Leaf Blister

On maples, oaks, and pears in the Puget Sound region, cool wet springs trigger large, light-green to yellow blisters on leaf surfaces, with a white fungal bloom appearing underneath the leaf. These blisters turn black after a few weeks, resembling necrotic leaf spots rather than the distinctive blisters they actually are. This confusion with other leaf spots, particularly those caused by eriophyid mites, makes positive identification important before assuming management is needed.

Heavy infection impairs tree appearance but poses no serious threat to tree health or survival. Red oak species are particularly susceptible while white oaks rarely develop symptoms even in conducive years. The fungus survives winter under bud scales and germinates on tender new leaves when spring conditions turn cool and wet. Since new leaves become more resistant to infection as they mature, and appearance recovers quickly once the season turns dry, management typically isn't necessary; growing resistant species provides the simplest long-term solution.