Late Blight is Back
Posted: June 30, 2012
Caption for the Late Blight photo: Symptoms of late blight on tomato include gray-brown lesions on the upper leaf surface and elongated chocolate lesions on the stems. Photo: Beth K. Gugino
Late blight can destroy a healthy tomato or potato plant in a matter of a few days. It does not attack other vegetable or fruiting plants. Late blight is a fungus that moves through the air. It can move long distances when the weather is cool, and wet. You will first notice a dark colored blotch with a light green hallow around it on an upper leaf. When you look on the underside of the leaf the lesion will have a light grayish colored fuzzy ring around the outside. This ring of fuzzy growth is the formation of spores that will be released into the air to infect additional plants.
Because late blight makes thousands of spores that can travel long distances it is critical that you watch your tomato and potato plants carefully. If you see what you think is late blight, please let your county Penn State Extension office know as soon as possible. A positive identification will be needed. Also, other gardeners and farmers need to know if late blight is in their area.
Fungicides are effective at preventing late blight if they are applied before the spore lands on the leaf. Late blight can’t be eradicated once it has infected a plant. Fungicides containing Chlorothalonil are the most effective at preventing late blight. Fungicides containing Mancozeb are second best. Copper fungicide is somewhat effective and some formulations of copper fungicides are approved for organic production.
Penn State Extension has a great factsheet on late blight that is available on the web at http://pubs.cas.psu.edu/freepubs/PDFS/ul215.pdf
For more information contact your local extension office. In Lackawanna County call 570-963-6842 or email LackawannaMG@psu.edu
John Esslinger, Extension Educator
Penn State Extension