Source: WNC Vegetable and Small Fruit News
Cucurbits or Cucurbitaceae are members of the gourd family and include the popular crops of cantaloupe, cucumber, pumpkin, squash and watermelon. In recent years Downy mildew has become an important disease of all these crops, especially in areas with high humidity and rainfall. Optimum conditions for sporulation are about 60 degrees with 6-12 hours of moisture present (usually in the form of morning dew).
The pathogen, Pseudoperonospora cubensis, causes angular, chlorotic, yellowing lesions on the foliage. These lesions appear angular because they are bound by leaf veins, not round as a fungal disease would show. During humid conditions, inspection of the underside of the leaf may reveal gray-brown to purplish-black ‘down’.
Eventually, leaves will turn necrotic and curl upwards. Symptoms on watermelon and cantaloupe are different than on other cucurbits; leaf spots are typically not angular and turn brown to black in color.
The disease is sometimes called “wildfire” because of how rapidly it progresses, as if burned by fire.
Sometimes it will infect certain cucurbits and not others. For example, it’s been observed that where cucumber and squash grown in close proximity perhaps only the cucumber is diseased. When a pathogen exhibits this type of host specificity, it is referred to as a pathotype.
Controlling downy mildew requires use of resistant cultivars, fungicide applications and early detection. Fungicides are most effective when applied prior to infection and reapplied at 5- to 7-day intervals.
Fungicides such as chlorothalonil and mancozeb can be used as a protectant. Other chemicals are registered for use against Downy Mildew so read the labels in your to see what else might be available to use on a rotation basis.
You may find other products at your garden center that are labeled for Downy mildew but make sure your crop is listed.
The pathogen overwinters in an area that doesn’t experience a hard frost (e.g., Southern Florida) and moves north carried by the winds. But that doesn’t mean that you should ignore sanitation!
Next year look for cultivars resistant to downy mildew that have been developed for cucumber and cantaloupe and to a lesser extent for squash and pumpkin.
Article written by Glenn Palmer, Extension Master Gardener Volunteer.