Differential response of four Californian native plants to worldwide Phytophthora cinnamomi genotypes: implications for the modeling of disease spread in California

Abstract

Multiple introductions in California of Phytophthora cinnamomi are severely threatening its native ecosystems. However, little is known about the consequences of outbreaks caused by different genotypes on endemic hosts. Virulence of 10 genotypes representative of the California and worldwide genetic variability of P. cinnamomi was studied on roots and stems of four native California plant hosts. Hosts differed in type of response and susceptibility level, and individual pathogen genotypes differed in virulence. Pathogen variability was clear when testing hosts of moderate susceptibility (Pseudotsuga menziesii and Umbellularia californica), but not so on highly susceptible hosts. Some genotypes are better adapted as root rather than stem pathogens, causing a different disease. Arbutus menziesii and Arctostaphylos viscida are hosts with aerial and root susceptibility, while U. californica acts mainly as root host. Four isolates representing three genotypes caused the highest level of disease: two of them were from a genetic lineage only recently identified in California and Mexico, but nowhere else. The results presented here are preliminary, however if confirmed, they would suggest that novel invasive genotypes of P. cinnamomi may require regulatory action to prevent their further spread. It also proves that highly susceptible hosts should not be used to identify phenotypic variability in pathogens.