Studies on the Effect of Phytophthora Ramorum on Madrone
P. E. Maloney1, S. F. Kane2, C. E. Jensen2, and D. M. Rizzo2
Symptoms and disease progression have not been well characterized for most of the hosts of Phytophthora ramorum. On madrone (Arbutus menziesii), P. ramorum has been confirmed from leaf lesions and stem cankers using molecular, and to a lesser extent, cultural techniques. Difficulty in culturing P. ramorum from lesions, in addition to lack of unique symptomatology, complicate diagnosis of P. ramorum infection on madrone. Presently, molecular PCR-based techniques are the best way to confirm presence of P. ramorum in madrone. To further complicate field diagnosis, madrone may also be infected with Nattrassia mangiferae and Botryosphaeria dothidea. These pathogens overlap in geographic range with P. ramorum. N. mangiferae causes distinct sunken cankers that look like open sores on the branches and trunk. B. dothidea causes dieback of twigs and leaves that could be confused with P. ramorum infection. Generally, neither pathogen causes death of madrone. Given that P. ramorum and B. dothidea co-occur and cause similar symptoms on madrone, laboratory and field studies were established to characterize symptoms of P. ramorum infection as opposed to those of B. dothidea. In artificial inoculation experiments, P. ramorum formed very dark lesions on leaves. On stems, the lesion initially was hardly apparent and appeared as a slight water soaking or darkening of the bark that traveled very quickly throughout the stem and then killed the tissue. In growth chamber experiments, P. ramorum was recovered from stems and leaves 28 days after plants were inoculated and at 42 days P. ramorum was recovered from stems and only 1 in 30 leaf samples. Because detection of P. ramorum in the field is difficult, we placed madrone saplings (approximately 1 m tall) in 10 plots in a redwood/tanoak forest at Jack London State Park to observe disease development over time. When exposed to natural levels of inoculum, five of the ten madrones became infected with P. ramorum within 2 months and 2 more became infected 3 and 6 months later. Both stems and foliage were infected. Five of the saplings died from P. ramorum infection within 6 months. Further laboratory experiments will be conducted to assess the interactions between P. ramorum and B. dothidea. Field experiments will be repeated and expanded during winter 2002-2003 to determine the effects of P. ramorum on madrone populations in redwood/tanoak forests.
1Department of Plant Pathology, University of California, One Shields Ave., Davis, CA 95616; (530) 754-9894; tbntm@telis.net
2Department of Plant Pathology, University of California, One Shields Ave., Davis, CA 95616
|
