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Additional key words: Fusicoccum, Phytophthora cinnamomi, Arctostaphylos viscida
We have determined that at least two different diseases are affecting the health of A. myrtifolia and A. viscida in the Ione area. The first is a branch canker disease caused by species of Fusicoccum, which has previously been identified on A. myrtifolia. We isolated two species of Fusicoccum from stem cankers and completed proof of pathogenicity on both A. myrtifolia and A. viscida. Most of the Fusicoccum cankers that we monitored expanded very little between March and October. Progressive dieback associated with Fusicoccum cankers probably contributes to and may sometimes cause the death of A. myrtifolia plants. Mortality associated with Fusicoccum canker tends to be scattered within affected stands.
The second disease, which is newly identified and documented in this report, is a root and crown rot caused by Phytophthora cinnamomi. P. cinnamomi root and crown rot causes large contiguous patches of mortality in stands of A. myrtifolia and A. viscida. Infected plants desiccate rapidly at the onset of hot weather. We isolated P. cinnamomi from symptomatic plants at two disjunct mortality centers and also recovered the pathogen from soil collected from one mortality center. We completed greenhouse tests to demonstrate pathogenicity of the P. cinnamomi isolates to both A. myrtifolia and A. viscida.
Although the two diseases have not been previously distinguished, they differ in symptomatology, etiology, and their potential for impact on A. myrtifolia and other species. P. cinnamomi root and crown rot is by far the more serious disease and has the potential to both eliminate entire A. myrtifolia populations and prevent recolonization of infested sites by A. myrtifolia. Successful conservation of this species will not be possible unless spread of this disease into noninfested stands can be stopped. Hence, a high priority should be placed on delineating the extend of this pathogen within the range of A. myrtifolia and implementing measures to restrict the movement of infested soil and plant materials from areas affected by P. cinnamomi.