Signs of tomato plant diseases include halted growth, yellowing leaves, premature fruit ripening, dark spots on leaves, leaf curling and white scabby lesions on tomatoes. Symptoms vary according to the type of infection present. Additional evidence can present itself as sticky excrement on leaves, brown or black leaves, discolored roots and yellow rings or spots on tomatoes.
Some diseases, such as curly top virus, are spread by parasitic bugs. In other cases, pests may not be the direct disease carrier, but they can leave plants open to infection when they eat and damage the plant's tissues. These bugs include beet leafhoppers, psyllids, flea beetles, whiteflies, aphids and tomato or tobacco hornworms. Diseases such as early blight, Fusarium wilt, Septoria leaf spot, Fusarium crown rot and root rot are caused by fungal infections. Cucumber mosaic virus, tomato spotted wilt virus and impatiens necrotic spot virus are common viral infections that affect the roots, leaves and fruits of tomato plants. Cankers on plant stems and vascular discoloration can be caused by a bacterial infection. Sometimes, the cause of plant decay is not caused by an infection, but by competition for resources from weeds, poor air circulation, phosphorus deficiencies, high nitrogen levels, inadequate sunlight and substandard watering practices.