Answer: Vegetable plants grow best in full sunshine, with ample room between plants for good air circulation. Keeping water off the leaves will also help your plants avoid diseases. You might also want to rotate your vegetable crops so soil borne pathogens don't compromise your crops.
Tomatoes are subject to a number of problems, but it sounds as though your plants are suffering from fusarium wilt. This is a soil borne disease and can be imported with transplants and can be inadvertently spread through the garden on dirt-encrusted tools. Unfortunately there is no cure, and so your efforts now will be aimed at avoiding it in future years. First off, you should probably dig up and dispose of the infected plants. (Before you do this, you might wish to take a sample to your County Extension for a positive diagnosis of the problem) From now on, always buy strongly disease resistant varieties, which will be marked twice with a letter code of "F", usually along with other capital letters such as "VFFNT" indicating resistance to verticillium, two strains of fusarium, and tobacco mosaic. (You will find this on the seed packet or on the plant label.) This should solve the fusarium problem. Since tomatoes are prone to diseases, many gardeners do rotate them through the garden so that there is at least a break of three years between tomato (and related) crops in any given patch of ground. Sanitation is another important step toward avoiding carry over of diseases; this means cleaning up, removing and destroying (do not add to your compost) all residue from infected plants both during the growing season and in the fall. Some gardeners have reported success with solarizing their soil for five weeks under a clear plastic sheet as a method of killing fusarium, however given the availability of plants with extreme resistance to the disease I think that using them is the better method of coping.
Hope this information helps you grow a healthier garden this summer!
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