Tomato plants in many parts of the North America can suffer from this fungal disease, which can appear at any time. Symptoms first appear on oldest, lower leaves. Gray-brown areas have gray centers and a darker border. The dark border lacks the rings typical of early blight. The centers of the discolored areas may have many small, black spores, and sometimes a yellowish area surrounds the darkened areas. It usually slowly defoliates plants, but a harvest is still possible.
To prevent septoria infection, use copper fungicides such as Bordeaux mixture or liquid copper as directed by the product label, or spray aerated compost tea every 7 to 10 days. Grow ″potato-leaved″ (leaves that have no indentations on the borders) and rugose-leaved tomato varieties (with a puckered leaf surface), which show better resistance.
Photo courtesy Thomas Zitter, Cornell University