Epidemiology and microevolution of Phytophthora ramorum during a controlled disease outbreak in a simulated plant production facility.
Phytophthora ramorum outbreaks have been documented to occur in natural settings during favourable environmental conditions, and to be caused by the overdominance of highly infectious genotypes. However, little is known about the dynamics of outbreaks in nursery settings. Through the description and quantification of symptoms, as well as through systematic pathogen isolation and genotyping, this study examines the scale and dynamic of spread of four different genotypes of P. ramorum in soil, water and leaves among Rhododendron plants in a nursery setting. Phytophthora ramorum isolation success was highest from leaves and intermediate from soil, reaching peak values at the end of the wet-warm season. The observed disease outbreak was of moderate intensity, and abundance among the four genotypes used as inoculum varied, depending on substrate and isolation time. The spread mechanism of the disease was mostly through leaf-to-leaf contagion, followed by leaf-to-soil, and the scale of pathogen spread was less than 2 m in the 20 months of the experiment. Surprisingly, a large number of novel genotypes were detected during the experiment, and all were clearly derived from the four used as inoculum. The frequency of two such novel genotypes in the post-outbreak phase was comparable to the frequency of some of the original four genotypes, suggesting they may be competitive. The creation of new genotypes in a nursery setting poses a threat to the industry itself, as well as to wildlands, due to the increase in pathogen adaptability often associated with new genetic variation.