|What causes Tomato leaves to get brown ?|
|Sometimes it's just old age. If only the lower leaves are turning brown, chalk it up to the fact that the plant is putting its energy into developing new leaves and fruit at the expense of the oldest leaves. It's nothing to worry about.
If, however, all of the leaves, old and new, are turning brown, it could indicate a disease. The most common is 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. If this is what your tomato plants have, your efforts shouldl 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.
Best wishes with your tomatoes!