Late Blight

This post is from Skippy's Vegetable Garden blog. Skippy's garden is located near Boston, Massachusetts. Late blight has struck the area and is spreading, so I wanted to keep you informed. We may not see much of it here, but you never know. They had a long, wet spring, where we have been drier.

"Late blight seems to be the only conversation topic I hear this week at our community gardens. I hear yelling from across the way about newly discovered dead tomato plants. I hear long technical discussions among groups of gardeners.

And everyone spends a fair amount of time walking around and looking at tomato and potato plants in other plots. People come by my plot to chat and they know whose plants look good and whose are dead. They remind me that I should be bagging and destroying the foliage so it doesn't infect everyone else's plants, which, of course are all already infected or dead or coated with fungicide.

And there are stories circulating about who has pulled everything, who won't pull anything, who's bagging and disposing, and probably one about me just throwing my dead plants into my compost bin.

I heard someone dug up a whole row of potatoes that were just mush from blight.

I emailed the extension school yesterday to see if there were any experts who could come talk to us. With so much interest, I think it would be a good way to get the gardeners together, though we can be an unruly bunch. We could have a nice relaxing coffee and donuts gathering, in a freshly mowed grassy area with beautiful mid-summer flowers, singing birds, and listen to tomato gloom and doom stories....

An article on Late Blight is in yesterday's New York Times: "Outbreak of Fungus Threatens Tomato Crop": "A highly contagious fungus that destroys tomato plants has quickly spread to nearly every state in the Northeast and the mid-Atlantic, and the weather over the next week may determine ... whether tomato crops are ruined.... described as an “explosive” rate of infection.....William Fry, a professor of plant pathology at Cornell, said, “I’ve never seen this on such a wide scale.”

"The current outbreak is believed to have spread from plants in garden stores to backyard gardens and commercial fields.... Some growers are talking about $40 boxes already."
Some facts I've learned about late blight:
- Late Blight was responsible for the Irish potato famine of the mid-19th century.
- It is one of the few plant diseases that can destroy an entire crop.
- The disease can wipe out entire tomato and potato fields within a week if conditions are wet.
- Late blight spores can travel over 40 miles under the right conditions.
- Powerful synthetic fungicides like chlorothalonil (not approved for organic farming) can protect unaffected plants from disease, but can't cure infected ones.
- Copper fungicides are officially listed as synthetics but organic certified farmers are allowed to use these after they have used all available alternative practices to manage late blight.
- Copper fungicides extend potato growing period by between 2-4 weeks.... estimated to result in 10 - 40% higher yields.
- Potato varieties with moderate levels of resistance include: Kennebec, Sebago, Allegany, and Rosa. Elba is currently the most resistant potato variety available.
- Few late blight resistant tomato varieties are available. The cultivar 'Legend' has some resistance, though not under high disease pressure. Some cherry tomato cultivars ('Red Cherry' and 'Sweetie') are more tolerant to late blight. 'Matt's Wild Cherry' is considered resistant.
- If late blight occurs when potatoes tubers have already 'sized up', harvest crop as soon as possible to avoid post-harvest tuber rot.
- Tubers become infected when spores wash down through the soil and come into contact with the potatoes. Tubers are not infected via their connection to plants with blighted foliage.
- Occasionally peppers and eggplants and mildly infected, as are a few nightshade weeds."