Azalea and rhododendron petal blight—Ovulinia azaleae
Azaleas and rhododendrons are highly susceptible to petal blight, also called flower blight, or Ovulinia petal blight.
The fungus infects only petals, causing white to brown spots that enlarge rapidly. Infected blossoms become droopy, limp, slimy, and sometimes cling to leaves after they die.
Ovulinia petal blight resembles Botrytis blight. However, Botrytis blight is a drier rot that also affects dying or inactive green tissue, while Ovulinia petal blight infects only blossoms. Botrytis and Ovulinia produce similar sclerotia (infectious structures) that are black, flattened, irregular shaped, and up to about 1/2 inch long. Sclerotia from fallen flowers produce very small, brownish, mushroomlike or wineglass-shaped apothecia (reproductive, spore-forming structures) about 1/10 inch diameter on stalks up to about 2/5 inch long.
The Ovulinia fungus infects wet blossoms when temperatures are mild, about 50° to 70°F. Black sclerotia develop at the base of infected flowers, then drop to the soil where they persist and produce apothecia for several years. Ovulinia petal blight also produces colorless conidia (asexual spores) on infected petals. These spores spread in wind and by the movement of flower-visiting insects, especially bumble bees. This allows the fungus to reproduce and spread between many blossoms within several days of an initial infection.
Sanitation is the primary management method. Remove and dispose of fallen, old, and infected flowers. Gently twist the flower stem and discard it in a covered trash container. Do not add azalea or rhododendron petals or leaves to mulch that will be used around those plants, even if it has been composted. It is difficult to expose plant debris to the temperature required to kill all of the Ovulinia propagules by composting. Avoid overhead irrigation and provide good air circulation.
Each year, when blossoms are no longer present, apply a fresh layer of uncontaminated organic mulch beneath host plants and maintain about a 4-inch mulch layer to help suppress Ovulinia propagules. Keep mulch thin near the trunk or several inches away from the trunk.
Application of an appropriate fungicide, such as triforine, prior to budbreak or prior to rainy weather can help to reduce infections. Fungicides often provide only partial control unless they are used in combination with the recommended sanitation and cultural practices.
Flower blight on azalea
Pale blotches from petal blight fungus