By Sandy Feather ©2009
Penn State Extension
Downy mildew is a fungal disease that first appears as pale green areas on the leaves that turn yellow as the disease progresses. The spots take on an angular appearance because the leaf veins delineate their spread, especially on cucumbers. The spots turn brown as the leaf tissue dies. During humid weather, you may see a purplish-grey downy growth on the underside of the leaves, but you cannot always see this symptom. When the leaves die, they often remain attached to the erect, still green petioles (leaf stems). The infection usually starts near the center of a planting and works its way out toward the ends of the vines. New growth is stunted, and fruiting is reduced. Those fruits that are produced are smaller than normal. When weather conditions are conducive, downy mildew can severely damage a cucurbit crop within a few days.
The causal organism, Pseudoperonospora cubensis does not overwinter in our climate. It is not able to overwinter north of the southernmost states and Mexico. Spores of the causal fungus are blown northward as the growing season warms and conditions become favorable for them to survive. It is favored by cool to moderately warm temperatures (60 - 80 degrees F), although it can flourish in hot weather as long as it is humid and/or wet. It is most severe when we have heavy dew, fog or frequent rain. Overhead irrigation that wets the foliage frequently can also create favorable conditions for downy mildew to develop. Working on or around infected plants, especially when they are wet, can spread downy mildew. Simply brushing against an infected plant can spread the disease; it is also spread on tools used around infected plants.
Managing downy mildew successfully takes a multi-pronged approach. Plant resistant varieties whenever they are available. Cucumber varieties that are resistant to downy mildew include slicers such as Fanfare, Marketmore 76, Dasher II, and Indy. Picklers that have resistance include Calypso, Patio Pickle, and Wellington. Burpless types that show resistance are Burpless 26, Green Dragon, Sweet Slice and Orient Express. Cantaloupe varieties that are listed as resistant include Allstar and Ambrosia.
It is also helpful to grow cucurbit crops as well as possible. Have your soil tested, and follow the recommendations to adjust soil pH and fertilize them for optimum vigor and productivity. Be sure cucurbits are planted in full sun, and that they are spaced properly to allow good air circulation around each planting. Use drip irrigation or soaker hoses, if possible, to avoid wetting the leaves more than necessary. If you must use overhead irrigation, water during the day so that the leaves will dry quickly. Avoid watering at night when the leaves will stay wet all night, or first thing in the morning when the leaves are already wet with dew. You can remove infected leaves, but be careful not to spread downy mildew to healthy plants as you work.
Fungicide applications are the main way to control downy mildew. They are best used preventatively, before the disease strikes. Frankly, the fungicides that commercial vegetable growers use are more effective than those home gardeners have access to. Daconil 2787 or Ortho Max Garden Disease Control (chlorothalonil) is a standard fungicide labeled to control downy mildew on cucurbits. Copper-based fungicides are often recommended for organic gardeners, but must be used with care because they can burn the leaves of cucurbits when applied during hot, humid weather. Other options for organic gardeners include BioNeem (azadirachtin) and Remedy (potassium bicarbonate.
Repeated applications of all of these products are necessary to control downy mildew. Check the label of the product you are using to determine the intervals between applications. Spray at the shortest possible interval allowed by the label when weather conditions are favorable to downy mildew.