Maple leaf blister
During June, the leaves of scattered maples in central Minnesota developed blackened patches as if they had been burned. Two fungi-Taphrina dearnessii on red maples and Taphrina sacchari on sugar maples- cause this disease. Infected areas on the leaves produce many extra cells that are larger than normal. As a result of this unusual development, parts of infected leaves become distorted and blackened and resemble shallow blisters. Spores of these fungi are liberated from diseased leaves in the summer and they lodge among bud scales where they lie dormant until new leaves emerge the following spring.