(Editor's Note: This article was originally published on September 3, 2009. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
I suppose a gardener's luck can only last, well, from one season to the next? I thought mine would never run out, until recently when I discovered my tomatoes had been infected with late blight. I first heard about the disease in mid-July when it was announced in the media that big-box retailers had sold tomato starts infected with the pathogen. Gardeners everywhere experience troubles in the veggie patch from time to time, but "the latest trouble is the explosion of late blight" as The New York Times Op-Ed Contributor Dan Barber put it in a piece published August 8 of this year.
A water mold (oomycete), Phytophthora infestans is the same disease that caused the Irish Potato Famine in 1845. Similar, but distinctly different evolutionarily, to fungi, the microscopic spores of P. infestans develop on leaves and stems when temperatures are above 50 °F and humidity stays at 75% for more than two days. The spores of the fungus develop rapidly in wet and cool weather.
Tell-tale signs of the disease show up as dark brown or gray blotches on leaf tips and stems of plants. Ideal weather conditions in the northeast over the summer, especially during late July and into August, caused a major outbreak of the disease which infected hundreds of tomato patches in backyard gardens. Market growers and farmers who grow sizable crops of tomatoes were also affected.
Light blight can be seen along the stem of this infected tomato plant. Affected areas can be dark and appear fuzzy with a moldy appearance.
It's not uncommon to misdiagnose the disease so before you take drastic measures and destroy your plants, be sure to correctly indentify the problem. Look for symptoms described above and visit online tomato disease sites such as those listed at the end of this article.
Tomato stem infected with late blight.
If after searching online you're unable to determine if late blight has infected your plants, take a specimen in to your local cooperative extension office. Cut the stem that you think might be infected, include as many leaves as are remaining on the stem and place the sample in a clear plastic zip-lock baggie.
Personal experience with the disease this year has shown that you can still harvest tomatoes from diseased plants. However, a certain level of comfort might alter your decision to use fruit from blight infected plants. Green and/or ripening tomatoes picked from diseased plants will continue to ripen, but may still become infected. Look for any unnatural blemishes or dark spots and discard fruit if signs of infection become apparent.
Preventing late blight is especially troublesome for backyard gardeners. Fungicides containing chlorothalonil (Bravo), and dithane (Mancozeb) have proven to be somewhat effective. A spraying program must be followed diligently every 5 to 7 days, and the frequency of sprayings can increase depending on the growth rate of your plants.
It's important to discard plants infected with late blight. Do not compost infected tomato or potato plants. Although late blight is not a soil borne disease, it can overwinter in infected tubers. During especially mild winters, fungal diseases such as late blight may find enough warmth within the compost pile to survive.
Use thick plastic garbage bags when discarding plants infected with late blight.
For more information about late blight:
Cornell University's Late Blight Factsheet
Disease Photos: Late Blight on Tomato, Cornell University