The lower, older leaves on your tomato plant begin to be shaded out, once the plant reaches about 3 ft. tall. Because it’s damp and shady down there, those leaves are usually the first to develop fungal problems. While they are spotting and yellowing, they are also spreading the disease. You can prevent this problem somewhat by removing the leaves on the lower 8 – 12 inches of stem. This way any soil born spores will have a harder time splashing up on the leaves.
Spraying the whole plant weekly with compost tea will give the plants an added boost of prevention.
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