Is variation in susceptibility to Phytophthora ramorum correlated with population genetic structure in coast live oak (Quercus agrifolia)?

Abstract

Summary• California coastal woodlands are suffering severe disease and mortality as a result of infection from Phytophthora ramorum . Quercus agrifolia is one of the major woodland species at risk. This study investigated within-and among-population variation in host susceptibility to inoculation with P. ramorum and compared this with population genetic structure using molecular markers.• Susceptibility was assessed using a branch-cutting inoculation test. Trees were selected from seven natural populations in California. Amplified fragment length polymorphism molecular markers were analysed for all trees used in the trials.• Lesion sizes varied quantitatively among individuals within populations, with up to an eightfold difference. There was little support for population differences in susceptibility. Molecular structure also showed a strong within-population, and weaker among-population, pattern of variation.• Our data suggest that susceptibility of Q. agrifolia to P. ramorum is variable and is under the control of several gene loci. This variation exists within populations, so that less susceptible local genotypes may provide the gene pool for regeneration of woodlands where mortality is high.

Sudden Oak Death: Interactions of the Exotic Oomycete Phytophthora ramorum with Naïve North American Hosts

“…Most of this variability may be explained by age of the infestation, by the amount of bay laurel intermixed with oaks (30,71,78,82), and by the number of other tree species present, with a higher diversity of species leading to lower overall infection rates (53). Additionally, intraspecific variability in oaks' susceptibility to P. ramorum has been observed in inoculation studies performed on detached branches of coast live oaks (32,33). Because these experiments were performed on excised plant parts of wild trees of unknown parentage, it has been impossible to determine whether the observed variation was genetically or environmentally determined, but it seems reasonable that both mechanisms may play a part.…”

Phosphonate controls sudden oak death pathogen for up to 2 years

“…The efficacy of these two chemical treatments was tested at 6, 12 and 18 months post-treatment by inoculating 10 cut branches per tree with P. ramorum as described in Dodd et al (2005). Terminal branches (0.8 to 1.2 millimeters diameter) were cut, immediately placed in water and transported to the UC Berkeley greenhouse.…”

Scientific Opinion on the Pest Risk Analysis onPhytophthora ramorumprepared by the FP6 project RAPRA

“…In the same study, Dodd et al (2008) report a genetic component in the size of lesions to coast live oaks caused by P. ramorum (already hinted at in a previous study, not considered in the RAPRA report, Dodd et al (2005)). Although for P. ramorum in Europe and America there has not been hostpathogen co-evolution, there is still likely to be intraspecific genetic variation in susceptibility of the various host species (Nettel et al, 2009).…”

“…This study confirms the previously reported (in Mascheretti et al, 2008 already cited in the RAPRA report) spatial genetic autocorrelation profile with an increase in relatedness from a few hundred metres to about one km, which could be related to wind-dispersal, but also to artificial movements of the pathogen. Dodd et al (2008) report a genetic component in the size of lesions to coast live oaks caused by P. ramorum (already hinted at in a previous study, not considered in the RAPRA report (Dodd et al, 2005)). Although for P. ramorum in Europe and America there has not been host-pathogen coevolution, there is still likely to be intraspecific genetic variation in susceptibility of the various host species (Nettel et al, 2009).…”

“…In itself, this is enough to warrant a special consideration for this pathogen, but really what needs to happen is to determine where sporulating hosts are sympatric with susceptible hosts (sometimes the two hosts can be the same) in an area with favourable environmental factors, and to determine whether favourable environmental factors occur when phenology of the host makes them most susceptible as it has been shown for California oaks (Dodd et al, 2008). Because most inoculation tests are inevitably performed on a few individuals only, it may also be necessary to assess whether putative susceptibility occurs at the same level throughout populations of the host as has been done for coast live oaks (Dodd et al, 2005) and bay laurels (Anacker et al, 2008) in California.…”