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What is it?
This tomato plant wilt disease is caused by the soil inhabitating fungi of the fusarium species known as fusarium oxysporum. This is a species of fusarium wilt which only attacks tomato plants.
What does it look like?
Yellowing begins on the lower tomato leaves, which then turn a dingy brown and begin to wilt. Next the tomato shoots become infected and begin wilting. Typically the wilting will begin on one side of the tomato plant’s leaves or shoots and spread to the other side as infection worsens. Full grown tomato plants have the biggest problem with severe wilting, it is not as severe in younger shoots. To confirm fusarium wilt in tomato plants, you can slice the stem of the plant lengthwise at the soil line, near the base. If fusarium wilt is the problem, the tissue inside the cut will reveal dark brownish plant tissue starting about 1/8 inch below the bark. Root systems of the tomato plants will be partially or totally reddish-brown upon digging them up. Eventually the tomato plant will die from the spread of the fungus and if not controlled, the fungus will quickly spread to nearby tomato plants in the garden.
How does it manifest?
Fungi of the fusarium oxysporum species penetrate tomato plant roots and spread up into the stems and leaves through the water conducting vessels in the tomato plant. As infection spreads, the plant vessels become plugged and damaged so that the waterfeeding system to the tomato plants becomes blocked. This is the process which causes the wilt to occur. Fusarium wilt in tomato plants favors temperatures between 70° and 80°, especially favoring weather that is wet which allows it to spread more quickly throughout the nearby tomato plants in the garden. The fusarium wilt disease is also spread by contact and transportation through contaminated seeds, soil, plants and equipment and as such it often enters a new plant through the roots when transplanting. The fungi of fusarium wilt live on organic matter in the soil and so can be spread by just about anything that comes in contact with any infected region of the garden. Once fusarium wilt infects a tomato plant, it is likely that the infected tomato plants may not produce adequate fruit. Typically, when infection is present the tomato plants either do not produce fruit at all, or they produce fruit which is deformed or has little to no taste at all. Typically, as the vessels become more plugged and less water and nutrients are available to the tomato plant, entire plants may die off.
What can you do about it?
Any plants, which show signs of infection, must be destroyed immediately. Unfortunately, no chemical control is available to treat the infection of fusarium oxysporum. Your best bet once plants become infected is to plant new tomato plants the following year in soil that has not been infected and where tomato plants have not been previously planted for a term of 2 to 4 years. If you choose to plant in infected soil, first fumigate the soil with meta-sodium two to three weeks prior to planting which is usually an effective measure for removing fusarium fungus from the soil. Alternately you may want to consider planting fusarium wilt resistant tomato plants in the garden, and avoid the problem entirely. Tomato plant varieties which are resistant to fusarium wilt are commonly marked with a capitol F after the name of the tomato variety on the seed packet or transplant.
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