Seasonal trends in response to inoculation of coast live oak with Phytophthora ramorum

We developed a branch cutting inoculation method to provide a controlled system for studying variation in response to inoculation of coast live oak (Quercus agrifolia) with Phytophthora ramorum. This method has advantages over inoculations of trees in the field, in containing the inoculum and in allowing high levels of replication and the possibility of time series of responses. We previously reported significant tree-to-tree variation, with little population variation in lesion size using this method (Dodd and others 2005). Here we report on a time series in which branch cuttings were collected from the same trees at eight dates through a full year cycle. Branch cuttings were sampled from 33 trees from two sites at China Camp in Marin County, California, including 18 trees from one site that had suffered heavy mortality from this disease (Miwok Meadows) and 15 trees from a second site that has had little infection (Chicken Coop Island)

Phenotypic variation among Phytophthora ramorum isolates from California and Oregon

To manage and control Phytophthora ramorum successfully, it is important to know the amount of phenotypic variation within a given pathogen population. Because the pathogen has only recently been described, there are few studies on morphological and pathological variation of isolates from the United States. One study has compared growth rate on agar, aggressiveness to Quercus rubra (Eastern American red oak), and developmental stability of European and U.S. isolates. In this study, the U.S. isolates were significantly slower growing, less aggressive, morphologically more unstable, and had a larger range in growth rates than the European isolates (Brasier and others unpublished data; Brasier 2003; Brasier and others, this Proceedings). However, there is no information on fungicide tolerance, spore production or aggressiveness to hosts from the western United States to U.S. isolates. This study examines these phenotypic characters among 12 isolates from a broad range of hosts, locations within California and Oregon, and AFLP genotypes. We examine variation in aggressiveness of isolate populations from California. Additionally, we determined whether measurement of lesions on inoculated coast live oak stems is a good measure of pathogenicity

Pittosporum undulatum is a potential Australian host of Phytophthora ramorum

Pittosporum undulatum is a potential Australian host of Phytophthora ramorum, the causal agent of sudden oak death in California. It was susceptible and supported sporulation in zoospore inoculations of detached leaves. Susceptibility and sporulation potential were low when compared to Umbellularia californica. Two independent positives were obtained from symptomatic trees in a PCR-based assay using species-specific primers. Foliar symptoms observed on the trees were replicated in the detached leaf inoculations

Variation in susceptibility of Umbellularia californica (Bay Laurel) to Phytophthora ramorum

Bay laurel (Umbellularia californica) is an important foliar host in terms of spore production and transmission of disease. We designed a bioassay to screen for variation in susceptibility to Phytophthora ramorum among populations of bay laurel collected along the coast of California to southern Oregon and also from Yosemite. Mature leaves of bay laurel were inoculated with zoospores and nine days later lesions and infection percentage were measured. Results indicated that each population comprises trees of different susceptibility. Data from three separate trials indicated that trees from Oregon were less susceptible than those from California. There was little variation among populations from California. Seasonal variation in susceptibility of bay laurel to P. ramorum was observed among populations. It is hypothesized that the interaction of a number of factors are responsible for the observed variation in susceptibility including host resistance, genetic structure of host species in the forests, pathogen variation, and environmental conditions

Evidence for the role of synchronicity between host phenology and pathogen activity in the distribution of sudden oak death canker disease

Variations in synchronicity between colonization rate by the pathogen and host phenology may account for unexplained spatial distribution of canker disease. The hypothesis that synchronous pathogenicity and host development are necessary for incidence of sudden oak death disease was tested by correlating seasonal variations in host cambial phenology and response to inoculation with Phytophthora ramorum. • Response to infection was estimated by inoculating branch cuttings from coast live oak (Quercus agrifolia) trees at nine dates through a full annual cycle in2003-2004 . Host phenology was estimated from measurements of bud burst and cambial activity in spring 2006. • Lesions were largest in the spring soon after the cambium resumed activity. A moderate genetic component to lesion size was detected. Variation among trees in date of largest lesions correlated with variation in timing of bud burst and cambial phenology. • The data support the hypothesis that active host cambial tissue is a necessary requisite for successful infection with the pathogen that causes sudden oak death canker disease. Genetic variation in host phenology will buffer coast live oak against epidemics of this diseas

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

* 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

Susceptibility to Phytophthora ramorum in a key infectious host: Landscape variation in host genotype, host phenotype, and environmental factors

• Sudden oak death is an emerging forest disease caused by the invasive pathogen Phytophthora ramorum. Genetic and environmental factors affecting susceptibility to P. ramorum in the key inoculum-producing host tree Umbellularia californica (bay laurel) were examined across a heterogeneous landscape in California, USA. • Laboratory susceptibility trials were conducted on detached leaves and assessed field disease levels for 97 host trees from 12 225-m2 plots. Genotype and phenotype characteristics were assessed for each tree. Effects of plot-level environmental conditions (understory microclimate, amount of solar radiation and topographic moisture potential) on disease expression were also evaluated. • Susceptibility varied significantly among U. californica trees, with a fivefold difference in leaf lesion size. Lesion size was positively related to leaf area, but not to other phenotypic traits or to field disease level. Genetic diversity was structured at three spatial scales, but primarily among individuals within plots. Lesion size was significantly related to amplified fragment length polymorphism (AFLP) markers, but local environment explained most variation in field disease level. • Thus, substantial genetic variation in susceptibility to P. ramorum occurs in its principal foliar host U. californica, but local environment mediates expression of susceptibility in nature

Susceptibility to Phytophthora ramorum in California bay laurel, a key foliar host of sudden oak death

Sudden oak death, caused by the water mold Phytophthora ramorum, is a plant disease responsible for the death of hundreds of thousands of oak and tanoak trees. Some foliar hosts play a major role in the epidemiology of this disease. Upon infection by P. ramorum, these foliar hosts express non-fatal leaf lesions from which large amounts of inoculum can be produced and spread to neighboring host individuals, including oak species. Umbellularia californica (California bay laurel) may be one of the most important foliar hosts of sudden oak death due its observed ability to produce inoculum and its high abundance in the woodlands of coastal California. While previous research on susceptibility to P. ramorum in U. californica has shown significant variability among trees, with more resistant individuals in northern areas of its range, little is known about the causes or extent of this variability. Here, we ask three research questions: (1) How does susceptibility vary among U. californica individuals and P. ramorum isolates? (2) Are U. californica phenotype and genotype related to susceptibility? (3) What factors influence disease expression in nature

Phytophthora ramorum: an emerging forest pathogen

Aided initially by taxon specific PCR primers, we have determined that Phytophthora ramorum, cause of Sudden Oak Death, is not restricted to oaks, but has a host range encompassing at least 11 families and 18 plant species, including dominant tree species such as redwood and Douglas-fir, as well as understory shrubs, and herbaceous plants