See:
Rhododendron - Ramorum Leaf Blight and Shoot Dieback
Cause Several fungi can cause leaf spotting including Pestalotia kalmicola (reported from Washington), Asteromella kalmiicola (formerly Phyllosticta kalmiicola) (isolated in the OSU Plant Clinic), Cercospora sp., and Septoria sp. (reported from Idaho). Moist conditions and locations favor disease development. Most leaf spot fungi can overwinter in infected leaves that remain on the plant or fall to the ground. Cercospora infections begin on newly expanding foliage just before and during bloom.
The cultivars Ostbo Red, Pristine, and Tinkerbell did not develop Mycosphaerella colorata leaf spots during a study in Georgia.
Symptoms Asteromella produces irregular to circular light gray spots with a purplish brown border. Black, fruiting bodies (pycnidia) usually form in the centers of these spots. Leaves may fall prematurely. Severe infections can stunt plants and suppress flowering. Cercospora infections have similar symptoms.
Cultural control
- Remove and destroy infected or fallen leaves.
- Avoid overhead irrigation.
- Increase plant spacing and improve air circulation.
Chemical control Apply at new leaf emergence and at flowering.
- 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.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
- Daconil Weather Stik at 1.38 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- 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.
- 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.
- Ortho MAX Garden Disease Control at 2 teaspoons/gal water. H
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Li, H., Chappell, M., and Zhang, D. 2018. Evaluation of Twenty-one Mountain Laurel Cultivars for Container and Landscape Performance in the Southeastern United States. HortTechnology 28:867-874.