Comparative Host Susceptibility and Sporulation Potential of Phytophthora ramorum

Abstract

Phytophthora ramorum, causal agent of ramorum blight of woody shrub species, has caused serious damage to cultivars and species of camellia in commercial nurseries. Reports of relative susceptibility of camellia to P. ramorum have indicated a range from high to low susceptibility, both in nurseries and under experimental conditions. We inoculated a series of cultivars of camellia to determine their relative susceptibility to infection, and then compared lesion size to the capacity of the pathogen to produce sporangia on the lesions. We found, as did others, a wide range of susceptibility among cultivars, but lack of correlation between susceptibility (lesion size) and potential to produce sporangia that might spread the pathogen within the nursery. These results indicate that on some cultivars the pathogen might produce small or inconspicuous lesions, yet still produce copious numbers of sporangia that could spread the disease, both within the nursery and from nursery to nursery. Accepted for publication 10 April 2007. Published 22 August 2007.