Abstracts of Presentations at the 2024 APS Potomac Division Meeting

The downy mildews on roses and brambles are sometimes considered to be the same species, Peronospora sparsa, and sometimes considered to be two species, P. sparsa on roses, and P. rubi on brambles.P. sparsa has also been reported from cherry laurel (Prunus laurocerasus).We have thus far collected two isolates of P. sparsa from roses, and, pending collection of additional isolates for molecular characterization, we have performed host range tests on detached leaves of multiflora rose (R. multiflora), wild raspberry (Rubus allegheniensis), wineberry (Rubus phoenicolasius), miniature hybrid teas, and Rosa 'Knock Out' incubated in moist chambers consisting of Petri dishes lined with moist Whatman Filter paper.Sporangia collected from rose petals were used as inoculum suspension (approx.15,000 spores/mL water), then pipetted as 20 uL droplets onto each leaf.After 8-10 days, the number of infected leaves or leaflets were counted.Miniature roses were most susceptible (20-100% of leaflets), followed by 'Knock Out' (30-55%) and wild raspberry (0-75%) with less infection of wild rose and wineberry (0-20%), and no infection of cherry laurel.Results suggest that while P. sparsa can infect other genera in the Rosaceae, it is most virulent on rose.