Downy Mildew of Melon

The pathogen, Pseudoperonospora cubensis, is an obligate parasite of the Cucurbitaceae family. Pathogen populations include pathotype 1, which affects melon, cucumber, and watermelon, and pathotype 2, which is additionally capable of infecting squash. The development of the pathogen is characterized by the formation of polygonal lemon-yellow spots that are strictly delimited by leaf veins. Under high humidity conditions, a gray-violet sporulation layer forms on the underside of the affected areas.

As the disease progresses, the spots enlarge and coalesce, causing browning and necrosis of the leaf blade, while the petioles often remain unaffected. Unlike bacterial diseases, downy mildew lesions have a strictly angular shape. Specific gene markers for resistance to this disease in melon are not widely used in breeding practice at present, and the resistance of cultivars is more often determined empirically through field trials.