This is caused by a fungus that infects plants through the root system, then spreads upward through the conduction system of the plant. The subsequent blockage of the water-conducting system then results in leaf wilting and scorching, branch dieback and often, plant death. Common factors for verticillium wilt development are moisture stress, transplant shock on younger trees, salt damage, and genetic susceptibility of certain plant species to the disease. Entomologist Dan Herms of Ohio State University points out that one of the key factors in the extent of damage from a season of defoliation by gypsy moths is how stressed the tree was prior to the defoliation. Oaks are among gypsy moth's favorite foods and studies show that the ability of an oak tree to survive a year or two years of total leaf loss depends upon how healthy it was before the infestation.
Drought stress can play a big role in that pre-infestation health.
Entomophaga fungus, which thrives on late spring moisture, is one of the main natural controls of gypsy moth caterpillars. While 1999 was a good year for the oaks, it was a bad year for this fungus, thus potentially a good year for survival of gypsy moths. Decline of woody plants is complex, and the history, including the drought of 1999, should be factored into what we observe in 2000 and subsequent years. This spring and summer, when trees show leaf scorch and branch dieback, do not jump to conclusions and blame the last person who visited your landscape, be it the lawn-care company or the window washer. The cause of tree decline has a much longer pedigree.