Downy Mildew on Cucumber in WNC

Well, this has been a very exciting week to be a plant pathologist! First, downy mildew on basil, now downy mildew on cucumbers.
*These are two different disease problems, just because they have the same name DOES NOT mean that these two disease are caused by the same organism and will cross-infect one another!*

Cucurbit downy mildew, caused by pathogen Pseudoperonospora cubensis, has been confirmed on pickling cucumbers in Henderson County. Growers are advised to scout their cucurbit plantings which include cucumber, squash, melons and other gourds. Last year we found the downy mildew in Henderson County late in July.

Symptomatic downy mildew cucumber leaf. Angular lesion are first light green and then turn brown with age.

The underside of a downy mildew infected cucumber leaf. You can see the dark spores of the pathogen as they arise from the angular lesions which are bound by the leaf veins.

Downy mildew of cucurbits has the potential of spreading very quickly. Its wind-borne spores can spread easily throughout the region - especially on cloudy days. The pathogen prefers cool temperatures and wet conditions, which we certainly have not had in WNC as of late! I believe that recent hot, dry and sunny conditions are the reason that the downy mildew on cucurbits is not widespread at this time. For example, large cucumber plantings just miles away from where I initially found the disease are unaffected at this time.

With rain possible this weekend, it is very important to prevent downy mildew from infecting your cucurbits, especially cucumber. Growers are advised to protect their plants using fungicides. For conventional producers, review the June 4th North Carolina Pest News or review my blogpost from June 2009 for control recommendations.

Management of downy mildew on cucurbits is tricky for organic producers. There are a number of OMRI-listed products that are labeled for the control of downy mildew on cucurbits, including copper, OxiDate, Serenade and neem. In research studies, copper products were proven to be the best option (only option, really) for managing downy mildew on cucurbits. Copper is only efficacious in plants that are not yet infected or exhibit only mild symptoms.

Visit the Cucurbit Downy Mildew Forecasting Website to see where the disease is currently located and where the team predicts it to show up next.

If you need help diagnosing a problem on your cucurbits, please contact your local NC Cooperative Extension Office.

To see more pictures of the downy mildew on cucurbits and to learn more about the disease, visit my previous posts.