Monday, July 6, 2009

Late blight of tomato and potato arrives early this year

I had hoped that I would not write about late blight this year. Usually it is August or September before late blight rears its head in our area. So writing about it in early July is a bad sign for gardeners and farmers. But it is here. A perverse combination of the widespread sale of infected tomato plants and abnormally cool, wet, weather in June was the perfect storm. The disease has been confirmed in almost all Northeastern and mid-Atlantic states, including in Lancaster and Lehigh counties in southeastern Pennsylvania.

This devastating fungal disease is a threat to all tomato and potato growers. It is the same fungus that was involved in the Irish potato famine of the 1840’s. While no one will starve in 2009 due to late blight, it appears that we are in for an unusually early and wide spread epidemic. Plant pathologists are reporting that this is the earliest the organism has been found in such a wide spread area of the Northeast U.S. The problem is, this fungus can travel many miles in the air, settling out on host plants and spreading infections. Apparently, widespread sale of infected transplants has inoculated the northeast. Many of these locations will be home gardens where it will go unnoticed until it is too late. Nearby commercial plantings are at risk.

The fungal pathogen involved in late blight disease is Phytophthora infestans. It thrives under cool (60-70 degree) moist (dew rain, humidity) conditions. Host plants include tomato, potato, other tomato family plants and, believe it or not, petunia. Foliage, stems and fruit blackens. Unlike the slow progression of early blight described in the previous blog, late blight moves like lightning. That’s why plant pathologists watch like hawks for its emergence and alert commercial growers to its presence.

Fungicides can protect plants and limit the spread of the disease. Famers have good options but must be vigilant and proactive. Gardeners have the fungicides chlorothalonil and copper. Copper is a poor second choice but some forms are organically approved. More details, instructions and pictures from Cornell University can be found here. And at this Penn State site. Plant pathologists recommend treating now, rather than waiting for signs of infection. This fungus is amongus.


Patrick said...

The main stems on my tomato plants also show the blight spots. My question is: Will the fruit be edible?
The ripening tomatoes are shiny. Some of the green fruit are quite dull in color.

Scott Guiser said...

Yes fruit is edible but your plants may not live long enough to ripen the fruit. Fried green tomatoes anyone? In additon, infected plants are a threat to other tomatoes and potatoes. Bite the bullet and sacrifice the crop. See this link for instructions.