Early necrotic symptoms are seen on the flower petals.
Cause Ovulinia azaleae, a fungus. The disease is important on many greenhouse-grown azaleas as well as field-grown rhododendron and azalea. Flowers are the only part of the plant affected. Most azaleas and rhododendrons are susceptible and may become infected if weather and inoculum conditions are favorable. The disease develops during rainy periods at flowering time. It overwinters as sclerotia on diseased petals on plants, on the soil surface, or in leaf mulch under bushes. Sclerotia germinate and give rise to stalked, cup-shaped apothecia.
Mature ascospores are discharged from their apothecium. After primary infection, colonization, and the breakdown of flower tissue, numerous conidia form and are spread by splashing rain, insects, and wind.
Infected flowers first show small spots, about 1 mm in diameter, which appear water-soaked. Spots can occur on petals before they are fully open. Spots enlarge rapidly and become slimy. As the disease progress, entire petals become slimy and limp. Infected areas of flowers soon become tan or light brown; eventually entire flowers turn prematurely brown. Infected flowers may last only 2 to 3 days after they are fully opened. Those on lower limbs usually are infected first. Nearly all flowers on a plant become infected when disease conditions are optimum. Infected flowers dry and cling to the plant longer than uninfected flowers. Small, black sclerotia form on infected flowers 8 weeks later. Sclerotia are buried in petal tissue and are most common in the thick portion of the flower throat or tube.
The disease produces no symptoms on the vegetative portion of the plant. Botrytis blight causes spots that initially are similar to those of Ovulinia petal blight but do not enlarge as much or as rapidly. Also, Botrytis blight does not produce slimy petals.
Remove and destroy infected flowers.
Remove and destroy floral debris from the base of plants or pots. Carefully remove 1 inch of the media, mulch, or soil from around base of plants. Replace with fresh, clean mulch.
Avoid overhead irrigation.
Maintain greenhouse humidity below 85%.
Inspect and isolate any new shipments of plants.
Pruning will help improve air circulation and drying conditions. Thin dense plantings and/or remove lower branches where first infections are likely to occur.