Growing tomatoes in the home garden can be a productive and rewarding endeavor. However, a gardener may become frustrated if their well-maintained tomato plant begins dying, starting with the bottom leaves and spreading upward. This type of death could be caused by a number of pathogens, most of them fungal. Unfortunately, most tomatoes affected by these fungi will eventually die and should be destroyed to prevent further disease spread.
Early blight, caused by the fungus Alternaria solani, first appears as small, irregular dead spots on older, lower leaves. These spots enlarge and develop concentric rings or yellow halos and as the disease progresses up the main stem the oldest leaves begin to dry and drop off. Stems may also develop lesions near ground level and fruit usually become infected and drop. The alternaria fungus survives on crop debris and spreads to healthy plants best during warm, humid weather. This disease is most serious if tomatoes are already stressed. Avoid this fungal problem by using proper crop rotation practices, avoiding late-day irrigation and planting resistant varieties.
Septoria Leaf Spot
Septoria leaf spot, caused by the Septoria copersici fungus, tends to appear first on lower leaves as small, water-soaked spots that turn brown and enlarge. This fungus, spread by splashing water or physical contact when plants are wet, reduces the number of good leaves on the plant, which negatively impacts fruit quality and yield. Avoid problems with septoria by using proper crop rotation, practicing good sanitation and applying a fungicide like benomyl, chlorothalonil, mancozeb or maneb.
Symptoms of the fungal disease verticillium wilt often become apparent during a period of stress or high fruit production. Lower leaves turn pale, often develop a V-shaped lesion, die and drop off of the plant. Verticillium-infected plants tend to survive through the season but may produce poor-quality fruit. Sometimes verticillium symptoms appear on only one side of the plant. This pathogen prefers soil temperatures between 65 and 80 degrees Fahrenheit. Avoid problems with verticillium wilt by using crop rotation, keeping areas free of weeds, destroying infected plant debris following harvest, avoiding overwatering and planting only resistant varieties.
Fusariam Wilt and Crown Rot
Fusarium wilt or crown rot, caused by the fungus of the same name, first appears as a yellowing of older leaves. Leaves on plants with crown rot generally turn brown or black and wilt while wilt-affected plants have yellow wilted leaves. Improper watering practices can imitate these symptoms. Identify fusarium by checking to see if the roots are rotting and cut open the lower stem to inspect the tomato's vascular tissue, which will be dark brown if the plant is infected. If fusarium is a problem in a garden, do not plant tomatoes, eggplant or potatoes in the area for a few years and only use varieties labeled as resistant to fusarium.
The fungus that causes late blight, Phytophthora infestans, does not survive in soil or on plant debris, so spores are introduced by rain events or on transplants. Late blight first appears on lower leaves, where pale, water-soaked spots appear, enlarge and darken. This fungus prefers cool, moist weather, which allows the growth of a cottony spore mass on the lower leaf surfaces. Avoid this disease by minimizing cool, damp conditions and applying a protective fungicide.