Thesis

Factors Influencing Phytophthora ramorum Infectivity on Umbellularia californica and Testing of a Defoliation-Based Control Method

The primary foliar host for Phytophthora ramorum is Umbellularia californica (bay laurel), a tree species that serves as a reservoir for infections in California woodlands. I investigated environmental and pathogen-mediated influences on the incidence and severity of P. ramorum infection of U. californica as well as developing non-destructive means for controlling P. ramorum in woodlands.
 The distribution and abundance of P. ramorum in California is typically assessed by counting symptomatic hosts and confirmed by culturing the pathogen from field-collected samples. I hypothesized that the probability of a successful culture depends on the local environmental conditions where the field samples are collected. In 2010, an extensive culturing study was conducted within a previously established plot network in Sonoma County, where P. ramorum has been studied since 2003. I collected symptomatic leaf tissue for 424 trees in 153 plots randomly distributed within a 275 sq km region. Phytophothora ramorum was successfully cultured from 138 trees (32.5%) and collected from 71 plots (46.4%). Culture success was greatest in the southwest portion of the study area and lowest in the northeast. Culture success was positively related to topographic moisture index and field count of symptomatic leaves at the site and negatively related to average mean temperature at the site. These data show that culture success in the laboratory could be used as an indicator of inoculum load in the field. Studies that rely solely on culture success to determine pathogen presence should use caution in interpreting results, as they may overlook the possibility of false negatives.
 Additionally, I developed a live plant model to assess the validity of the commonly used detached leaf method for predicting interactions that occur between
 P. ramorum and foliar hosts. Specifically, I assessed infectivity of detached leaves and attached leaves from the same U. californica trees in a growth chamber and compared this to infectivity of detached leaves in an incubator. After seven days, lesions were scored. Mean infection score did not differ between detached leaves and attached leaves in the growth chamber. Detached leaves in the growth chamber and detached leaves in the incubator also did not differ significantly. Despite differences in light and humidity between the growth chamber and the incubator, no differences in infection score were found between any of the treatments. These results suggest that the detached leaf assay is a good indicator of infectivity in live trees.
 Finally, I examined controlled defoliation as an alternative to the current practice of managing P. ramorum through the destruction and removal of U. californica trees near symptomatic plants. Twenty-four U. californica seedlings were placed in six exclosures under infected canopies in Fairfield Osborn Preserve in February 2011. Two trees in each exclosure were sprayed in May and July 2011 with Ethephon, which releases ethylene upon decomposition, thereby inducing leaf abscission. Lesions were counted post treatment in January 2012. Leaves on the trees that received treatment developed significantly fewer lesions compared to the control group. Control saplings had 3.5 fold more infections than defoliated saplings. Defoliated saplings did experience excess lateral shooting, a side effect of Ethephon, as well as a dead zone at the crown. Further testing could determine the long-term effects of Ethephon on U. californica saplings and whether this dead zone is permanent or temporary. Ethephon does show promise as an alternate control method for P. ramorum.

The primary foliar host for Phytophthora ramorum is Umbellularia californica (bay laurel), a tree species that serves as a reservoir for infections in California woodlands. I investigated environmental and pathogen-mediated influences on the incidence and severity of P. ramorum infection of U. californica as well as developing non-destructive means for controlling P. ramorum in woodlands. The distribution and abundance of P. ramorum in California is typically assessed by counting symptomatic hosts and confirmed by culturing the pathogen from field-collected samples. I hypothesized that the probability of a successful culture depends on the local environmental conditions where the field samples are collected. In 2010, an extensive culturing study was conducted within a previously established plot network in Sonoma County, where P. ramorum has been studied since 2003. I collected symptomatic leaf tissue for 424 trees in 153 plots randomly distributed within a 275 sq km region. Phytophothora ramorum was successfully cultured from 138 trees (32.5%) and collected from 71 plots (46.4%). Culture success was greatest in the southwest portion of the study area and lowest in the northeast. Culture success was positively related to topographic moisture index and field count of symptomatic leaves at the site and negatively related to average mean temperature at the site. These data show that culture success in the laboratory could be used as an indicator of inoculum load in the field. Studies that rely solely on culture success to determine pathogen presence should use caution in interpreting results, as they may overlook the possibility of false negatives. Additionally, I developed a live plant model to assess the validity of the commonly used detached leaf method for predicting interactions that occur between P. ramorum and foliar hosts. Specifically, I assessed infectivity of detached leaves and attached leaves from the same U. californica trees in a growth chamber and compared this to infectivity of detached leaves in an incubator. After seven days, lesions were scored. Mean infection score did not differ between detached leaves and attached leaves in the growth chamber. Detached leaves in the growth chamber and detached leaves in the incubator also did not differ significantly. Despite differences in light and humidity between the growth chamber and the incubator, no differences in infection score were found between any of the treatments. These results suggest that the detached leaf assay is a good indicator of infectivity in live trees. Finally, I examined controlled defoliation as an alternative to the current practice of managing P. ramorum through the destruction and removal of U. californica trees near symptomatic plants. Twenty-four U. californica seedlings were placed in six exclosures under infected canopies in Fairfield Osborn Preserve in February 2011. Two trees in each exclosure were sprayed in May and July 2011 with Ethephon, which releases ethylene upon decomposition, thereby inducing leaf abscission. Lesions were counted post treatment in January 2012. Leaves on the trees that received treatment developed significantly fewer lesions compared to the control group. Control saplings had 3.5 fold more infections than defoliated saplings. Defoliated saplings did experience excess lateral shooting, a side effect of Ethephon, as well as a dead zone at the crown. Further testing could determine the long-term effects of Ethephon on U. californica saplings and whether this dead zone is permanent or temporary. Ethephon does show promise as an alternate control method for P. ramorum.

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