OAK LEAF BLISTER
Prepared by William M. Johnson, Ph.D.
Leaf tissue cells in the diseased spots multiply more than surrounding cells and result in a raised blister-like buckling of the leaf.
As the spots age, their upper surfaces become covered with a buff-white coating of fungal growth that later turns brown.
The leaves remain on the tree and do not fall prematurely.
PLANT HEALTH MANAGEMENT OPTIONS:
To Treat or Not Treat? - In most cases, chemical control is generally not necessary for near-mature and mature trees. Fungicide application is generally not necessary because the trees are seldom severely infected and most leaves do not fall prematurely. Although infections may be extensive some years, little harm to plant health actually results. Treating young (up to 8-to-10 years old) trees may be warranted to provide optimal growing conditions. However, all fungicide treatments must be applied as a dormant treatment shortly before bud swell in order to be affective. In other words, fungicide treatments applied after the onset of visible symptoms have little to no effect on disease control.
Fungicide Application - To control oak leaf blister (during next spring's growing season), a fungicide must be applied prior to bud break. Daconil 2787* (ex., Ortho's Multi-Purpose Fungicide) or Dithane* or Mancozeb* are applied late in dormancy (* trade name). All these compounds must be applied as a dormant treatment shortly before bud swell in order to be affective. Once bud break has occurred and symptoms are visible, it is too late to spray.
Sanitation - Rake and properly dispose of infected leaves. Leaves should be bagged for curbside garbage pickup. Do place leaves in the compost pile.
Maintain Healthy Trees - Proper fertilization and watering during low soil moisture conditions help to maintain trees in a healthy, vigorous state thus rendering them more resistant to infection.
SEASON OF OCCURRENCE:
Cool, moist conditions are required for this disease to occur. The fungus only infects young emerging leaf tissue in the spring, entering the stomates (small openings) and growing between the leaf cells. The new spores of the fungus formed on the surface of the spots are blown to new buds and remain dormant there until the next spring. Thus, the fungus has only one infection period in the spring and generally does not continue to cause new spots to form later during the growing season.
This web site is maintained by Master Gardener Laura Bellmore, under the direction of William M. Johnson, Ph.D., County Extension Agent-Horticulture & Master Gardener Program Coordinator.
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