Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Cucurbit downy mildew has been reported on cucumbers in North Carolina (Wilson County). This is a very early detection for this area and a threat to Delmarva pickle production if weather conditions favor spore movement. I know this is early and the weather has been dry in most downstate areas but this is a very mobile disease. The disease forecast is showing that there is no current risk of spread to Delaware at this time but growers and fieldmen need to be vigilant. As in past years the Cucurbit Downy Mildew PIPE Forecast website will let you know where the disease is present. Visit the site at http://cdm.ipmpipe.org/.
In the past several years my fungicide research as well as that of Steve Rideout in VA has shown that spray programs that include Presidio (4 fl.oz/A) and Ranman (2.75 fl.oz/A) provide the best control. Previcur Flex (1.2 pt/A) has been very effective in the past but we are a little hesitant to wholeheartedly recommend it because our results were not as good as in past years. As a result growers should not rely on this product alone. These fungicides should be rotated/tank mixed with protectants containing mancozeb or chlorothalonil. Applications should be made before symptoms appear and apply on a 7 to 10-schedule. Good coverage is very important so apply the fungicides in at least 20 gallons/A, 30 would be better.
Upper leaf surface showing angular yellow spots of downy mildew.
Lower surface of cucumber leaf with gray-purple fungal growth. These are the spore bearing structures called sporaniophores with sporangia (spores).