The Purdue Plant and Pest Diagnostic Laboratory

What's Still Hot at the P&PDL for
September 2009

Late Blight Confirmed on Tomato in Indiana

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

(For up to date information on late blight, see our Late Blight Resources & News page)

Late blight, a plant disease caused by the fungal-like organism Phytophthora infestans and last reported in Indiana in 1998 on potatoes, was microscopically confirmed on a tomato sample submitted to the P&PDL on August 7th, 2009. As of October 29th, the P&PDL has confirmed late blight on tomatoes in thirty-one counties in Indiana, including vegetable gardens as well as commercial fields of processing and fresh market tomatoes.

Blighted plant parts include leaves, stems and fruit. Infected plants in home gardens should be removed immediately and either burned or put in a plastic bag for disposal. DO NOT put the removed plants in a compost pile as spores will spread from this infected debris to other healthy tomato plants.

When conditions are cool and wet, this extremely destructive disease quickly kills foliage and rots tomato fruit and potato tubers if not managed. Rainy, cloudy conditions have provided a favorable environment for the pathogen to be successfully dispersed, including long distance spread for infection.

All tomato and potato crops are at high risk of developing late blight this season, especially if the rainy weather continues. All growers should assume their crops eventually will be affected and thus should be on a weekly schedule to both thoroughly inspect their potato and tomato plantings and apply fungicides.

Classic symptoms are large (at least nickel sized) 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. Leaf lesions begin as tiny, irregularly shaped brown spots. Brown to blackish lesions also develop on upper stems. Firm, brown spots develop on tomato fruit. Photographs are posted on the web at: Late blight on tomato and Late Blight of Potatoes and Tomatoes (Cornell University)

Commercial growers have a number of fungicides that if applied early and often, can reduce the spread of Late Blight. Homeowners have a few products that are registered for use; the common name of chlorothalonil should appear on the product label. These products are only effective if used before the disease appears and should be reapplied every 5-7 days if wet weather persists. Chlorothalonil is a protectant fungicide, with no systemic movement in the plant, so thorough coverage is necessary. For organic growers the options are very limited, since only copper fungicides can be used, and they are not very effective.

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.


Other pertinent links:

Map of Indiana Late Blight Confirmations - Updated 10/29/09

Late Blight Q&A - Ag Answers

Late Blight Confirmed in Indiana - Vegetable Crops Hotline

Late Blight on Tomato Plants at Local Large Stores in Most States in the Northeast

Tomato Diseases and Disorders (pdf file) - Iowa State University

Take steps now to help prevent late blight next year - Purdue University News Service

Late blight: Kill diseased tomatoes and potatoes to prevent disease in your gardens next year - Vegetable Crop Advisory Team Alert, Michigan State University

Click image to enlarge

Late blight

Late blight on fruit

Late blight

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service