See:
Azalea (Rhododendron spp.) - Leaf and Flower Gall
Cause Exobasidium spp., fungi that are believed to overwinter as spores adhering to bark and bud scales and to cause infection with the opening of buds in spring. Older leaves are resistant to infection. Infection is dependent on high humidity and moisture during bud break. Occurrence in the Willamette Valley has been quite variable from severe for a few years, back-to-back, to hardly noticeable for several years in a row. Hosts of this fungus include Camellia, huckleberry (Vaccinium spp.), Kalmia, kinnikinnick, Leucothoe, madrone, Pieris, and salal.
Symptoms In contrast to bladder-like galls on azalea, certain evergreen rhododendrons can show a variety of symptoms from yellow leaf chlorosis, leaf thickening, or yellow leaf spotting. Either all or part of a leaf may become thickened, fleshy, and covered with a white bloom of spores. As spots age the center of the spot begins to turn brown.
Cultural control
- Remove and destroy affected shoots before they turn white.
- Reduce greenhouse humidity.
- Select less susceptible cultivars.
Chemical control Apply the first spray before budbreak in spring and a second spray 2 to 3 weeks later. Some references call for regular applications until all infected tissues have dried up. Symptom reduction is then seen the following spring. Other references indicate chemical control is unsuccessful.
- Captan 80 WDG at 1.5 lb/100 gal water. Group M4 fungicide. 48-hr reentry.
- Copper-based fungicides may discolor foliage and blooms. Group M1 fungicide. O
- Badge X2 at 1.5 to 2 lb/A. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
- Bonide Copper Fungicide at 8 oz/3 gal water. Group M1 fungicide. H
- Bordeaux 6-2-100.
- Monterey Liqui-Cop at 3 Tbsp/gal water. H
- Nu-Cop 50 DF at 1 lb/100 gal water. 48-hr reentry general or 24-hr reentry for greenhouse.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80WP Rainshield 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.
Reference Linderman, R.G., and Benson, D.M. 2014. Compendium of Rhododendron and Azalea Diseases and Pests, 2nd ed. St. Paul, MN:APS Press.
Wolfe, L. M. and Rissler, L. J. 1999. Reproductive consequences of a gall-inducing fungal pathogen (Exobasidium vaccinii) on Rhododendron calendulaceum (Ericaceae). Canadian Journal of Botany 77: 1454-1459.