Early blight on potatoes

Early blight on potatoes is so common that most gardeners accept some degree of it as a normal part of the growing season. The challenge is that it begins on the oldest lower leaves in midsummer and progresses upward precisely as tubers are sizing -- reduced photosynthesis during this period.

Early blight on potatoes is so common that most gardeners accept some degree of it as a normal part of the growing season. The challenge is that it begins on the oldest lower leaves in midsummer and progresses upward precisely as tubers are sizing — reduced photosynthesis during this period directly reduces tuber yields. Managing early blight is therefore a yield management question, not just an aesthetic one.

I don't grow potatoes at my Long Island property, so this guide draws on University Extension research.

The pathogen

Early blight of potato is caused by Alternaria solani, the same species that causes early blight of tomato. Per Penn State Extension, the fungus overwinters in infected plant debris in soil and on infected tubers; seed potatoes infected with the pathogen can introduce it to new garden sites. Conidia (spores) are splash-dispersed from soil and infected leaves to healthy foliage.

A related species, Alternaria alternata, causes similar but less severe lesions on potato under some conditions.