Tomatoes are popular vegetables to grow in the home garden since they are relatively easy to grow and produce a bountiful harvest. However, just like other plants, they can be subject to parasitic viruses, fungus and pests that make the plant unhealthy. Parasites on tomato plants can cause numerous effects, from yellowing leaves to complete plant death. According to the Colorado State University Extension, problems can be minimized by ensuring that the plants are receiving proper nutrition and adequate water, keeping the leaf debris out of the garden and checking the plants frequently for signs of disease or illness. Destroy plants you think are infected with a virus or fungus to avoid spread to other plants.
Yellowing leaves can be the first sign of trouble with your tomatoes. Unfortunately the reason for the yellowing leaves can be numerous, from lack of nutrients to parasitic infection. Most parasitic infections, however, have more than one symptom. A psyllid infection will cause yellowing leaves, as well as give a tinge of purple to the veins and cause zigzag growth. Curly top virus turns the leaves yellow and stops growth. Small holes in the leaves signify the flea beetle; extensive leaf damage indicates the tomato hornworm.
When a tomato plant stops growing, there are several possibilities as to why. The curly top virus is spread by beet leaf hoppers and causes yellow leaves as well as stunted growth. The cucumber mosaic virus causes stunted growth and mottled leaves. Infection most often is already present in seeds. There is no cure for either virus. Infected plants should be destroyed.
Leaves spotted with black, target-like spots can indicate early blight. The spots will mostly occur on older plants, but if the infection is severe, younger leaves can be infected as well. Spots that are white or gray and surrounded by a brown margin point to septoria leaf spot. It is also a fungal infection. Prevention is the best cure for these fungal parasites. Plants should be checked frequently for signs of disease and all suspect leaves should be removed. Make sure the plants are spaced far enough apart to allow for air circulation.
Fruit With Spots
Spots on the fruit itself can indicate bacterial canker, tomato spotted wilt/impatiens necrotic spot tospoviruses, sun scald or stink bug infestation. Bacterial canker also affects the leaves and stem; the lower leaves turn inward and start to brown. The fruit has small, white "bird's-eye" spots on it. Yellow rings or spots on the tomato are classic symptoms of tomato spotted wilt, also known as impatiens necrotic spot tospoviruses. Sun scald presents as spots on the fruit where the skin in thin and transparent. This usually occurs comorbidly with hornworms or other defoliating parasites. If your plant has dark spots surrounded by light discolored areas, suspect stinkbugs, which damage the tomato when they feed on it.