Late blight on tomato. Photo courtesy Purdue University/Janna Beckerman
Late blight disease was confirmed this week on several tomato samples from Tippecanoe County in west-central Indiana, leading the Purdue University Plant and Pest Diagnostic Laboratory to urge growers to inspect their plants for the destructive disease.
Symptoms of late blight, caused by the fungal-like organism Phytophthora infestans, include olive green to brown spots on leaves with slightly fuzzy-white fungal growth on the underside. Humid conditions, such as in the early morning or after rain, is a primary contributing factor.
The lesion border sometimes is yellow or has a water-soaked appearance. Brown to blackish lesions also develop on upper stems, and brown spots develop on tomato fruit.
The disease can spread quickly in tomato and potato plantings in cool and wet conditions. The spread is 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,” said Tom Creswell, lab director, and Gail Ruhl, senior plant disease diagnostician, on the PPDL’s website.
They said 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,” they said.
Because there are many similar diseases on tomato leaves, identification of late blight requires examination by microscope. Samples can be submitted for analysis to Plant and Pest Diagnostic Laboratory [1].
More information on late blight is available in the Purdue Extension publication Late Blight on Tomato and Potato [2].