See:
Azalea (Rhododendron spp.) - Ovulinia Petal Blight
Greenhouse Plants, Ornamental - Gray Mold
Cause Botrytis cinerea, a fungus. This disease often follows mechanical damage or environmental stress. It can also develop on plants overwintered in unheated plastic houses when moisture develops on the canopy. Long wet periods favor disease development. The fungus colonizes dead or dying tissue such as fallen leaves and petals. This material may serve as a source of inoculum.
Symptoms Brown, water-soaked spots on leaves, which later produce a gray-brown mass of spores. Small, water-soaked lesions on petals also develop the characteristic gray fungal growth. Spots are similar to those produced by Ovulinia but do not enlarge as much or as quickly.
Cultural control
- Select disease-free cuttings.
- Reduce humidity by increasing air circulation, temperature, and plant spacing.
- Remove fading flowers and yellowing leaves. Place diseased debris in a closed container if not immediately removed from the house.
Chemical control Spray while flowers are opening. Best when used with cultural controls. Tank-mix and/or alternate products from different groups with different modes of action to prevent the build-up of resistant fungi.
- Astun at 10 to 17 fl oz/100 gal water. Group 7 fungicide. 12-hr reentry.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. Group M5 fungicide. H
- Broadform at 4 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Captan 80 WDG at 1.5 lb/100 gal water. Group M4 fungicide. 48-hr reentry.
- Chipco 26019 FLO at 1 to 2.5 quarts/100 gal water. Limit to two (2) applications per year to aid resistance management. Group 2 fungicide. 12-hr reentry.
- Daconil Weather Stik at 1.38 pints/100 gal water. Blooms may discolor. Group M5 fungicide. 12-hr reentry.
- Fixed-copper. Foliage or blooms may discolor. Group M1 fungicides. O
- Nu-Cop 50 DF at 1 lb/100 gal water. 24-hr reentry for greenhouse.
- Hi-Yield Captan 50 W Fungicide may be used in home gardens at 1.5 Tbsp/gal water. H
- Mancozeb-based products can be used as mixing partners and provide some protection. 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.
- Medallion WDG at 2 to 4 oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
- Orkestra at 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Pageant at 12 to 18 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
- Regime at 20.5 to 45.7 fl oz/A. Reapply if rain occurs within 12 hours of original application. Group BM01 fungicide. 4-hr reentry.
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Spirato GHN at 2 to 4 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
- Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Biological control
- Prestop (Gliocladium catenulatum strain J1446) at 0.33 oz/5 gal water. Do not use with other products in the tank. 4-hr reentry. O
Reference Linderman, R,G., and Benson, D.M. 2014. Compendium of Rhododendron and Azalea Diseases and Pests, 2nd ed. St. Paul, MN: APS Press