Late blight crisis: Keep an eye on your tomato plants

Serious bad news warning: Late blight, perhaps the most devastating disease that can affect food growers, appears to be widespread in tomato plants in all six New England states and New York, with additional cases confirmed in virtually every other East Coast state, as well as Ohio and West Virginia.

Cornell University released the story on July 1. People not familiar with the various diseases that can strike plants may have felt that Cornell plant pathologist Meg McGrath (no relation that we know of) was exaggerating the seriousness of the problem, but nothing could be further from the truth.

Although this outbreak seems to be limited to tomatoes so far, late blight is the same organism that caused the Irish potato famine. The disease is equally deadly to tomatoes, thanks to the close botanical relationship between the plants.

Like all plant diseases, late blight doesn't directly affect anything outside the world of plants. But it is deadly to the plants it infects. Most diseases that affect tomatoes generally only cause some discoloration of leaves. Late blight affects the fruits as well as the foliage, and almost always causes rapid death of the entire plant.

Late blight spreads like wildfire. If people don't promptly remove and destroy infected plants, the spores will travel with the wind to the next garden or farm. The cool, damp weather that plagued our area throughout much of June provided the 'perfect' conditions for this nasty actor to spread rapidly. And this is the earliest we've ever seen this disease, whose common name tells you that it generally shows up late in the season.

The Associated Press has done has an excellent job of reporting on this potential crisis. According to its story of July 3, the disease appears to have originated in tomato plants shipped from a large-scale grower down South to 'Big Box' stores on the East Coast -- specifically Lowe's, Home Depot, Wal-Mart and KMart. Be especially vigilant if you bought plants from those stores.

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

Now, there are many common disease symptoms that don't indicate late blight. For instance, the soil-borne wilts that are virtually unavoidable if you grow your tomatoes in the same spot year after year cause the bottom leaves of the plants to turn yellow. The fruits are never affected, the progress of the disease is generally very slow and there's no reason to do anything other than trash those discolored leaves.

Here are the symptoms of late blight as described by Cornell's Meg McGrath: Brown spots (lesions) on stems are one of the most visible early symptoms. They begin small and firm, then quickly enlarge. Under moist conditions, a white fungal growth develops and a soft rot collapses the stem of the tomato plant, which turns black.

Other classic symptoms include large (nickel-sized or bigger) olive-green to brown spots on leaves; sometimes with slightly fuzzy white fungal growth on the undersides of those leaves, especially when 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.

Here's a link to detailed photos of late blight. Scroll down -- the first few aren't all that helpful, but the ones that follow are very informative.

I'm sorry, but if your plants come down with this problem, you MUST pull them up and destroy them. I suggest 'bagging' the top of the plant in a plastic trash bag (to minimize spore drop), and then pull it out by the roots. Put the bagged plant into a bigger bag and put it out with the trash. Under no circumstances, should you hope that 'it will get better.' (It won't and those spores are exploding exponentially every day.) Don't try to compost the remains. Trash, trash, trash.

You will see recommendations for fungicides that can be used. Don't delude yourself and become the vegetative equivalent of Typhoid Mary; these chemicals have little to no ability to affect this truly virulent actor. Do the right thing and destroy infected plants ASAP.

Prevention: If your tomato plants are 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 and replanting 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 compost; buy some. Don't water with an overhead sprinkler; only water at the base.

And finally, if you want to spray something for prevention, spray your tomato (and potato) leaves with compost tea or the Cornell Formula - a baking soda based disease preventative - first thing in the morning, once a week.

The Cornell Formula

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.

In one gallon of water, mix and repeatedly shake:

• 1 tablespoon baking soda
• 2 drops dishwashing liquid or insecticidal soap
• 1 tablespoon oil. You can use vegetable oil, but horticultural oil (available at better garden centers) 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.)

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 it's city water, let it sit for a day first and stir it a few times to dissipate the chlorine. Let it steep for 24 hours, then strain the liquid that next morning and spray immediately. 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 tea

Again, 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, such as The Soil Soup machine or Gardens Alive's Compost Tea. That extra air will greatly multiply the number of helpful little compost guys in your tea.

(Copyright 2009 by WTOP and Mike McGrath. All Rights Reserved.)