Late blight is a disease that most commonly affects potatoes, but can affect tomatoes (and sometimes petunias) when the weather is cool, rainy and humid. The pathogen is called Phytophthora infestans and is well-known to potato growers. Late blight was very troublesome in 2009 for tomato and potato growers in regions that had an especially wet spring followed by an unusually cool and wet summer. Late blight symptoms include blighting on all aboveground parts of the tomato plant. Lesions on leaves often appear dark and oily with production of sporangia (a.k.a. seeds of the pathogen) occurring on the undersides of the leaves, resulting in a whitish/purplish appearance especially when conditions are wet and humid. These sporangia can be carried long distances from diseased plants to nearby healthy plants via wind currents and storm fronts. Blackened lesions on the stems also occur and are typical of late blight disease. Late blight affects green and ripe tomato fruit. The blighting on the fruit appears as dark, greasy areas that enlarge rapidly, encompassing the entire fruit. During wet and humid conditions, white masses (sporangia and threads) of the late blight pathogen can be seen on the diseased leaves and fruit.
Between cropping seasons, this fungal-like mold survives on volunteer and abandoned potatoes in cull piles. The potato pathologist from MSU, Dr. Willie Kirk, has reported that 2010 is turning out to be a very bad volunteer year for potato tubers that are emerging in fields that had late blight last year. Control measures include eliminating all potato and/or tomato cull piles and destroying volunteer potato plants that grow from overwintered tubers. Infected potato plants established from diseased seed potatoes are another source of late blight.
Each year, tomato transplants growing in the greenhouse environment should be routinely protected with fungicides because they are vulnerable to a number of diseases including Botrytis, Alternaria, Septoria and three types of bacterial blight. Given the increase of late blight occurrences in the United States over the last year, it is imperative that greenhouse tomato transplants (and petunias) also be protected against late blight. Although late blight does not overwinter in Michigan on diseased tomatoes, late blight spores can develop on nearby cull or volunteer potatoes and blow into greenhouses and cause infection on susceptible crops. Tomatoes represent the largest group of greenhouse-grown, late blight-susceptible crops. However, in years past, petunias have served as a host of late blight in other regions of the United States.
Use a copper-based organic approved fungicide to help reduce the
infection level but be sure to apply on a consistent basis and follow
This article by Dr. Mary Hausbeck was posted on June 1, 2010 at http://ipmnews.msu.edu/greenhouse/greenhouse/tabid/99/articleType/ArticleView/articleId/2730/Protect-tomatoes-in-the-greenhouse-from-late-blight.aspx
We edited the article to remove references to fungicides not appropriate for organic systems and we added the recommendation to use a copper-based organic-approved fungicide.