Helpful Hints: Tomato Transplant Alert! Late Blight on Tomato and Potatoes

Garden retailers and landscapers should be aware of Late Blight caused by Phytophthora infestans – a very destructive and very infectious disease that kills tomato and potato plants in gardens and on commercial farms in the eastern U.S.

Late blight is the same disease that caused the Irish Potato Famine in the 1840s. It has been in the US for over a century, but it has never occurred this early and this widespread. It not only threatens home gardens, but also the thousands of acres of commercial potatoes and tomatoes that are grown in Massachusetts and across New England. The disease has been diagnosed on tomato transplants throughout the Northeast. Infected plants were distributed throughout the region by several plant retailers this spring. This disease is not seed borne however, it is exceptionally contagious, and can spread to tomato plants on retail shelves not involved in the original and initial source of the inoculum.

Late blight, affects both potato and tomato crops. It produces spores very rapidly and these move very easily from one garden or field to others, because the spores are easily carried in wind currents to infect susceptible plants in even the most remote area in our region.  All tomato and potato plants grown in home gardens and in commercial fields are susceptible to late blight! 

What to Look For:

The most common early symptoms on tomato transplants are brown lesions on stems, with white fungal growth developing under moist conditions.

Symptoms appear as 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 or wet. 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. Late blight can be confused with early blight and Septoria leaf spot, two common diseases found in home gardens.  If the lesion has a yellow border and is occurring on the bottom of the plant, it is likely due to infection of either early blight or Septoria leaf spot.

Information provided by: UMass Extension Landscape/Nursery/Urban Forestry Program

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