The Purdue Plant and Pest Diagnostic Laboratory

Late Blight Resources & News

Late blight of tomato causes large, brown, wet-looking lesions on tomato leaves, dark lesions on tomato stems and leaf petioles and causes tomato fruit to turn brown and rot. The same fungal-like organism that causes late blight of tomato also causes late blight of potato, the disease that was responsible for the Irish potato famine in the mid 1840’s. In late summer and fall of 2009, late blight of tomato was reported in Indiana for the first time in many years. Tomatoes grown by homeowners and commercial growers were affected by the disease. In all, late blight was confirmed in over 30 counties in Indiana. It is likely that late blight arrived in Indiana in 2009 on tomato transplants. Late blight has not been observed in the past to live through Indiana winters.

Late blight, caused by the fungus-like organism, Phytophthora infestans, occurs in moist weather with cool nights and moderately warm days. Dark-green to nearly black wet-looking spots begin spreading in from the leaf edge. In wet weather, the spots may have a downy, white growth on the lower leaf surface near the outer portion of the spot. Spots also develop on the fruits. At first, the spots are gray-green and water-soaked, but they soon enlarge and turn dark brown and firm, with a rough surface. When conditions are favorable, the disease may progress very rapidly.


2010 States with Confirmed Reports of Late Blight

Indiana - June 30

  • Dearborn Co (6/30)
  • Wayne Co (7/16)

Ohio - June 23

Michigan - June 21

Kentucky- June 2


Maryland - May 2

Louisiana - May 4


Homeowner Links


Commercial Links





Click image to enlarge

Late blight on tomato

The large brown area of this tomato leaf is affected by late blight. The edge of the lesion has a white coloration which is the fungal-like organism which causes
late blight.

Close up of leaf

Fruit on plant

Fruit on plant

3 tomato fruit

Late blight

Late blight on fruit

Late blight

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service