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Project description

Spatial distribution and host associations of Phytophthora ramorum, causal agent of sudden oak death in California. (01XN021)
Program Exotic Pests and Diseases Research Program
Principal
investigators
D.M. Rizzo, Plant Pathology, UC Davis
M. Garbelotto, Environmental Science, Policy and Management, UC Berkeley
Host/habitat Oak; Bay Laurel; Madrone; Tanoak
Pest Sudden Oak Death Phytophthora ramorum
Discipline Plant Pathology
Review
panel
Natural Systems
Start year (duration)  2001 (Three Years)
Objectives Determine the fine scale spatial distribution of Phytophthora ramorum in two forest types.

Determine spread of P. ramorum over a two-year period within permanent plots.

Determine the incidence of P. ramorum in open grown trees.

Final report At total of 360 field plots were established in the mixed-evergreen and redwood-tanoak forest types of the coastal ranges of California to monitor the distribution and spread of Phytophthora ramorum, the cause of sudden oak death. More than 9,000 trees and shrubs, encompassing more than 30 plant species, have been examined on these plots to date. Plants are examined in the field for symptoms of infection by P. ramorum; infection is confirmed by laboratory analysis using culturing and molecular diagnostics. On the two of the three mixed-evergreen forest sites, it appears that about 20% of all coast live oak trees were infected by the pathogen on the sites. A much higher percentage (70-80%) of all bay laurel trees in these forests are showing symptoms of infection. On the redwood-tanoak sites, 12 to 23% of tanoak trees and up to 70% of bay laurel trees were infected by the pathogen. P. ramorum, is consistently recovered from soil at all locations and spores have been detected in rainwater at all sites, with greater numbers found in redwood-tanoak forests. Spores produced on foliar hosts, such as bay laurel, serve as the main source of the pathogen to infect oaks and tanoak. Spores of P. ramorum are also produced on tanoak, but in lower numbers than bay laurel. The pathogen does not appear to produce spores on oaks; therefore, this host may be considered an ecological dead end. Overall, infections in tanoak forests increased up to 70% between 2002 and 2003. Disease increases over the two-year period were less in oak forests, but at least one site had an additional 7% of oaks became infected. A number of new hosts for P. ramorum were discovered during this research including hazelnut, wood rose, and starflower. Finally, a range of susceptibilities to infection was noted for most hosts, but especially fir oak and bay laurel.

Second-year
progress
Field plots have been established in the mixed-evergreen and redwood-tanoak forest types of the coastal ranges of California to monitor the distribution and spread of Phytophthora ramorum, cause of sudden oak death. Over 7000 trees and shrubs, encompassing 23 plant species, have been mapped on these plots to date. Plants are examined in the field for symptoms of infection by P. ramorum; infection is confirmed by laboratory analysis. On the two of the three mixed-evergreen forest sites, it appears that about 20% of all coast live oak trees are infected by the pathogen on the sites. A much higher percentage (70-80%) of all bay laurel trees in these forests are showing symptoms of infection. On the redwood-tanoak sites, 12 to 23% of all tanoak trees are infected by the pathogen. P. ramorum is consistently recovered from soil all locations and spores have been detected in rainwater at all sites, with greater numbers found in redwood-tanoak forests. It appears that spores produced on foliar hosts, such as bay laurel, serve as the main source of spores to infect oaks and tanoak.

First-year
progress
Field plots have been established in the mixed-evergreen and redwood/tanoak forest types of the coastal ranges of California to monitor the distribution and spread of Phytophthora ramorum, cause of sudden oak death. Over 4000 trees and shrubs, encompassing 18 species, have been mapped on these plots to date. Plants are examined in the field for symptoms of infection by P. ramorum; infection is confirmed by laboratory analysis. On the two mixed-evergreen forest sites, it appears that about 20% of all coast live oak trees are infected by the pathogen on the sites. A much higher percentage (70-80%) of all bay laurel trees in these forests are showing symptoms of infection. Infection data is still being compiled for the redwood/tanoak sites. P. ramorum is consistently recovered from soil in all locations and spores have been detected in rainwater at all sites. The main hypothesis we are testing is that spores produced on foliar hosts, such as bay laurel, serve as the main source of spores to infect oaks and tanoak.

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