A foliar disease called leaf blister has been common on silver and red maples and their hybrids during the past few weeks. The causal agent is the fungus Taphrina carveri. The disease may go unnoticed until a large number of leaves are severely infected and begin to fall from the tree. Symptoms include grayish brown-to-black irregularly-shaped, slightly-raised but not always obvious blisters on the leaves. The leaf blisters turn black, often lighter-colored in the center. (Fig. 1) Multiple infections will cause the leaf to become distorted. (Fig. 2) The rounded shape of the spots and blistering distinguish this disease from maple anthracnose, which produces irregularly shaped brown spots or blotches that follow the veins of leaves and is caused by a different fungus. Leaf blister and anthracnose can occur on the same tree and even on the same leaf.
Taprhina spp. overwinters on bud scales. Infection occurs on developing leaves early in the spring when buds open if environmental conditions are favorable for the disease. Optimal conditions for this pathogen are over 12.5 hours of leaf wetness and temperatures below 61 degrees F. Severe infection, along with defoliation, may occur if these weather patterns persist. Less severely affected leaves may remain on the tree all season. If defoliation does occur, the lost leaves will almost always be replaced by new ones within several weeks to a month. The high-risk period for infection is when leaves are very young; later in the season, leaves become resistant to the fungi, so there are no repeat cycles of infection in the same season.
Leaf blister is an occasional problem in Iowa and elsewhere in the Upper Midwest (see University of Minnesota Yard and Garden Line News, July 1, 2002