Late blight on tomatoes

Late blight is the disease that caused the Irish Potato Famine of the 1840s. The same pathogen -- Phytophthora infestans -- attacks tomatoes as well as potatoes, and in favorable years it can destroy a healthy tomato planting within days to two weeks. The 2009 epidemic in the northeastern US,.

Late blight is the disease that caused the Irish Potato Famine of the 1840s. The same pathogen — Phytophthora infestans — attacks tomatoes as well as potatoes, and in favorable years it can destroy a healthy tomato planting within days to two weeks. The 2009 epidemic in the northeastern US, spread through infected transplants sold at big-box retailers, demonstrated that the disease can arrive in suburban gardens unexpectedly and spread rapidly through an entire region.

I don't grow tomatoes at my Long Island property, so this guide is sourced entirely from University Extension and USDA publications.

The pathogen

Late blight is caused by Phytophthora infestans, an oomycete (not a true fungus) in the same pathogen group as downy mildews. Per Cornell Cooperative Extension, in much of the northern US, P. infestans does not survive in outdoor soil over winter — it overwinters primarily in infected potato tubers (cull piles, unharvested potatoes, seed tubers). New strains introduced from nursery-level infected transplants, as in 2009, can initiate epidemics in seasons and regions where the disease would not normally appear.

Per Penn State Extension, the pathogen produces airborne sporangia that are dispersed by wind over long distances — a single infected plant can spread the disease to gardens and farms miles away.