Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu and Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Downy mildew was observed on cucumber in our sentinel plots on Wednesday, July 9 near Georgetown, DE by Emmalea Ernest. She found several small spots on 3-4 leaves on the susceptible slicing variety ‘Straight Eight’. This indicates that we had a transport event of spores from the South over the weekend as predicted by the NC State Plant Disease Forecast Center. We are recommending that cucumbers be sprayed at this time to protect plants that are at a vulnerable stage of development. We are recommending fungicide programs that utilize fungicides that have provided very good control in the past several years. Previcur Flex or Ranman combined with Bravo and alternated with Tanos combined with mancozeb or Bravo have done well in regional trials. Ranman/Bravo alternated with Previcur Flex/Bravo has also been a very effective combination. Other fungicides that are good alternating partners have been Curzate and Gavel. Another very good product that was just registered is Presidio from Valent. It had very good activity against downy mildew in tests conducted in North Carolina and could be used in a program as you would Previcur Flex or Ranman, or in alternation with these products. Presidio also needs to be tank mixed with a protectant fungicide. Be sure to check the labels for specific instructions and resistance management strategies.
For other cucurbit crops we are suggesting that you wait until the disease is seen on crops other than cucumbers. Be sure to be checking your fields daily especially in areas that had heavy rainfall Sunday. Our sentinel plots have other cucurbits including cantaloupe, pumpkin, watermelon and winter squash. To repeat, downy mildew was only seen on cucumber near Georgetown, DE and it is not present on any crop in the sentinel plots in Salisbury, MD. To track disease development visit the Downy Mildew Forecast website often http://www.ces.ncsu.edu/depts/pp/cucurbit/
Downy mildew caused by Pseudoperonospora cubensis on the upper leaf surface
Downy mildew infection on the corresponding lower leaf surface – purple fuzzy growth is the fungus sporulating from the infected leaf tissue.