This pernicious plague caused by the pathogen Phytophthora infestans was responsible for the 19th-century Irish potato famine, but also targets the tomato, queen among backyard crops.
In its modern incarnation, fortified by strains from New World spots like Mexico, late blight is an aggressive disease resistant to traditional chemical controls.
Late blight arrived early this season, thriving in cool, wet conditions that persisted through early summer. The epidemic was possibly helped along by the wide distribution of infected seedlings to chain stores, but even that factor can’t totally account for the way the blight spread like wildfire along the East Coast from Maine to the Carolinas. It was a devastating season for potato farmers and a bad, sad year for tomato-lovers.
If your plants succumbed and your tomatoes rotted on the vine, my sympathies. But onward, backyard warriors! What about next season? Are we doomed to a repeat performance of the Black Death?
I’ve been scouring the literature from university researchers for the answer to these questions, prompted in part by reader James Cassella of Parsippany, who lost his whole crop this season and spent the summer bereft of home-grown tomato joy.
First things first — prevention begins with a thorough cleanup of any remaining tomato debris. At the first sign of late blight, conscientious gardeners should have removed, bagged and thrown away (not composted) any infected plants.
This assumes that the disease was recognized and correctly identified, something not a given in a year when backyard food-raising jumped 20 percent in a recession. First-timers who knew not what they saw and failed to pull affected plants may have unwittingly contributed to the spread of disease spores, which can travel up to 40 miles on the wind. (Identification help is available from county offices of the Rutgers Cooperative Extension Service; you can find yours at
If you grew potatoes, discard the plants and dig up and throw away all tubers, since the disease can overwinter on them. And here’s something to keep in mind next spring: If you see any volunteer tomato or potato seedlings, pull them up and discard them, too. According to the Ohio State University Extension Office: "Volunteer potatoes and tomatoes can be a significant source of spores of the late-blight fungus."
Late blight can’t survive in the soil, only on plant tissue, so there’s no recommended treatment for purging the earth of spores. Freezing temperatures should do them in. Nonetheless, it’s a very good idea to plant tomatoes in another location next year.
"Rotating" crops, or moving them around from season to season, is a good hedge against the wide range of diseases (some 19 in all) that can afflict tomato plants. If you don’t have a good alternative spot, you might do what I plan to do — grow your tomatoes in big containers next season.
Next spring, plant only certified disease-free seed potatoes and choose resistant varieties for at least some of your tomato crop. There are no guarantees in any year with high disease pressure like this one, but there are a few cultivars that have shown promise.
‘Legend,’ ‘Ferline’ and ‘Fantasio’ have demonstrated late-blight resistance, according to the University of Maine. An even newer cultivar is ‘Mountain Magic,’ a cross between large-fruited and grape tomatoes released by Randy Gardner of North Carolina State University.
You’re unlikely to see these varieties in your local garden center, which brings us to another point: Blight was identified on mass-produced seedlings but seed-grown plants raised at home were at least initially uninfected.
You might try raising your own tomato plants to insulate them from this contagious disease, which can spread rapidly through garden center inventories. If you do purchase seedlings, examine them closely for signs of disease, and discard any that develop cankers or lesions once planted.
Another recommendation is to avoid overfertilization with nitrogen, which causes dense leaf growth (often at the expense of fruit production). Thick foliage tends to retain moisture when humidity is high, and that dampness can create favorable disease conditions.
Avoid overhead watering for the same reason, and don’t irrigate in rainy weather or during cool, cloudy periods. Remember these words from Cornell University scientists: "Infection only occurs when leaves are wet."
Fungicides are a preventive, not a cure. If conditions favor late blight next year, you might want to mount a spraying program. The thing is, you’ll have to be consistent since only continuous coverage will fend off problems. Choose a fungicide that includes the active ingredients maneb, mancozeb, chlorothalonil or fixed copper, and is labeled for tomato and potato late blight.
These measures are about all you can do to keep the evil blight from ruining your crop next year. God speed, tomato-lovers, and good luck.
Garden Diary appears every other week in H&G Today. You may reach Valerie Sudol at email@example.com.