Answer: You didn't say whether or not the plant eventually dies. If so, the problem may be Timber Rot, caused by Sclerotinia sclerotiorum.
Timber rot causes loss of plant vigor, followed by wilting and eventual death of the plant. Lesions on the stem show hollow pith when cut open, and white moldy growth is usually visible at the edges of the lesions. Small black spherical fruiting bodies (sclerotia) are usually observed in or on the lesion as the condition advances.
Sclerotinia occurs when tomatoes are planted after susceptible crops such as lettuce, cabbage, beans, peppers or eggplants. Soil sterilization is effective on a small plot basis, but good cultural practices and attention to sanitation and crop rotation is important to avoid this problem.
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