Raywood ash canker and decline—Botryosphaeria stevensii
Raywood ash (Fraxinus oxycarpa ‘Raywood’) commonly suffers from dieback of multiple branches throughout the canopy. The primary cause is plant stress due to drought (insufficient soil moisture), which allows the weak pathogen Botryosphaeria stevensii to kill infected limbs.
Identification
Dieback of multiple branches throughout the canopy is indicative of Raywood ash canker and decline. On scattered limbs foliage turns yellow and wilts, then becomes brown and dies. Affected limb bark develops reddish brown discolored cankers and dies. Bark then cracks and may slough off portions of the infected limbs.
Entire trees are generally not killed by Raywood ash canker and decline. But where people and property occur, the dead limbs pose a hazard of breaking and dropping. Severely affected ash are often removed because of unsightly dieback, reduced shading, and their potential limb drop hazard due to Raywood ash canker and decline.
Life cycle
Raywood ash is apparently less drought tolerant than previously believed. Stressful conditions at the growing site, and especially moisture deficit, predispose Raywood ash to infection and limb death by B. stevensii. This fungus is a weak pathogen that does not affect vigorous trees but becomes aggressive and pathogenic when trees are stressed, commonly from insufficient soil moisture.
Solutions
Protect trees from injury to their limbs, roots, and trunk. Provide a good growing environment and appropriate cultural care. Provide occasional deep watering during the drought season. Pruning to thin canopies and reduce transpiration (water) demand may improve the performance of Raywood ash. Where trees are a hazard and should be removed, green ash (Fraxinus pennsylvanica) is a similar-looking alternative for planting and appears to not be susceptible to this disease.
Adapted from Investigation into the Etiology of Decline of Raywood Ash in Northern California, UC Davis and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM). |
Brown foliage due to Raywood ash canker and decline.
A reddish brown canker at the base of a limb (right) killed by Raywood ash canker and decline.
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