Lilly Schinsing Phytophthora ramorum in Tanoak and California Bay 5 9 05 The Effects of Temperature and Inoculum Concentration on Susceptibility to Infection by Phytophthora ramorum in Tanoak and California Bay Laurel Lilly Schinsing Abstract 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 of temperature and zoospore concentration two factors hypothesized to limit infection rates and the extent of disease caused by Phytophthora ramorum in tanoak Lithocarpus densiflora and intermediately resistant California bay laurel Umbellularia californica Three trees of each species were sampled in the fall of 2004 in Marin County CA Leaves were inoculated at concentrations 3 3x10 3 3x102 3 3x103 and 3 3x104 spores ml then incubated at ideal growth conditions 19 C In a separate experiment leaves were inoculated with 3 3x104 spores ml at temperatures 5 C 12 C 19 C 26 C and 33 C To determine how lesion size varied within these conditions lesion area was measured Both experiments yielded leaf infection at all conditions including the lowest concentration levels Increasing zoospore concentration led to a statistically significant increases in infection rates and b increasing mean lesion sizes for both species Infection rates in bay leaves were greatest at intermediate temperatures although unexpectedly relatively high infection rates were found at the highest and lowest temperatures Significant differences in lesion size in bay were found between intermediate temperature groups and those at the extremes Infection rates and lesion size in tanoak suggested an inhibitory effect of heat but not of cold These experiments demonstrate a remarkable ability of P ramorum zoospores to infect leaves even at suboptimal conditions p 1 Lilly Schinsing Phytophthora ramorum in Tanoak and California Bay 5 9 05 Introduction Sudden oak death SOD is a recently discovered epidemic that occurs throughout the coastal regions of California and Oregon from the counties of Monterey to Mendocino in California and is isolated in Curry County in southwest Oregon McPherson et al 2003 Over the past ten years SOD has had a devastating effect on oak and tanoak species in forests along 185 miles of the Central Coast in California Garbelotto et al 2003 and could dramatically impact ecosystem structure and function in the future Kelly and Meentemeyer 2002 Currently SOD is a threat to native oak forests and the ecosystems supported by these forests In addition it is a disease that has serious implications for land management policy M Garbelotto per comm April 2004 It is especially important to study this disease as little is known about how it spreads across the stand and the ecosystem level Given that tanoak coast live oak and black oak trees are distributed along 1 500 miles of the coast of California and Oregon Garbelotto et al 2001 the potential for huge negative ecosystem impact is high Sudden oak death is caused by the pathogen Phytophthora ramorum a new species of Phytophthora in the phylum Oomycota Werres et al 2001 Garbelotto et al 2003 identified via DNA 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 phylum Oomycota Garbelotto et al 2001 that causes two types of diseases in hosts one that lethally infects branches and stems and the other that non lethally infects foliage and twigs Rizzo and Garbelotto 2003 Garbelotto et al 2003 This study examines two possible conditions that affect infection rates and lesion size of disease caused by P ramorum in tanoak Lithocarpus densiflora a tree species that is fatally affected by the pathogen and intermediately resistant California bay laurel Umbellularia californica a non lethally damaged tree host for the pathogen The lethal form of the disease is an infection of the trunk of oaks and tanoaks that creates black 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 parvula var 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 and understory hosts These foliar hosts which are affected by pathogen infection in leaves only are p 2 Lilly Schinsing Phytophthora ramorum in Tanoak and California Bay 5 9 05 thought to be the main source of disease spread of P ramorum which is then spread by wind and rain to susceptible trees and subsequently leads to infection and eventual mortality Kelly and Meentemeyer 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 Garbelotto et al 2003 Over the last few years there have been increasing discoveries of foliar host species associated with the spread of P ramorum a good example is the California bay laurel In a 2002 report 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 and SOD They hypothesized that this connection was due to the creation of favorable microclimates for pathogen infection Swiecki and Bernhardt 2002 later it was found that the California bay is a foliar host Rizzo et al 2002b Currently the known geographic P ramorum spread consists of forest types that have cool and moist climates Rizzo et al 2002a Studies done on inoculum control in P ramorum have shown 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 Bernhardt 2002 Garbelotto et al 2002a Garbelotto et al 2003 Because foliar hosts have been hypothesized to be important in spreading the pathogen to oaks forests that have a high diversity of plant hosts may be more at risk for invasion by P ramorum Rizzo and Garbelotto 2003 In contrast a lower density of favored hosts may result in a more gradual spread of the disease and allow forest managers to identify and protect stands before they are fully destroyed In this light studies need to be conducted that examine the conditions that affect the
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