During periods of high humidity, which includes most of the spring, apply a fungicide weekly. Another destructive foliage disease of tomatoes is Septoria leaf spot. In checking plants for this disease, look at the older foliage. The fungus is characterized by circular lesions with gray centers surrounded by dark margins. With age, the spots become covered with tiny black specks from which spores grow.
Lesions are smaller and more numerous than the early blight organism.
The fruit is rarely affected, but stems and blossoms are attacked. The disease overwinters on old tomato vines and wild relatives of the tomato family. Apply a fungicide containing benomyl. Because benomyl is a systemic fungicide which goes into the plant, it lasts longer and does not have to be applied as often. To provide complete fungus protection from Septoria and early blight during spring periods of high humidity, mix benomyl with the weekly chlorothalonil every other week.