Late blight on potatoes

Phytophthora infestans on potatoes is a special case in plant pathology: it is the disease that re-shaped modern agriculture. The same pathogen that caused the Irish Famine remains a serious disease today, requiring active management in any season with cool, wet summers. What makes potato late.

Phytophthora infestans on potatoes is a special case in plant pathology: it is the disease that re-shaped modern agriculture. The same pathogen that caused the Irish Famine remains a serious disease today, requiring active management in any season with cool, wet summers. What makes potato late blight different from late blight on tomatoes is the tuber dimension — infected tubers in the ground or in storage serve as overwintering inoculum for the following season, creating a year-to-year carry-over problem that does not exist with tomatoes in most northern gardens.

I don't grow potatoes at my Long Island property, so this guide draws on Penn State Extension, Cornell, and USDA late blight research.

The pathogen

Per Penn State Extension, Phytophthora infestans is an oomycete that overwinters primarily in infected potato tubers. In the northern US, the pathogen does not survive in soil absent host tissue; infected cull piles, unharvested potatoes, and infected seed pieces are the primary inoculum sources for each season's epidemic.

When infected tubers sprout, the emerging shoot carries the infection and begins producing sporangia. Sporangia are wind-dispersed, travel long distances, and infect new plants through stomates when conditions are favorable.

Per Cornell Cooperative Extension, the USAblight.org network (usablight.org) tracks confirmed late blight reports and provides regional forecasts.