The Purdue Plant and Pest Diagnostic Laboratory

What's Hot at the P&PDL on
August 19, 2013

Late Blight Confirmed on Tomato in Indiana

Tom Creswell and Gail Ruhl; Plant and Pest Diagnostic Lab, Purdue University

Late blight, a plant disease caused by the fungal-like organism Phytophthora infestans was confirmed on a tomato sample submitted to the PPDL on August 19th, 2013.

Symptoms include olive green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid (early morning or after rain). Sometimes the lesion border is yellow or has a water-soaked appearance. Brown to blackish lesions also develop on upper stems and brown spots develop on tomato fruit.

When conditions are cool and wet, this extremely destructive disease can quickly spread in tomato and potato plantings, but disease spread will be slowed by hot sunny weather. All growers should assume their crops may eventually be affected and thus should be on a weekly schedule to both thoroughly inspect their potato and tomato plantings and apply fungicides if the weather remains cool and cloudy.  See our publication on late blight for more information: BP-80-W: Late Blight on Tomato and Potato (pdf file).

Infected plants in home gardens should be removed immediately and either burned or put in a plastic bag for disposal. DO NOT compost affected plants as spores will spread from this infected debris to other healthy tomato plants.

Since there are many look-alike diseases on tomato leaves identification requires microscopic examination, not visual determination. Suspect samples may be submitted to the Purdue Plant and Pest Diagnostic Lab for confirmation.

AG Answers: FAQ's for Late Blight

Click image to enlarge

Late blight symptoms on tomato leaves and stems. Photo by Janna Beckerman

Late blight lesions appear olive green to brown on tomato leaves

Spore production on late blight lesions gives a fuzzy appearance following a rain or dew.

A dissecting microscope or good hand lens reveals the spores in the “fuzz”.

Compound microscope view of Phytophthora infestans sporangia.

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service