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From pictures and description of this disease, I suspect they've always plagued my cucumber plants. What is the best fungicide can I use against it?

Copper-based fungicides like copper oxychloride, copper hydroxide, copper acetate etc are effective. You may combine them with mancozeb for better action.

Thanks. Pls which systemic fungicide can i use?

Check this thread http://www.farmersjoint.com/thread-28687.html.

I appreciate your immense help so far.

Here is an important info I came across. But this one is about Downy mildew.

Once it starts, protectant fungicides like mancozeb and captan will not control it. Phosphorus acid in combination with metalaxyl will control it if you start spraying before it starts or 3-4 days after it starts.

Strobilurin is good even when the disease is becoming severe but beware of resistance development (that is, the downy mildew microbes can eventually learn how to survive it). To prevent this, rotate Strobilurin with other fungicides like metalaxyl (combine with mancozeb) hexaconazole (combine with mancozeb) etc.

Metalaxyl combined with Mefonoxam ( Rodomil Gold M2) and Rodomil copper are also effective.