Summary: At the beginning of this project there had been a forty-year research gap on X-disease, which led to significant confusion between X-disease, caused by ‘Candidatus Phytoplasma pruni’ and little cherry disease, caused by little cherry viruses 1 & 2. In this study we identified differences in the symptoms induced by little cherry virus and the X-disease phytoplasma, and most notably, found that the reason that the diseases caused by the two pathogens were being confused was that the phytoplasma strain present in the PNW is not the same as described from previous outbreaks. What we have is a strain that is both genetically and biologically different from the ‘older’ strains, and most importantly, induces clear symptoms only on fruit, and does not cause appreciable foliar or dieback symptoms. We found that all common commercial sweet cherry cultivars in production are susceptible to these pathogens, and that any differences in symptom severity were subjective and influenced by infection progression state and pathogen concentration, as well as environmental/management influences. No sources of tolerance or resistance were observed in sweet or sour cherry, and from this and other work we have performed, any tolerance would have to be introduced from distantly related Prunus species. Next, we found that the development of fruit symptoms is the result of changes across a broad range of gene and genetic pathways. These were most notable from pit hardening to harvest, but stress-related responses by the plant were observed throughout the fruit development cycle. These produce a picture of the plant upregulating plant defense and stress pathways at the expense of normal fruit development. The degree to which this is the result of phytoplasma effector activity versus plant response to infection requires further research, and cumulatively this data will be used by pathology researchers at WSU and USDA-ARS to study disease expression and potential control measures, and by the WSU cherry breeding program to screen lines for disease susceptibility.
