Cause Phyllosticta spp., a fungus found on wild as well as cultivated rhododendron. It can enter through wounds. Six species have been reported in Washington. Cladosporium sp. have also been found frequently associated with leaf spots by the OSU Plant Clinic. Both fungi have been found when samples are sent in with concerns about Ramorum Leaf Blight. In addition there are many different fungi that have been found less frequently in association with leaf spots on rhododendron in the PNW.
Symptoms Spots are dark brown, zonate, and frequently cover half the leaf area. The spot's upper surface may feel bumpy due to the fungal fruiting bodies (pycnidia).
- Remove and destroy affected leaves and fallen leaves.
- Prevent injury and give proper cultural care.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Monterey Liqui-Cop at 3 Tbsp/gal water. H
- Nu-Cop 50 DF at 1 lb/100 gal water. 48-hr reentry.
- Pageant at 8 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Tourney 50 WDG at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Linderman, R.G. and Benson, D.M. 2014. Compendium of Rhododendron and Azalea Diseases and Pests, 2nd ed. St. Paul, MN:APS Press.