Downy mildew

 

   

   Optimal conditions for Pseudoperonospora cubensis are cool nights between 55-75F and relative humidity >95. All cucurbits are susceptible to this fungus but yield losses are most common in cucumbers and muskmelons. Several strains of this disease may exist in cucurbits. Initial lesions, which are limited by the small leaf veins, include irregular to angular, pale green spots which appear on the upper side of leaves near the plant crown. The lesions then turn into yellow angular spots. The underside of leaves later develop a downy white to gray mold, which may turn gray to purple during wet weather. Spores develop in this mold. The spores can be transmitted from plant to plant and from field to field by wind. After spores infect a new leaf, lesions develop in four to six days. The entire infected leaves eventually wither and die. Plant stunting and poor fruit growth results under heavy disease pressure.

   The disease can be prevented with weekly fungicide treatments. Sprays are less effective once the disease has appeared in the field. Area-wide monitoring programs exist in other states in which weather is monitored and growers are warned to start their spray programs when the weather is conducive to downy mildew outbreaks. Resistance is available in some commercial cultivars such as Dasher II, Marketmore 76, Poinsett 76, Sweet Slice, Slice Master, Sprint 440 II, and Gemini 7. Downy mildew resistance in cucumbers is complex and involves several genes.