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Late Blight Crisis; Keep an Eye on Your Tomato and Potato Plants!

Serious bad news warning: As of early July 2009, Late Blight, perhaps the most devastating disease of food plants, appears to be widespread in tomato and potato plants in all six New England states and New York; with additional cases confirmed in virtually every other East Coast state, Ohio and West Virginia.

Persons not familiar with disease issues may have felt that plant pathologist Dr. Meg McGrath (no relation) was exaggerating the seriousness of the problem in news releases from Cornell University, but nothing could be further from the truth.

Late Blight is the same pathogen that caused the Irish Potato Famine. The disease is equally deadly to tomatoes. Like all plant diseases, late blight doesn't directly affect humans or other non-plant organisms, but it is deadly to the plants it infects. A lot of the diseases that affect tomatoes only cause some discoloration of leaves and a corresponding loss in solar powered photosynthesis. Late blight affects the fruit as well as the foliage, and almost always causes rapid death of the entire plant. Same with potatoes.

And late blight spreads like wildfire on a windy day in bone-dry weather. If people don't promptly remove and destroy infected plants the spores will travel with the wind to the next garden—or farm. A single spore can travel many miles. And the cool, damp weather that plagued the affected parts of the country throughout much of June provided the 'perfect' conditions for this nasty actor to spread rapidly. Oh and just to make SURE you don't sleep well tonight, this is the earliest and most widespread it's ever appeared.

The Associated Press has done an excellent job of reporting on this potential crisis. According to their story of July 3rd, the disease appears to have originated in tomato plants shipped to 'Big Box' stores on the east Coast—specifically Lowe's, Home Depot, Wal-Mart, and Kmart. Infected plants were found at almost every store that was checked, so be especially vigilant if you or a neighbor got plants from such a store.

If you grew your own plants from seed or got them from a purely local grower—who didn't mix their stock with bulk-purchased plants—your only risk is the spores blowing into your garden. If you're in a community garden or other 'shared' situation, make sure your fellow growers are informed, and agree up front that any infected plants will be immediately destroyed.

Now, don't panic completely. Many common disease symptoms don't indicate late blight—like the soil-borne wilts that discolor the bottom leaves of tomato plants grown in the same spot year after year. And many diseases in addition to late blight cause leaf spots to appear; they're smaller and lack the accompanying symptoms of late blight.

Cornell's Meg McGrath says the most visible early symptom of late blight is the appearance of brown spots or lesions on the plant's stems. They quickly enlarge, and under moist conditions develop a white fungal growth. A soft rot soon collapses the stem, which turns a dark brown or even black. The spots that appear on the plant's leaves are large (at least the size of a nickel), olive-green to brown, and sometimes have a fuzzy white fungal growth underneath, especially if conditions have been humid. The borders of the spots may be yellow or have a water-soaked appearance. Eventually, brown spots develop on the fruit as well. Here's a link to more descriptions and lots of photos from the Cornell website.

I'm sorry, but if your plants come down with these symptoms, you MUST pull them up and destroy them. I suggest you 'bag' the top of the plant in a plastic trash bag (to minimize spore drop), and pull it out by the roots. Put the bagged plant into a bigger bag and put it out with the trash. Meg McGrath—whose own Long Island, NY garden has been hit—additionally recommends you 'cook' the bag in the sun for a day first. I know it's tough to sacrifice your precious plants so early in the season, but you must. Under no circumstances, should you hope that 'it'll get better' (it won't; and those spores are exploding exponentially every day) or try and compost the remains. Trash, trash, trash.

You will see recommendations for chemical fungicides that can be used. Don't delude yourself and become the vegetative equivalent of Typhoid Mary; the chemicals available to home gardeners have little to no ability to affect this truly virulent actor. Neither do organic solutions once the disease has taken hold. Do the right thing and destroy the plants ASAP.

If your plants are clean so far, try and keep them as healthy as possible. If they're sprawling, get them up off the ground. If they're crowded—that's less than a foot between the outside edges—move or lose a few (do any moving in the evening). If they're mulched with anything but compost, remove the mulch and trash it. Then cover the entire area underneath the plant with an inch or two of high-quality compost. If you don't have any, buy some. Water only at the base of the plants; not with an overhead sprinkler.

And finally, if you want to try some basic disease-prevention on healthy tomato and potato plants, you have four basic alternatives.

In their extremely helpful Bulletin on late blight in potatoes, the absolutely amazing organic resource for farmers ATTRA (National Sustainable Agriculture Information Service) suggests sprays of compost tea, copper, or the relatively new (and really cool!) natural disease-preventing agent Bacillus subtilis, a naturally-occurring organism sold under a number of different brand names, including Serenade and Plant Guardian Biofungicide. I like this new player in the disease-fighting field a lot, it's approved for use in organic agriculture without any restrictions and is effective against a wide variety of disease problems.

I would also add The Cornell Formula, a baking soda-based disease preventative (recipe below) that I've found to be very effective at preventing disease on roses. No matter what you choose to spray, always spray in the morning, always remove any discolored leaves before spraying, always make sure to soak the undersides of the leaves, and never use a sprayer that has held herbicides, pesticides or other chemicals.

The Cornell Formula.

In one gallon of water, mix and repeatedly shake:

  • 1 tablespoon baking soda
  • 2 drops dish washing liquid or insecticidal soap
  • 1 tablespoon oil. You can use vegetable oil, but horticultural oil will work better, especially one of the new lighter-weight "summer oils". (Cowboy Gardeners: Do NOT use motor oil or WD-40 or any other such foolish thing.)
Regular compost tea
Early in the morning, place some of your finest quality compost in a porous cloth container and put it in a container full of cool water; an old sock for a gallon of water; a pillowcase or burlap sack in a clean trash can full of water. If its city water, let it sit for a day first and stir it a few times to dissipate the chlorine. Let your tea steep for 24 hours, then strain the liquid and spray immediately the next morning; you want to use it right away to get the maximum number of little compost guys fighting for you. (Return the contents of your 'tea bag' to your compost pile.)

Aerated compost teaAgain, make a batch of compost tea in the morning, but drop some aquarium bubblers in there to add air as it brews—or use one of the commercial devices that do this, like The Soil Soup machine or Gardens Alive's "Compost Tea kit". That extra air will greatly multiply the number of helpful little compost guys in your tea.

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