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For early blight, once the tomato plant is established, I always put a piece of copper wire through the stem, just above the soil. However, this will not stop late blight, as it comes from the air not the soil.
If you have a problem with tomato plants contracting a blight that starts at the bottom and works its way up until the plant is dead, or even the beginning of a blight, you need to cook your garden bed. Once tomatoes are infected, they cannot be helped. The key is solarizing the soil to kill the bacteria before they get to the plants. As soon as you can work the soil, turn the entire bed to a depth of 6 inches, then level and smooth it out. Dig a narrow 4 to 6-inch deep trench around the whole bed and thoroughly soak the soil by slowly running a sprinkler over it for several hours.
Cover the bed with a clear, heavy plastic painters drop cloth. Lay the edges of it in the trench and cover with soil to keep heat from escaping. The sun should heat the area for at least 6 weeks. The longer you leave the cover in place, the better. In the meantime, try growing some of those new verticillium and fusarium-resistant varieties in another location, or in containers of sterile potting soil, as you let your infected tomato bed cook.
This gardening information comes from Veggie Grow How, by Glen O. Seibert, "The Greenman". http://www.backyardlivingmagazine.com/podcasts.aspx
By Connie from Oden, AR
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Here are questions related to Treating Soil After Tomato Blight.
My tomatoes had blight last year. Can I safely plant other plants in this spot, such as cucumbers? Thank you.
By Sandi 03/27/2010
Rotate the veggies is always a good idea. Never plant the same thing in a spot you did the previous year. We had tomato blight here despite the fact I started my own plants. I understand it came up from the south via infected plants. You were supposed to burn the infected plants which we did. This year I am using big pots for my tomatoes and not planting them in the ground, just in case. Last year I lost all my tomato plants, but not the ones in pots. Good luck.