On Crops: Tomatoes
Worldwide in warm humid climates
In midsummer when temperatures hover in and 80F (27C) range and it rains for more than two days, plants infected with septoria leaf spot develop hundreds of small dark spots all over tomato leaves, especially near leaf tips. Leaves that have very numerous spots often turn yellow between the spots, then wither to brown.
As tomatoes lose leaf area to septoria and other leaf spot diseases, the plants are weakened and may not make much new growth. If the plants are holding fruit, it may be smaller than usual and lack flavor.
A few tomato varieties are tolerant of this disease, but none are truly resistant. Stake or cage tomatoes to help keep the foliage dry, and use mulches to keep water from splashing onto foliage.
When the foliage is dry, clip off spotted leaves and compost them in an active compost pile. Prune away low leaves to help air circulate more freely through the plants. Fruit from infected plants is edible and may be good when fully ripe.