Disease Progression of Phytophthora ramorum and Botryosphaeria dothidea on Pacific Madrone.

Infection by Phytophthora ramorum was associated with stem and leaf lesions of Pacific madrone (Arbutus menziesii) seedlings and saplings. In addition, a common and native pathogen, Botryosphaeria dothidea, caused similar leaf and stem lesions. When exposed to natural levels of inoculum in forests infested with P. ramorum, 50 to 66% of madrone saplings used as bait died. Recovery of P. ramorum from colonized plant tissue on culture media was generally low. From initial infection, P. ramorum was not culturable from leaf tissue after a mean of 3.5 weeks or from stem tissue after a mean of 8 weeks. Generally, B. dothidea was recovered more frequently from necrotic stems and leaves than was P. ramorum. Experimental inoculations of madrone seedlings showed that leaf and stem lesion lengths were, on average, greater on tree seedlings inoculated with P. ramorum than on those inoculated with B. dothidea. P. ramorum and B. dothidea appear to coexist in stem and leaf tissue, forming a novel pathogen complex, affecting growth and reproduction of Pacific madrone.