By Mark Gleason and Fanny Iriarte
Plant Pathology Extension
Iowa State University
Blisters are something you get from applying tender hands or feet to tough jobs in the garden. But can trees get blisters too?
The answer is yes. Leaf blisters are a common springtime curiosity on oaks and maples in Iowa. But 2009 has been a leaf blister bonanza, with far more blisters than I’ve seen in my 24 years at ISU. Prolonged periods of cool, rainy spring weather set the stage.
The culprits are fungi in the genus Taphrina. One species has caused spectacular symptoms on silver and red maples over the past month or so. Grayish brown, sometimes rounded spots often cluster around the main veins of leaves. Heavily spotted leaves can become twisted and drop off the tree, which is when the blister problem may become obvious. The blister effect may not always be clear on these spots, but the spots are sometimes raised or curved.
The rounded shape and blister-like deformation distinguish this disease from anthracnose, another fungal disease of maple that produces large, irregularly shaped brown leaf spots in the springtime. Both leaf blister and anthracnose can occur on the same tree, and even on the same leaf.
Oak leaf blister is caused by another species of Taphrina. It shows up most often on the red oak group, which includes red, black, and pin oaks. The spots look different than on maple: more like real blisters. They are circular in shape, up to half an inch across and more clearly raised than on maple. The blisters shift in color as they age from yellowish to reddish brown to dull brown. When blisters merge together, the leaf often curls up.
The leaf blister fungi spend the winter in the scales of the leaf buds, so they are ready to rumble as soon as the leaves peek out in the spring. When you have spring weather as wet as in 2009 (and in 2008 for that matter), it’s a perfect setup for a leaf blister outbreak.
Despite their unpleasant appearance, the leaf blisters are not a threat to the health of trees. There are no repeated cycles of infection during summer rains, and the trees pretty much shrug off the annoyance of some blistered leaves. In fact, fallen leaves are usually replaced by trees within a few weeks. Even though 2009 was a banner year for blister spot diseases, they are fairly rare in so-called normal springtime weather.
The bottom line for management is that there almost never a need to do anything. The leaf blister diseases can run their course without chemical warfare or other homeowner responses.
If you have a beloved specimen tree on which you refuse to allow a bit of cosmetic damage, you can apply a single fungicide spray in the spring, just before bud break, to prevent the spots. A number of products containing the fungicide chlorothalonil will be effective and are available in most garden centers. But if you are willing to tolerate an interesting natural oddity – plant blisters – you don’t need to fight it with band-aids or fungicides. In fact, you might start to enjoy it!