Genetic and pathogenic characterization of Phacidiopycnis washingtonensis from apple and Pacific madrone from the western United States

Authors: SIKDAR, PARAMA - Washington State University; Mazzola, Mark; Xiao, Chang-Lin

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2018
Publication Date: 2/1/2019
Citation: Sikdar, P., Mazzola, M., Xiao, C. 2019. Genetic and pathogenic characterization of Phacidiopycnis washingtonensis from apple and Pacific madrone from western United States. Phytopathology. 109(3):469-479. https://doi.org/10.1094/PHYTO-10-17-0358-R.
DOI: https://doi.org/10.1094/PHYTO-10-17-0358-R

Interpretive Summary: Phacidiopycnis washingtonensis is a recently reported fungal pathogen causing a postharvest fruit rot of apple and a leaf blight of Pacific madrone in Washington State. In this study, we used microsatellite markers to investigate the genetic relationships among the pathogen populations from different hosts and geographic areas. We found no genetic differentiation between the apple and Pacific madrone populations, but the apple population possessed higher genotypic diversity than the Pacific madrone population, suggesting that P. washingtonensis isolates from apple may represent an older population and could have been introduced into the native habitat of Pacific madrone through accidental movement of infected plant materials. Our findings also indicate that P. washingtonensis isolates are capable of infecting both apple and Pacific madrone irrespective of their host of origin.

Technical Abstract: Phacidiopycnis washingtonensis had been reported to cause a postharvest fruit rot of apple and a leaf blight of Pacific madrone in Washington State. To investigate the genetic relationships among the pathogen populations from different hosts and geographic areas, 316 isolates of P. washingtonensis were collected from different apple production areas in eastern Washington and from Pacific madrone in western Washington. Using eight microsatellite markers designed in this study, 58 unique multilocus haplotypes were identified. Of these haplotypes, 52 were widely distributed in the apple population, five in the Pacific madrone population. Twelve of 58 multi locus microsatellite genotype (MLMG) or haplotypes were shared among the populations from apple and Pacific madrone from different geographic locations. Only one of the 12 haplotypes (H45) was shared between isolates from the apple population and Pacific madrone population and constituted nearly 30% of the isolates from the latter population. Analysis of molecular variance (AMOVA) showed no genetic differentiation between the apple and Pacific madrone populations and that maximum percent genetic variation was present within each subpopulation of apple from different geographic locations and within the Pacific madrone population. The apple population possessed higher genotypic diversity than the Pacific madrone population, suggesting that P. washingtonensis isolates from apple may represent an older population and could have been introduced into the native habitat of Pacific madrone through accidental movement of infected plant materials. P. washingtonensis is likely reproduced asexually as linkage disequilibrium indices indicated that the populations examined in this study were not in linkage equilibrium. Isolates representing haplotype H31 (from apple), and H45 and H47 (from Pacific madrone) were able to incite leaf blight on Pacific madrone and speck rot on apple fruit. Overall, our findings indicate that the P. washingtonensis population in Washington State is largely asexual with high genotypic flow and isolates are capable of infecting both apple and Pacific madrone irrespective of their host of origin.