Basil plants are threatened by downy mildew this year, according to Margaret McGrath, a Cornell Plant Pathologist. The disease has been documented in New York, New Jersey, Ohio, and Florida.
The most obvious signs of the blight, according to McGrath, are yellowing leaves. But the telltale marks of downy mildew are the dusty spores on the leaves' undersides. The disease was first described in Africa in the 1930s, and didn't appear in the US until just a few years ago.
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