This is Alternaria leaf spot, or early blight. It is a common problem on tomatoes and causes defoliation, usually during periods of high rainfall. Plant tomatoes on a raised bed to improve water drainage. They can be spaced adequately to allow air to move and foliage to dry, and to prevent diseases. Follow a fungicidal spray program beginning when the fruit is set and continuing at one- to two-week intervals during the growing season until harvest.
Contact your local Extension office for a list of the recommended fungicides.
