Lilly Schinsing Phytophthora ramorum in Tanoak and California Bay 5/9/05p. 1The Effects of Temperature and Inoculum Concentration on Susceptibility toInfection by Phytophthora ramorum in Tanoak and California Bay LaurelLilly SchinsingAbstract Despite widespread concern about risk factors associated with Sudden Oak Death,little is known about the conditions that limit infection. This study examines the effect oftemperature and zoospore concentration, two factors hypothesized to limit infection rates and theextent of disease caused by Phytophthora ramorum in tanoak (Lithocarpus densiflora) andintermediately resistant California bay laurel (Umbellularia californica). Three trees of eachspecies were sampled in the fall of 2004 in Marin County, CA. Leaves were inoculated atconcentrations 3.3x10, 3.3x102, 3.3x103 and 3.3x104 spores/ml, then incubated at ideal growthconditions (19 °C). In a separate experiment leaves were inoculated with 3.3x104 spores/ml attemperatures 5 °C, 12 °C, 19 °C, 26 °C and 33 °C. To determine how lesion size varied withinthese conditions, lesion area was measured. Both experiments yielded leaf infection at allconditions, including the lowest concentration levels. Increasing zoospore concentration led to(a) statistically significant increases in infection rates and (b) increasing mean lesion sizes forboth species. Infection rates in bay leaves were greatest at intermediate temperatures, althoughunexpectedly, relatively high infection rates were found at the highest and lowest temperatures.Significant differences in lesion size in bay were found between intermediate temperature groupsand those at the extremes. Infection rates and lesion size in tanoak suggested an inhibitory effectof heat, but not of cold. These experiments demonstrate a remarkable ability of P. ramorumzoospores to infect leaves, even at suboptimal conditions.Lilly Schinsing Phytophthora ramorum in Tanoak and California Bay 5/9/05p. 2IntroductionSudden oak death (SOD) is a recently discovered epidemic that occurs throughout the coastalregions of California and Oregon, from the counties of Monterey to Mendocino in California andis isolated in Curry County in southwest Oregon (McPherson et al. 2003). Over the past ten yearsSOD has had a devastating effect on oak and tanoak species in forests along 185 miles of theCentral Coast in California (Garbelotto et al. 2003) and could dramatically impact ecosystemstructure and function in the future (Kelly and Meentemeyer 2002). Currently, SOD is a threat tonative oak forests and the ecosystems supported by these forests. In addition, it is a disease thathas serious implications for land management policy. (M. Garbelotto, per. comm. April, 2004) Itis especially important to study this disease as little is known about how it spreads across thestand and the ecosystem level. Given that tanoak, coast live oak, and black oak trees aredistributed along 1,500 miles of the coast of California and Oregon (Garbelotto et al. 2001), thepotential for huge negative ecosystem impact is high.Sudden oak death is caused by the pathogen Phytophthora ramorum, a new species ofPhytophthora in the phylum Oomycota (Werres et al. 2001, Garbelotto et al. 2003) identified viaDNA-based phylogenetic analysis in 2002 (Garbelotto et al. 2002a, Rizzo et al. 2002a).Although P. ramorum looks and behaves like a fungus it is actually a water mold, phylumOomycota (Garbelotto et al. 2001), that causes two types of diseases in hosts, one that lethallyinfects branches and stems, and the other that non-lethally infects foliage and twigs (Rizzo andGarbelotto 2003, Garbelotto et al. 2003). This study examines two possible conditions that affectinfection rates and lesion size of disease caused by P. ramorum in tanoak (Lithocarpusdensiflora), a tree species that is fatally affected by the pathogen and intermediately resistantCalifornia bay laurel (Umbellularia californica), a non-lethally damaged tree host for thepathogen.The lethal form of the disease is an infection of the trunk of oaks and tanoaks that createsblack bordered sap-seeping cankers (Rizzo et al. 2002a). P. ramorum kills tanoak, coast live oak(Quercus agrifolia), California black oak (Quercus kellogii), and Shreve’s oak (Quercus parvulavar shervei) in the greatest numbers (Rizzo et al. 2002). The disease girdles the infected tree,effectively cutting off its’ vascular transport mechanism for water supply (Rizzo et al. 2002a). P.ramorum non-lethally colonizes the foliage of an ever increasing number of overstory andunderstory hosts. These foliar hosts, which are affected by pathogen infection in leaves only, areLilly Schinsing Phytophthora ramorum in Tanoak and California Bay 5/9/05p. 3thought to be the main source of disease spread of P. ramorum, which is then spread by wind andrain to susceptible trees and subsequently leads to infection and eventual mortality (Kelly andMeentemeyer 2002). Foliar hosts have been hypothesized to play a key role in the spread of P.ramorum, and could be solely responsible for triggering the current forest epidemics (Garbelottoet al. 2003).Over the last few years there have been increasing discoveries of foliar host speciesassociated with the spread of P. ramorum; a good example is the California bay laurel. In a 2002report of stem water potential as a risk factor for P. ramorum infection, Swiecki and Bernhardt(2002) found that there was a positive association between the density of California bay trees andSOD. They hypothesized that this connection was due to the creation of favorable microclimatesfor pathogen infection (Swiecki and Bernhardt 2002); later, it was found that the California bayis a foliar host (Rizzo et al. 2002b).Currently, the known geographic P. ramorum spread consists of forest types that have cooland moist climates (Rizzo et al. 2002a). Studies done on inoculum control in P. ramorum haveshown that the pathogen is sensitive to high temperatures (higher than 35 °C) (Garbelotto et al.2001), and is therefore thought to favor wet and cool environments (Swiecki and Bernhardt2002, Garbelotto et al. 2002a, Garbelotto et al. 2003). Because foliar hosts have beenhypothesized to be important in spreading the pathogen to oaks, forests that have a high diversityof plant hosts may be more at risk for invasion by P. ramorum (Rizzo and Garbelotto 2003). Incontrast, a lower
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