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Defeating Tomato Disease


Q. I have 4'x 8' raised beds in my garden. My peppers, squash, and beans grow well, but my tomatoes have a horrible disease. The leaves get spotty, turn yellow, and eventually fall off. The fruits do fine, but the vines look horrible. Through the help of garden books, I think it's Verticillium Wilt, Fusarium Wilt, or Septoria Leaf Spot. The cure in all cases seems to be to "sterilize the soil" by covering the beds in black plastic and hoping that temperatures get high enough to bake. I don't think I can rely on this method at this point in the season, and have thought of two other possibilities.

1): Oven bake. The potential downside here is the unknown smell factor, but I imagine it would just be earthy. And 2): Bleach. If I put the soil in a trashcan with some water and bleach, would it sterilize itself? Would a little bleach in the soil hurt the plants next spring? Everyone recommends washing cutting boards in bleach…
    ---Andy in Ambler, PA
A. Yikes, Andy! You seem determined to prove the old adage that a little knowledge is a dangerous thing. I'm tempted to revoke your library card!

Fusarium is common down South; you are more likely to have Verticillium. Both are wilts that affect tomatoes grown in the same spot year after year, which is why rotation is so important to these treasured fruits of summer. If you have nowhere to rotate to, try growing in large containers filled with 2/3 of a 'soil-free' potting mix (like THIS one from Gardens Alive, ProMix or one of the Fafard products) and 1/3 compost. Do not use your infected garden soil.

Or remove the soil from one of your raised beds, replace it with a mixture of compost and screened topsoil and grow wilt-resistant varieties; these will be designated by the letters V and F after the tomato's name on the seed packet or plant tag. There may be other letters as well; that's fine.

Leaf spots are often caused by overhead watering. You should only ever water tomatoes at the base of the plants. You can't stop rain from wetting the leaves, but you can stop YOU from wetting the leaves. Positioning the plants where they get morning sun can also be crucial—especially in wet years.

Good disease prevention also includes providing plenty of room in between the plants, and keeping them upright with stakes and cages. (Tomatoes left to sprawl on the ground have no hope, even in a good year.) And although people SAY they will continually tie the new plant parts to stakes throughout the season, few actually do, and those that do do, kill the plants by stepping on the roots and compacting the soil. Do be do be do be do.

In addition, mulch the surface of the soil with an inch of compost at planting time, freshen it up monthly, and promptly pull off any infected leaves. Do this and you will see no more disease.

Sterilization? That's an extreme measure used to reclaim badly diseased soil. It takes a month to eight weeks of hot weather to achieve the necessary temps, and no one outside of Phoenix could hope for success in the Fall. You'd have to give up your beds for the whole summer next season. People in New England can't do it at all.

And NEVER bake soil indoors! Take it from someone who foolishly tried it when he was a newbie gardener; the stench is unbelievable—and long-lasting. We almost had to buy a new oven.

And bleach??!! Yikes, man—why not see if you can buy some Plutonium; that'll sure kill those disease spores! Mixing bleach into your soil would create dioxin-laden toxic waste, not clean soil. Keep it out of the garden. And out of the house, as well. Scrub your cutting boards with soap and hot water; use vinegar if you feel you must de-germ.

Q. My tomato plants grow to about two feet high, then the leaves develop brown and yellow spots, and die from the bottom up. By August they are half to completely dead. Is this Early Blight? I have heard you mention using oak leaves for mulch. I was told that trees contain a blight that can spread to a garden. A few years ago I used compost from leaves that included oak. Might I have gotten the blight from them?
    ---Michael in Greeneville, TN
A. No, Michael; leaf composts and mulches are a CURE for disease, not a cause. Early blight, which does cause spots, is also often a result of overhead watering. But it acts more like the wilts, with new growth looking fine and the affected plants often producing lots of tomatoes. But your language is confusing; to fully understand what's going on here, we need to know if your observations are from a 'glass half empty or glass half full' type of reporter.

If by 'half to completely dead' you mean that the bottoms look ratty, but the tops are green and productive, you may have early blight or one of the wilts. If entire plants are dead, you may have a more serious problem, like bacterial wilt.

Luckily, the remedies are the same: Try and grow in different spots next season, in raised beds amended with lots of compost, with lots of room between plants that are held upright, only water at the base, and don't use chemical fertilizers; the rapid, unnatural growth they cause makes plants much more susceptible to disease.

Q. I have problems with my tomatoes every year; a yellowing of the leaves at the bottom of the plant that works itself up. At times my tomatoes have little water spots and the plants die. I don't have anywhere else to plant; please help my tomatoes!!
    ---"Sweet Cindy" in Worcester, Mass
A. Yellow leaves at the bottom of plants are a classic sign of verticillium wilt, but spots on the fruits themselves are not. If you're a smoker, it could be Tobacco Mosaic Virus ("TMV"), a disease that tobacco users transmit to their plants. Similar problems are also caused by anthracnose and late blight—the dreaded Irish Potato Famine disease organism; both are worst in really wet years.

I strongly suggest you replace as much of your garden soil as possible with a mix of high-quality compost and screened topsoil; and build raised beds if you've been growing in flat ground. Or buy BIG containers, fill them with compost and a soil-free potting mix and grow only one tomato plant per container. And, of course, keep them upright, with lots of airflow between plants.

And because you're growing in such a cool climate, you should seek out disease-resistant varieties (you want the whole tomato alphabet, VFNT, after the variety name) that are also either bred specifically for the North or for early harvest. Don't set them out until at least June 1st, to insure that the soil is warm, be sure to provide lots of air circulation, and if you're a smoker, quit—before your own personal leaves start to turn yellow.

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