Raywood Ash Die-back

Raywood ash (Fraxinus oxycarpa) is a popular shade tree in Northern California. It is has an attractive appearance, and is resistant to most insect and disease problems affecting other varieties of ash grown in the state. However, during the past decade it has become increasingly apparent that Raywood ash is prone to a dieback problem of unknown etiology. The manifestation of this disease can range from a minor cosmetic problem to a loss of major scaffold branches and eventual death or removal of the tree.

In 2002, we initiated a study of this problem in order to determine the extent to which trees in Northern California were affected and to better understand its cause. Surveys revealed the problem to be widespread, with diseased trees being common in warm inland locations in the central valley and also in cooler areas near the coast. Although the cause is yet to be determined, Botryosphaeria stevensii has been identified on most affected branches and experiments are in progress to determine if it is capable of causing the die-back in otherwise healthy trees.