Camellia petal blight—Ciborinia camelliae
Camellia petal blight, also called Ciborinia 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. Prominent dark brown veins give infected petals a netted appearance.
Damage resembling that of camellia petal blight is also caused by Botrytis blight, frost, old age (overmature blossoms), and injury due to chemicals, rough handling, or wind. Symptoms that distinguish camellia petal blight from these other causes include petal veins darker than the surrounding tissue, infections beginning near the central part of the flower (not appearing first near petal margins), and symptoms that occur only on petals.
Ciborinia camelliae produces dark, hard, irregular-shaped sclerotia at the base of infected flowers, where they replace the calyx lobe. Depending on the extent to which nearby sclerotia unite, they typically range in size from 1/12 to 1 inch. Sclerotia can lie dormant for several years on or near the soil surface.
During winter and spring when camellias blossom, sclerotia produce light brown saucer-shaped apothecia (inverted mushroomlike bodies) about 1/5 to 3/4 inch in diameter. 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.
Pathogen development is favored by wet or humid conditions and mild temperatures (about 59° to 70°F) during bloom. Outbreaks are initiated by sclerotia-infested soil received with new plants and by sclerotia persisting beneath established plants that have previously been infected. Sclerotia continue to produce apothecia for 3 to 5 years after being introduced into soil and the pathogen cannot be eradicated once present.
Prevention is the best control. Remove the top layer of potting soil when new plants are purchased and replace it with pathogen-free soil. Plant camellias in a well-ventilated location and avoid overhead irrigation. Pull off infected flowers as they appear and collect fallen blossoms and dispose of them in a covered location away from camellias.
Do not add camellia petals or leaves to mulch that will be used around camellia, even if it has been composted. It is difficult to expose camellia debris to the 140°F required to kill all of the Ciborinia propagules by composting.
Each year, when blossoms are no longer present, apply a fresh layer of pathogen-free organic mulch and maintain a 4-inch layer of organic mulch beneath and somewhat beyond plants to suppress pathogen spore production. Remove fallen petals and other camellia debris before applying fresh mulch, but otherwise avoid moving or disturbing existing mulch where fungi may be present. Keep mulch thin near the trunk or several inches away from the trunk.
Spraying an appropriate fungicide during bloom can help to reduce infections. Depending on the fungicide, reapplication may be needed every 10 to 14 days while conditions remain suitable for the pathogen. Use fungicides only in conjunction with recommended sanitation and cultural practices.