![]() 1,2,5 Sporangia deposited on watermelon leaf surfaces can germinate in as few as two hours if water films are present. The sporangia can survive for one or two weeks and travel hundreds of miles in air currents. The sporangia are airborne and spread northward from southern growing regions as the season progresses. 1,5,6ĭowny mildew is spread by structures called sporangia, which function as spores. It then moves northward as the season progresses, typically arriving in the Northeast in July and August. In the U.S., the downy mildew pathogen typically overwinters on cucurbit plants growing in the gulf states. It overwinters on live cucurbit plants in frost-free areas or in greenhouses. It cannot overwinter on crop debris or in the soil. 5,6,7 The downy mildew pathogen is an obligate parasite that needs a living host to survive. Isolates belonging to clade 1 preferentially infect watermelon, pumpkin, and squash, while members of clade 2 preferentially infect cucumber and melon. Pathotypes of the cucurbit downy mildew pathogen are separated into two clades (genetic groups). However, the rapid defoliation can lead to sunscald of the newly exposed fruit, and reduced photosynthesis can result in fewer, smaller, lower-quality fruit. 3,5,6ĭowny mildew does not affect the fruit directly. ![]() The collapsed leaves can have a brown to gray, scorched appearance (Figure 1B). Eventually, the leaf blades curl upward, wilt, and die, leaving the petioles attached to the stem. 3,5 The lesions can expand and coalesce to cover most of the leaf surface. 5,6 The pathogen forms spores on the undersides of leaves, which can give the lesions a fuzzy brown to gray to purple appearance. Lesion expansion can be limited by major veins giving the lesions an angular appearance. 1,2,4,6 The lesions enlarge, becoming brown to black, surrounded by yellow halos (Figure 1A). Small, chlorotic to yellow spots that are circular to irregular in shape develop on the corresponding upper leaf surfaces. The initial symptoms are light yellow, water-soaked spots on the lower leaf surface. 1,3,6 Symptoms develop three to twelve days after infection. Leaves of all ages can be affected, but the symptoms typically develop first on older leaves near the crown before spreading to younger leaves. Downy mildew only infects the watermelon leaves. ![]()
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