These spots may be more reddish colored on some crabapples. Captan is the most commonly used chemical for control. They are also characterized by a target pattern of lines within the spots . Many spots may merge together to cause larger areas of the leaves to die. Like Septoria leaf spot, this disease overwinters on diseased plant debris from last year and is then spread to the lower leaves by splashing rain or irrigation water.
Symptoms are numerous, small, raised, rust-colored spots on the undersides of the leaves .
Thorough cleanup of old plant debris in the fall helps to reduce the disease next season. Fungicide treatment now would be of little value. This is a disease that can kill entire potato and tomato plants in a few days under cool, wet weather. While late blight has occurred in the United States since 1992, the pathogen has become more important in the past 5 years. About that time a new variant of the fungus developed and spread across North America.