Cucurbits at Risk for Downy and Powdery Mildew

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Note: Please read labels carefully as some of the fungicides mentioned in this article are not labelled on all cucurbits.

Powdery Mildew on Cucurbits
Powdery mildew on cucurbits is now beginning to progress. The powdery mildew pathogen is windborne and, unlike many other pathogens, can efficiently infect during dry periods such as we’re experiencing. Effectively managing powdery mildew requires fungicides. However, because the pathogen is prone to resistance, fungicide resistance within the pathogen population (Podosphaera xanthii) must be considered. Bioassays to evaluate the presence of resistance throughout the region have been conducted in the past year. Quintec (FRAC 13), Luna products (FRAC 7), and Vivando (FRAC U8) were all highly effective throughout the Mid-Atlantic and Northeast. Torino (FRAC U6) was effective in some locations but moderately effective in others including in my Maryland trials. Remember that the FRAC group 11 and 1 fungicides, which include strobilurin fungicides like Flint, and Topsin M, are ineffective. FRAC group 3 and 7 fungicides, which include Myclobutanil (Nova), boscalid (one of the active ingredients in Pristine), Fontelis and Folicur are in an intermediate group. We do know that resistance to FRAC groups 3 and 7 can be found in our pathogen populations, but if these products are used judiciously and in rotation with other effective products, they can be useful. Always tank mix fungicides with broad spectrum materials such as chlorothalonil, and alternate with a fungicide that has a different mode-of-action (FRAC group). A good strategy is to use moderately resistant cultivars and then alternate fungicides in FRAC groups where resistance has not been detected with fungicides in FRAC groups 3 or 7.

Pumpkin leaf that has both white powdery lesions, and brown downy mildew lesions. It is important to be sure that you are spraying for the correct disease because the most effective fungicides for each disease are different.

Downy Mildew on Cucumber
Downy mildew on cucumber has now been confirmed in Salem County, NJ. All cucumbers should be protected with targeted fungicides. Other cucurbits should be scouted aggressively for the presence of downy mildew.