Common Name: Late blight of tomatoes
Scientific Name: Phytophthora infestans
Late blight is a destructive tomato and potato disease. It is sometimes confused with early blight but is a very different fungal-like pathogen. It generally develops at cooler temperatures than early blight. In the U.S. it occurs primarily in the Midwest and northeastern states. It is also found across Canada.
The pathogen needs living tissue to survive. Generally speaking it is not a soil borne disease. However, this can be misleading, because the organism can survive mild winters on small bits of plant debris in the soil.
This pathogen is the same as the one that led to the Irish Potato Famine of the 1840's.
Plant symptoms: The first symptom on tomato plants is often a brown/black lesion on the stem (see above photo). Leaves develop large brown/black blotches. In humid weather and in early mornings, a fuzzy growth containing spores of the fungus can often be seen on the underside of the brown/black blotches or on the stem lesions.
Fruit symptoms: Infection causes a brown/black, leathery rot. It may later become soft and mushy.
The disease occurs sporadically because the pathogen is usually not present. But if the disease is present in the area it's very probable that wind-dispersed spores will distribute the pathogen over wide areas. Some plants may escape if the pathogen happens not to land on them.
For late blight to develop in a particular garden the pathogen must be present. It usually finds its way there by being brought in on infested potato seed tuber pieces or infected tomato transplants. It also may be blown in as spores from affected plants in another location.
Plants in warmer, drier, sunny spots will have less occurrence of the disease. Plants grown in a high moisture-holding soil and more humid environment will be more likely to have the disease than plants grown in sandy soil or plastic mulch.
A combination of practices can help keep the fungus under control. The best method is prevention. Once it's started the disease is very difficult to control. Fungicide may be your only option.
For more information on Phytophthora infestans and other tomato diseases, contact your county Extension office.