Management of powdery mildew affecting Monarda plants

Powdery mildew was observed on susceptible cultivars of Monarda didyma (Monarda) plants in a commercial nursery in British Columbia during 2021. PCR of the ITS1-5.8S-ITS2 region of rDNA revealed a 98% sequence identity to Golovinomyces monarda and G. biocellatus. Disease management strategies, both preventative (two trials) and eradicative (five trials), were assessed using four disease control products during 2022. These included Rhapsody ASO™ (dried cells of Bacillus subtilis QST 713), Senator 50SC® (50% thiophanate-methyl), Milstop® (potassium bicarbonate) and ZeroTol® (hydrogen peroxide). Applications were made to ‘Pocahontas Purple Rose’ plants at 2-week intervals either before the onset of disease (preventative) or when mildew colonies were visible (eradicative). Disease levels, expressed as percentage of leaf area affected by powdery mildew, were variable among the trials, and hence, the data from each trial were analysed separately. As a preventative treatment, Senator 50SC® was the most effective at controlling spread of powdery mildew when applied at 14-day intervals in both trials, reducing disease severity by 73–78% relative to the water control. The remaining products provided low to moderate disease control, with Milstop® reducing disease by 28–32% compared to the water control. In the eradicative trials, Senator 50SC® was again the most effective in all five trials, providing a 31–51% relative reduction in disease severity, followed by Milstop®, where a 24% relative reduction in disease severity was observed following applications at the first sign of disease development. This study has identified products that can be used to manage powdery mildew on Monarda plants.