When a tomato plant's leaves become droopy, dry or discolored, it often points to a problem that will affect the plant's fruit productivity. Yellow leaves in tomato plants can be cause by a variety of fungi, viruses and insects. Some of these issues can be treated, but others will destroy the plant.
Curl- top virus causes tomato plants to turn yellow and eventually stop growing completely. The leaves progress from yellow to purple and curl under, and the fruit ripens prematurely. The virus, which is transmitted by the beet leafhopper, is usually fatal to tomato plants. There is no treatment that can stop the progression of the virus, according to the Colorado State University Extension Service, and standard pesticides rarely prevent the bug from infecting the plant. The best way to prevent curly-top virus is to protect the tomato plants with row covers.
Psyllids are insects that feed on tomato plants. While munching on the plants they inject a toxic saliva into the green areas. The saliva causes yellowing tomato plant leaves, and often makes the veins turn purplish. The stems of the plant will take on a zig-zag appearance. Once the saliva has taken over the tomato plant, it is difficult to save. For best results, dust the leaves with sulfur as soon as you notice symptoms.
Early blight is caused by Alternaria solani, a fungus that is most prevalent during warmer months. The affected leaves will first turn yellow, then drop. This leaves the fruit on the diseased tomato plants susceptible to sunburn. The leaves may also develop brownish-black, target-style spots. The best way to control the tomato plant fungus is to remove all afflicted leaves from the plant and the ground immediately. Dusting the remaining leaves with sulfur powder can help prevent further outbreaks. Because the fungus can overwinter, make sure to plant your tomato seedlings in a different spot the following year.