Azalea petal blight and rhododendron petal blight—Ovulinia azaleae
Azaleas and rhododendrons are highly susceptible to petal blight, also called flower blight or Ovulinia petal blight. The causal fungus, Ovulinia azaleae, infects only petals, causing white to brownish spots that can enlarge rapidly.
Damage from Ovulinia petal blight resembles that of Botrytis blight. However, Botrytis blight is a drier rot that also affects blossoms and dying or inactive green tissue. Ovulinia petal blight infects only blossoms. Botrytis and Ovulinia produce similar sclerotia (infectious structures) that are black, flattened, irregular shaped, and about 1/8 to 1/2 inch long. Ovulinia sclerotia from fallen flowers produce very small, brownish, wineglass-shaped apothecia (reproductive structures) about 1/12 inch diameter on stalks about 1/8 to 2/5 inch long.
The biology of Ovulinia petal blight is nearly identical to that of camellia petal blight. The fungi infect wet blossoms when temperatures are mild, about 50° to 70°F.
Ovulinia azaleae persists and reproduces by forming apothecia and sclerotia described above. Ovulinia petal blight also produces colorless conidia (asexual spores) that develop on infected petals. Apothecia forcibly discharge large numbers of spores that are carried by wind onto emerging blooms where they germinate and infect flowers when they are wet. These spores also spread by movement of flower-visiting insects, especially bumble bees. The insect- and wind-borne spores allow the fungus to reproduce and spread between many blossoms within several days of an initial infection.
The disease causes premature death of blossoms of Rhododendron species. Infected blossoms develop brownish or white spots that spread over entire petals and flowers. Infected blossoms become droopy, limp, and slimy and sometimes cling to leaves after they die.
Sanitation is the primary management method. Remove the top layer of potting soil when new azalea or rhododendron are purchased and replace it with pathogen-free soil. Plant azaleas and rhododendrons in a well-ventilated location. Avoid over-head irrigation and instead install drip or microsprinkler irrigation.
Remove and dispose of fallen, old, and infected flowers. Do not add azalea and rhododendron petals or leaves to composting piles or mulch. Even if the organic material is composted, it is difficult to expose all plant debris to the 140°F required to kill the Ovulinia propagules by composting.
Each year when blossoms are no longer present, apply a fresh layer of uncontaminated organic mulch beneath host plants. Maintain about a 4-inch mulch layer to help suppress Ovulinia propagules. Keep mulch thin near the trunk or several inches away from trunks.
Application of appropriate fungicides prior to bud break or before rainy weather or both can help to reduce infections. Where petal blight has been a problem, the fungicides chlorothalonil, thiophanate methyl, or triforine can be applied before disease develops.
Depending on the fungicide, reapplication may be warranted every 10 to 14 days while conditions remain suitable (foggy or rainy) for the pathogen. Be aware that fungicides often provide only partial control unless they are applied preventatively in combination with the recommended cultural and sanitation practices.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).