Early Blight is superficially similar to the more serious Late Blight.
It is a common fungal disease of some types of potato. The leaves become infected with small sunken black spots, especially on old leaves. It is often worse during dry weather.
The dots may join up to form dry blackish patches, but by then the crop is generally ready anyway.
I have seen it on many earlies, especially Anya and Swift, and a number of later potatoes including Markies, Maris Piper and Sarpo Mira. It doesn't seem to affect yield very much.
The fungus responsible is Alternaria solani. I generally pull off the affected leaves and isolate the plants if they're in containers.
The fungus overwinters either on potato tubers or on dead, infected plant debris. Infection is difficult to predict since early blight is less dependent upon specific weather conditions than late blight.
Nigel Deacon / Diversity website
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