P&PDL Picture of the Week for
May 17, 2010

Late Blight of Tomato

Dan Egel, Extension Plant Pathologist, SWPAC, Department of Botany & Plant Pathology, Purdue University

Late blight of tomato causes large, brown, wet-looking lesions on tomato leaves and causes tomato fruit to turn brown and rot.   The same fungus that causes late blight of tomato of 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.  Homeowners and commercial growers were affected by the disease.  In all, over 30 counties were affected.  The reason late blight doesn’t affect tomatoes every year is that the fungus doesn’t usually live through Indiana winters.  It is likely that the late blight fungus arrived in Indiana in 2009 on tomato transplants.

Whether or not late blight will show up this year is of interest to tomato growers all over Indiana.  It is doubtful that the fungus that causes late blight will survive the Indiana winter unless the tomato or potato tissue has been sheltered as in a greenhouse.  Another way in which the late blight fungus might survive is on potato tubers in the soil where the fungus is protected from the cold. 

Details on how to avoid late blight, what to do if late blight is suspected and how to manage late blight can be found in Ag Answers Frequently Asked Question’s, archives of Ag Answers and an extension bulletin found below. 

Frequently Asked Questions about late blight of tomato

Late blight confirmed in Indiana in 2009

Purdue University Extension Bulletin about late blight (pdf file)

Potato Late Blight Webcast - Plant Management Network

Late Blight Update for Indiana - June 1, 2010

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 fungus which causes
late blight.

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service