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Oak Leaf Blister

Oak Leaf Blister, Bur Oak

Oak Leaf Blister

Oak leaf blister - (Taphrina caerulescens)

 Host(s): Bur and other oaks

Description/Biology: Fungal spores infect new leaves as buds begin to open in spring. Following infection, the fungus grows intercellular within the leaf tissue, causing leaf deformities termed blisters to form. Under favorable conditions, these blisters may continue to expand as new leaf tissue is invaded by the fungus. New leaves that are formed in mid-summer may also be infected, causing a second cycle of this disease. The spore-bearing structures of this fungus are produced on the lower surface of blisters and can only be seen with a microscope. The fungus overwinters on bud scales and in bark crevices.

Damage/Symptoms: This disease often goes unnoticed except during occasional moist years or localities that favor heavy infections. Leaf blisters appear as wrinkled, distinctly raised bulges on the upper leaf surface and are lighter in color compared to non-infected portions of the leaf. The tissue on the upper surface of the blister may stay green for several weeks before desiccating and turning brown, while the lower leaf surface may appear gray. Typically blisters are 3 to 20 mm across but may coalesce and encompass the entire leaf. Often, severely blistered leaves may curl and fall from the tree prematurely. These disease symptoms are more prominent in the lower, more shaded portions of the crown. Bur oak is a hardy, resilient species and this particular foliar disease rarely causes substantial physical damage. Despite this, oak leaf blister can cause stress to young trees, trees that have been severely infected for several consecutive years, or newly transplanted trees. It can cause aesthetic damage, but this is infrequent, except where the tree’s genotype and local environmental conditions are particularly favorable for the disease.

Comments: Cultural practices such as proper watering and mulching may increase the tree’s vigor and may sufficiently minimize the damage caused by this pathogen. High-value ornamental trees may benefit from chemical control in areas of high disease pressure. Protectant fungicides must be applied prior to and during bud break to prevent infection. Fungicides are not effective once leaves begin to expand because infection has already occurred. Various formulations of chlorothalonil and mancozeb are registered for this disease. Some authors also recommend either lime sulfur at a rate of 10 tablespoons per gallon, or Bordeaux mix. Removal of infected leaves has not been shown to effectively reduce disease development. (Information provided by Joe Zeleznik submitted by Jackie Buckley)



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