Phlox species show quantitative and qualitative resistance to a population of powdery mildew isolates from the eastern United States.

Ornamental plants in the genus Phlox are extensively planted in landscapes and home gardens around the world. A major limitation to a more widespread use of these plants is their susceptibility to powdery mildew (PM). In this study, we used Multilocus Sequence Typing (MLST) analysis to gain insights into the population diversity of 32 Phlox PM pathogen (Golovinomyces magnicellulatus and Podosphaera sp.) isolates collected from the eastern U.S. and relate it to the ability to overcome host resistance. Low genetic diversity, and a lack of structure, were found within our population. Whole genome comparison of two isolates was used to support low genetic diversity evidence found with the MLST analysis. Recombination was suggested by the incongruences observed in the phylogenetic trees generated from the housekeeping genes TEF-1alpha, CSI, ITS, IGS, H3, and TUB. Contrasting with low genetic diversity, we found high phenotypic diversity when using ten of the 32 isolates to evaluate host resistance in four different Phlox species (P. paniculata 'Dunbar Creek', P. amoena OPGC 3598, P. glaberrima OPGC 3594, and P. subulata OPGC 4185) using in vitro bioassays. We observed quantitative and qualitative resistance in all Phlox species and a consistent low disease severity in our control P. paniculata 'Dunbar Creek'. Taken together, the results generated in this study constitute a robust screening of popular Phlox germplasm, that can be incorporated into breeding programs for PM resistance, and it also provides significant information on the evolution of PM pathogens.