Good tomato culture, including removing garden debris and spacing plants to permit air circulation, prevents tomato problems before they begin, notes the Colorado State University Extension. Practices such as using black plastic mulch under plants can prevent disease-causing fungi, for example, from splashing up on plants when it rains and infecting them. When problems do occur, you'll need to undertake a little detective work to identify the cause.
Assess the symptoms of the problem. Note whether the leaves, stems, crown area (where the stem meets the roots), fruit or general health of the plant seem to be the main problem.
Note the leaf color. Yellow lower leaves bearing black or dark brown spots indicate early blight. Spots on yellow leaves with white or gray centers surrounded by dark margins indicate septoria leaf spot. Purple leaves indicate either phosphorus deficiency due to early cool temperatures or curly top virus if the veins are purplish. Wilted leaves can indicate fusarium wilt.
Study the leaf shape. Curled or cupped leaves indicate injury from a recent application of herbicide. Distorted leaves on a stunted plant indicate cucumber mosaic virus.
Examine green fruits for signs of insect damage. Dark dots surrounded by a light, discolored area indicate stink bug feeding.
Examine ripe fruits for flattened and black blossom ends, indicating blossom end rot. Ring spots on the fruit are a sign of tomato spotted wilt virus. White leathery areas indicate sun scald. Distorted fruit with yellow or dying areas are said to be cat faced.
Check the stems for black or dark cankers that can indicate early blight.
Examine the roots for discoloration or mushiness, indicating fungus-caused root rot.
Scrutinize the plant for general decline. While this can indicate any number of problems, the tomato may be suffering from bacterial wilt or a nutritional disorder, including a lack of nitrogen or phosphorus.