Cause Sphaerulina azaleae (formerly Septoria azaleae), a fungus that overwinters on dead azalea leaves or in infected leaves on plants in the greenhouse. It may cause infection by gaining entry through stomata. Wet leaves encourage spore germination and infection.
Symptoms Yellow leaf spots develop initially but turn dark, reddish-brown in early summer. Spots have red-to-purple margins. As spots enlarge, leaves may fall to the ground. Older spots are light brown; surrounding leaf tissue is yellow.
Cultural control
- Resistant cultivars are available.
- Rake and destroy fallen leaves.
- Keep greenhouse plant foliage dry.
Chemical control Apply in 14-day intervals as necessary. Many of the products used for Botrytis or Cylindrocladium will also be effective against this disease.
- 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.
- Badge X2 at 1.5 to 2 lb/A. 24-hr reentry. O
- Broadform at 4 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- 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.
Reference Linderman, R.G., and Benson, D.M. 2014. Compendium of Rhododendron and Azalea Diseases and Pests, 2nd ed. St. Paul, MN: APS Press