Hello Gardeners, I’m Amanda McNulty with Clemson Extension and Making It Grow . Certain strains of downy mildew are serious diseases of cucurbit family members – squash, cucumbers, melons and watermelons. Watermelons, in particular, are very important economically for some South Carolina farmers. Infected plants have angular lesions on the tops of leaves but the reproductive structures, resembling a grey, white mold, are on on the underside of the leaves. To spread, these spores needs free water