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morphologically more unstable, and had a larger range in growth rates than the European isolates (Brasier and others unpublished data; Brasier 2003; Brasier and others, this Proceedings).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

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