Camellia petal blight affects all cultivars of Camellia japonica. Camellia sasanqua is infected less often in California.
Infection by the Ciborinia camelliae fungus initially causes small, brown, irregularly shaped blotches in petals. Spots enlarge rapidly until the entire flower is brown and dead. Except when wet, blighted petals are dry or leathery but do not crumble when handled. Blossoms drop prematurely to the ground, often as intact flowers.
During the winter and spring when camellias blossom, fungus discharges large numbers of spores that are carried by wind onto emerging blooms, where they germinate and infect flowers when they are wet.
Prevention is the best control. Remove the top layer of soil when new plants are purchased and replace it with pathogen-free soil. Plant camellias in a well-ventilated location and avoid of overhead irrigation. Pull off infected flowers as they appear and collect fallen blossoms and dispose of them in a covered location away from the plants.
DO NOT add camellia petals or leaves to mulch that will be used around camellias.
Each year, when blossoms are no longer present, apply a fresh layer of pathogen-free mulch and maintain a 4-inch layer of mulch beneath and somewhat beyond plants to suppress the disease. Remove fallen petals and other debris before applying fresh mulch.
There are no fungicides that eliminate the disease but there are some that can reduce the infections.
The above sanitation methods are the best way to control camellia petal blight.
— For more information visit: ipm.ucanr.edu