Cause Several fungi have been reported on spirea but only Podosphaera clandestina, P. tridactyla, P. minor var. longissima and P. spiraeae have been reported from the PNW. Podosphaera clandestina is also found on stone fruit trees such as cherry and peach. Podosphaera minor var. minor is reported as common. These fungi are obligate parasites that generally overwinter as chasmothecia or in infected buds. The fungus is favored in shady, humid environments.
Symptoms A dull, whitish fungal growth can be seen on leaves. Colonies start out white but leaves can turn a dull purple color later. Young leaves may be distorted and plants may have premature defoliation.
- Plant in a sunny location.
- Space plantings for good air circulation between plants.
- Raking and destroying fallen leaves may be helpful.
Chemical control Tank-mix and/or alternate products from different groups with different modes of action to prevent the buildup of resistant fungi. Limit the use of any one group during the growing season.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Broadform at 2 to 4 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Pageant at 6 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
- Seido at 4 to 5 fl oz/100 gal water plus an adjuvant. Group 50 fungicide. 4-hr reentry.
- Tourney 50 WDG at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trigo at 3 to 9 oz/100 gal water. Group 3 + 11 fungicide. 12-hr reentry.
- Trinity at 4 to 12 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Moparthi, S., Grove, G.G., and Bradshaw, M. 2018. First report of powdery mildew on Spiraea japonica caused by Podosphaera spiraeae in the United States. Plant Disease 102:682-683.