Answer: Fusarium Wilt is indicated when the lower leaves get brown and die, followed by the upper shoots. Eventually,the whole plant goes. Generally it occurs on one side, then the other. By slicing the stem lengthwise near the soil line you can see some dark brown tissue about an 1/8th of an inch under the surface.
There are many strains of the Fusarium fungus, and each strain is specific to a different type of plant. Fusarium Wilt will stay indefinitely on plant debris or in the soil. It enters the plants through the roots and can be spread by seeds, tools, soil and plants. It spreads via the water-conducting vessels in the plant. When these vessels get clogged it prevents nutrients from getting to the leaves and fruit.
There is no control available. You need to destroy the infected plants. Do NOT put them in your compost bin! The best solution is to rotate the crops (which you appear to be doing already) and use plants that are resistant to fusarium wilt. This will be indicated on the plants when you purchase them.
Another common problem is leaf spot. Moisture on the leaves encourages the growth of fungal diseases that cause leaf spot. Water the soil, not the leaves, and water in the morning, if possible, so that any water splashed on the leaves can evaporate quickly. In addition, a good preventative measure is to spray your plants (this is the only time the leaves should be wetted intentionally) with a compost tea solution. To make it, mix one part mature compost that contains some manure with five parts water. Let the mixture sit in the shade for two weeks, then filter through cheesecloth and spray on plants. You can dilute the solution by half or use it full strength. Spray every two or three weeks. The residue leftover after filtering can be spread around your plants.
Hope this helps you grow healthier tomato plants!
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