spore trap counts from the Hausbeck Lab at MSU have been very low – and only a few (8) P. cubensis spores were counted from one location on June 28, and none before or since.
We have seen many, many examples of bacterial diseases in vegetable crops this season, a consequence of excessive rainfall this spring and summer. Some bacterial diseases cause leaf spots that can be mistaken for those caused by fungi or oomycete pathogens. Angular leaf spot of cucumber is a prime example: the angular lesions are very similar to those of downy mildew. In downy mildew, the spores of the pathogen may be observed on the lower side of leaves with a good hand lens, but sometimes they are difficult to find. If downy mildew is suspected but can’t be confirmed with a hand lens, it should be confirmed with a lab microscope. Downy mildew spores can be visualized easily with a microscope. If on the other hand the symptoms are caused by bacteria, bacterial streaming from the lesions can be seen using a microscope. This process can be completed in minutes in an experienced lab. Ohio vegetable growers can send or drop off samples for diagnosis to the OSU Vegetable Pathology Lab in Wooster; a fillable PDF sample submission form can be found here. Samples can also be dropped off at OSU-OARDC Experiment Stations in Celeryville or Fremont, or on the OSU main campus in Columbus (Kottman Hall).
Applying fungicides is an important tactic in downy mildew management, but spraying them when the disease is caused by a bacterial pathogen is a waste of time and money. Regular scouting, confirmation of symptoms and being alert to reports of outbreaks of downy mildew in the vicinity should be the focus now.