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Phytophthora ramorum
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- Resource Type:
- Text
- Identifier:
- CATI Publication #100701
- Campus Tesim:
- Fresno
- Creator:
- Kuljian, Howard Gregory
- Description:
- The introduction of non-native pathogens can have profound effects on forest ecosystems resulting in tree mortality, changes in species composition, and altered fuel structure. The 1990’s introduction of Phytophthora ramorum, the pathogen recognized as causing the tree and plant disease known as sudden oak death, has caused rapid decline and mortality of tanoak (Lithocarpus densiflorus) in forests of coastal California, USA. To understand the potential effect that mortality could have on fuel structure and fire behavior, foliar moisture content of uninfected tanoaks, sudden oak death-infected tanoaks, sudden oak death-killed (dead) tanoaks, and surface litter was tracked for 12 months. Foliar moisture content of uninfected tanoaks averaged 82.3% for the year whereas foliar moisture content of infected tanoaks had a lower average of 77.8%. Dead tanoaks had significantly lower foliar moisture content than uninfected and infected trees, averaging 12.3% for the year. During fire season (June through September), dead tanoak foliar moisture content reached a low of 5.8%, with no significant difference between dead canopy fuels and surface litter. Remote automated weather station (RAWS) 10-hour timelag fuel moisture data corresponded to foliar moisture content of dead leaves, holding promise as a predictor of seasonal crown fire hazard. Decision support tools, based on Van Wagner’s (1977) crown ignition equation, can predict canopy base height values to escape crown ignition, however the Van Wagner equation was developed for conifers, not broadleaf trees (such as tanoak). No empirical data exist to corroborate ignition thresholds for extremely low foliar moistures found in dead foliage. To quantify crown base height ignition thresholds, a laboratory experiment was employed to measure foliar ignition and consumption at crown base heights from 0.5 m to 1.5 m across the range of foliar moistures found in healthy, sudden oak death-infected, and sudden oak death-killed tanoaks. Results from laboratory burning showed all foliage was consumed at the lowest simulated crown base heights in the laboratory, however consumption of live foliage dropped off quickly with increasing crown base height, with minimal consumption occurring at 1 m and above. Consumption of dead foliage declined with increasing simulated crown base heights, with some consumption still occurring at the highest crown base height tested (1.5 m). Using logistic regression, variables of crown base height, temperature, and duration of temperatures above 320 ºC or 410 ºC were used to predict crown ignition probabilities for all foliar moisture treatments tested (80%, 70%, 9%, and 5% foliar moisture content). Crown base height performed well as a predictor of crown ignition with correct predictions 87% to 91% of the time. Minimizing the probability of live tanoak foliage ignition results in a crown base ignition threshold lower than the Van Wagner model, while the dead tanoak foliage ignition threshold is considerably lower than an extrapolated Van Wagner equation. This suggests that tanoak will resist crown ignition at a lower threshold than conifers across the crown base heights tested. Results from this study will help refine the decision support tools for fire managers in sudden oak death-affected areas as well as serve as a model for other forests where diseases and insect epidemics have altered forest crown fuels.
- Resource Type:
- Masters Thesis
- Campus Tesim:
- Humboldt
- Department:
- Forestry
- Creator:
- Deshais, Janelle
- Description:
- Phytophthora ramorum, an aggressive introduced plant pathogen, has caused the death of several million tanoak (Lithocarpus densiflorus) and oak (Quercus spp.) trees throughout coastal forests of California and Oregon. In the United States, P. ramorum infections occur in 14 contiguous coastal California counties, from Monterey to Humboldt, and in Curry County, Oregon. Currently, P. ramorum has not been detected in the forests of Redwood National and State Parks (RNSP), however, the pathogen occurs in close proximity. An infestation in Curry County, Oregon is 17 km north of RNSP’s Jedediah Smith State Park, and a recent detection near Redwood Valley (Humboldt County, California) is less than 8 km southeast of the Parks’ southern border. This closeness is concerning because much of RNSP is vulnerable to P. ramorum infestation. Using Maxent, a niche-based modeling program, I determined that 60% of RNSP has the presence of at least one key pathogen-transmission species: California bay (Umbellularia californica), tanoak, and Pacific rhododendron (Rhododendron macrophyllum). Tanoak, which is also highly susceptible to P. ramorum-induced mortality, occurred in 47% of RNSP. All species’ models showed sufficient sensitivity and accuracy to create reliable distribution maps at 30 m x 30 m resolution. These data will be used in future efforts to predict potential P. ramorum disease spread throughout the Parks.
- Resource Type:
- Masters Thesis
- Campus Tesim:
- Humboldt
- Department:
- Forestry
- Creator:
- Rank, Nathan E., Meentemeyer, R. K., Rizzo, D. M., Cushman, J. Hall, and Anaker, B. L.
- Description:
- Human-caused changes in land use and land cover have dramatically altered ecosystems worldwide and may facilitate the spread of infectious diseases. To address this issue, we examined the influence of land-cover changes between 1942 and 2000 on the establishment of an invasive pathogen, Phytophthora ramorum, which causes the forest disease known as Sudden Oak Death. We assessed effects of land-cover change, forest structure, and understory microclimate on measures of inoculum load and disease prevalence in 102 15315 m plots within a 275-km2 region in northern California. Within a 150 m radius area around each plot, we mapped types of land cover (oak woodland, chaparral, grassland, vineyard, and development) in 1942 and 2000 using detailed aerial photos. During this 58-year period, oak woodlands significantly increased in area by 25%, while grassland and chaparral decreased by 34% and 51%, respectively. Analysis of covariance revealed that vegetation type in 1942 and woodland expansion were significant predictors of pathogen inoculum load in bay laurel (Umbellularia californica), the primary inoculum-producing host for P. ramorum in mixed evergreen forests. Path analysis showed that woodland expansion resulted in larger forests with higher densities of the primary host trees (U. californica, Quercus agrifolia, Q. kelloggii) and cooler understory temperatures. Together, the positive effects of woodland size and negative effects of understory temperature explained significant variation in inoculum load and disease prevalence in bay laurel; host stem density had additional positive effects on inoculum load. We conclude that enlargement of woodlands and closure of canopy gaps, likely due largely to years of fire suppression, facilitated establishment of P. ramorum by increasing the area occupied by inoculum-production foliar hosts and enhancing forest microclimate conditions. Epidemiological studies that incorporate land-use change are rare but may increase understanding of disease dynamics and improve our ability to manage invasive forest pathogens.
- Resource Type:
- Article
- Campus Tesim:
- Sonoma
- Creator:
- Colijn, Christine Windsor
- Description:
- 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.
- Resource Type:
- Thesis
- Campus Tesim:
- Sonoma