The tomato plant is a warm-season perennial that exhibits hardy growth under ideal growing conditions. Tomato plants are, however, susceptible to a few serious diseases during the growing season. Brown spots on tomato plant leaves are a symptom of bacterial and fungal plant diseases. Understanding the diseases that cause brown spots on tomato leaves and how to control them is important for reducing damage to tomato plants.
Early blight is a common disease of tomato plants, caused by the fungus Alternaria solani. The disease favours weakened tomato plants, with infection occurring at any point during the growing season. Symptoms of infection consist of irregular brown spots on leaves surrounded by yellow halos, defoliation, sunken spots on stems and stem death. Brown spots on leaves measure 1/2 inch in diameter, reports the Virginia Cooperative Extension.
Septoria Leaf Spot
Septoria leaf spot is a fungal disease of tomatoes, caused by the fungus Septoria lycopersici. Septoria fungi favour temperatures between 15.6 to 26.7 degrees Celsius and heavy rainfall. Symptoms of infection consist of small, dark-brown spots on leaves surrounded by yellow halos, visible fungi on the leaf spots and leaf drop. Septoria leaf spot differentiates itself from early blight due to the size of the brown spots, measuring 1/16 to 1/4 inch in diameter.
Bacterial Leaf Spot
Bacterial leaf spot of tomato, caused by the bacterium Xanthomonas campestris, is a serious leaf spot disease. It spreads through water movement from an infected plant to another and from infected seedlings. Symptoms of infection consist of angular, brown spots on leaves, leaf death or necrosis at the source of infection, and brown to black spots on tomato fruit. The spot sizes are similar to Septoria leaf spot, but there are no fruiting fungi on the leaf spots. University-based and private labs can test infected leaves to determine if the brown spots are the cause of bacterial or Septoria leaf spot.
Provide good air circulation to dry moisture on tomato plant leaves and remove host weeds, such as black nightshade and Jerusalem cherry to prevent early blight. Preventive fungicidal control is an effective method for controlling early blight. Preventive fungicidal spray and the control of surrounding weeds are effective methods for controlling Septoria leaf spot. Fungicidal spray is an effective method of control for severe infections. The North Carolina State University Plant Pathology Extension recommends copper spray for control and prevention of bacterial leaf spot. Avoid splashing water from infected plants to reduce bacterial leaf spot infections.