My cucumber leaves have angular yellow patches on the upper side that are bounded by the veins, and underneath there is a greyish purple fuzzy growth. The yellow turns brown and leaves dry up. It rained a lot recently. What is this?
Angular yellow patches limited by the leaf veins with a greyish fuzzy growth on the underside, appearing after heavy rain and humidity, are signs of downy mildew, a fast moving disease of cucurbits in the wet season. Unlike powdery mildew which is white powder on top, downy mildew shows fuzz on the underside and is driven by wetness. Improve airflow by spacing and trellising the vines off the ground, avoid overhead irrigation, and remove badly affected leaves. At first signs spray a protectant such as mancozeb or copper, and if it is advancing use a systemic like metalaxyl combined with mancozeb, or cymoxanil, rotating groups to avoid resistance. Acting early matters because downy mildew can strip a crop quickly in wet weather. Since gourds and cucumber are eaten fresh, follow the label dose and waiting period, and confirm the diagnosis with your KVK before spraying.
Trellising my cucumber off the ground helped a lot. When vines were lying flat the disease just exploded in the rains.
The fuzz underneath is the giveaway. I used to confuse it with powdery mildew and used the wrong spray. Now I always turn the leaf over to check.
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