To get the week off to a good start our “disease du jour” comes to us courtesy of Russell Harris in Orleans parish. This is oak leaf blister caused by the fungus Taphrina caerulescens. The disease develops on a wide variety of oaks and is seen here on a water oak (Quercus nigra). The blisters typically bulge upward with a depression visible on the lower surface of the leaf. Initially, the blisters may be whitish or light green, but as they age the tissue dies and becomes brown, as seen here. The pathogen attacks the young, expanding leaves when temperatures are mild and there is plenty of moisture to keep the foliage wet. Once the leaves are fully expanded, they become resistant to disease. When disease is severe, it may cause some defoliation, but this does not generally affect the overall health of the tree. Fungicides applied prior to bud break and during early leaf development may aid in the control of this disease, but spraying large trees in residential neighborhoods is generally problematic.
Donald M. Ferrin, Ph.D., Louisiana State University Agricultural Center, Baton Rouge, LA