Tomato/potato late blight update

Posted: May 12, 2011

The first late blight update of the 2011 season! No confirmed outbreaks in Pennsylvania.
Late blight stem lesions on tomato transplant.

Late blight stem lesions on tomato transplant.

Continued vigilance will be important in managing late blight (caused by Phytophthora infestans) in 2011. In late April, late blight was confirmed on tomatoes grown from farmer-saved seed and on potatoes grown from organic seed pieces in Connecticut. In Wisconsin, late blight was also confirmed on potato seed collected from culls that primarily had soft rot identified using highly sensitive molecular and biochemical tests. Most recently, late blight was confirmed on volunteer tomatoes in a soil bin in an organic greenhouse in Maine. Although these sources are not close by and no outbreaks of late blight have been confirmed in PA, it is still important to be scouting transplants and checking your seed pieces for symptoms. The cool wet weather this spring has been favorable for late blight development.

Symptoms on tomato transplants include lesions that are either pale green or water-soaked and gray in color on the leaves, petioles and stems. Under humid conditions, white sporulation (fuzzy growth) can develop especially on the underside of the leaves (see picture). When released, these spores can spread the pathogen to near-by plants. When dried out, the lesions appear necrotic and brown-black in color.

For potato, it is important to plant pathogen-free seed pieces. Visibly infected seed pieces will have shallow, brownish lesions on the tuber surface and a reddish-brown granular rot under the surface. Only plant certified, healthy seed pieces.

Remember that late blight is a community disease! If you suspect late blight please contact your local Penn State Cooperative Extension Office, the Penn State Plant Disease Clinic or Beth Gugino at or 814-865-7328. Updates will continue to be posted on our Vegetable and Small Fruit Extension Team website ( and audio-messages posted on the 1-800-IPM hotline under the vegetable extension.