Late blight of tomato plants has recently been confirmed in a commercial field in Montgomery County, MD.

Late blight is a potentially destructive disease of tomatoes (and potatoes) caused by the fungal disease, Phytophthora infestans. This pathogen is referred to as a ‘water mold’ since it thrives under cool, moist conditions. Avoid sprinkler irrigation if possible, because it favors the development of late blight. The disease can wipe out an entire crop within just a few weeks of infestation and was responsible for the infamous Irish potato famine of the 1840’s.

In 2009, the late blight pathogen infected tomato plants in many home and community gardens in the mid-Atlantic region. Southern grown tomato transplants sold through retail stores were the source of the original infection. Cool, wet spring weather conditions allowed the pathogen to flourish.


Late blight tomato leaf, image from Cornell University

Late Blight

Late blight can affect all parts of the plant; whereas some similar looking diseases cannot. Leaf symptoms of late blight first appear as small, pale green or olive green areas that quickly enlarge to brown-black, water-soaked areas and rapidly enlarge to form purple-brown, oily-appearing blotches.  Large, dark brown lesions develop on stems and petioles. On tomato fruit, late blight causes a firm, dark, greasy looking lesion. Both green and ripe fruit can be infected.


Late blight on tomato fruit, image from Cornell University


Unlike most pathogenic fungi, the late blight fungus cannot survive in soil or dead plant debris. The pathogen can overwinter on infected plant material that is kept alive through the winter, such as blight infected tomato plants kept warm in a compost pile. For this reason, do not compost late blight infected tomatoes. Instead, pull up plants by the roots, bag them and leave in the sun for a few days for the plant and pathogen to die; then put out for trash pickup. Established lesions cannot be controlled, even with the most effective systemic fungicides.


Lesions on stems, image from Cornell University


The spores of late blight are easily dislodged by wind and rain and can be blown into neighboring fields within 5-10 miles, or more. You can play an important role in minimizing the spread of late blight by controlling it in your home garden. This will prevent spores from being produced that could cause infection to nearby gardens or commercial fields. If you need more information, contact your local extension office. 

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