Space plants farther apart to improve air circulation. Avoid overhead irrigation. If the infestation is heavy, sulfur dust may help protect new leaves from infection. Septoria leaf spot is less common in Colorado than early blight. It, too, is a fungal disease.
Characteristic symptoms are white or gray spots on leaves, surrounded by a black or brown margin.
Control is similar to early blight. Leaves appear shiny and are somewhat sticky when honeydew is present. Damage usually is minimal on tomatoes and often can be ignored. If aphids become a problem, some applications of insecticidal soap are quite effective. Cucumber mosaic virus and herbicide injury from 2,4-D are almost impossible to tell apart without previous knowledge of chemicals applied or laboratory confirmation.