THE UNDERSTORY: Blighted Camellias



By Susan Starr.

Almost all of us are familiar with that most common of camellia diseases, camellia flower or petal blight. At our February meeting, Tom Nuccio explained that it is almost impossible to eradicate camellia blight since the source is a mushroom whose spores live in the soil and are borne on the wind.

The Fungus Cycle

Curious to learn more, I discovered that Ciborinia camelliae is the organism that causes camellia flower blight. The blight, characterized by small, brown blotches on the flower, is initiated by spores distributed from apothecia. Apothecia look somewhat like tiny (1/5 to 3/4 of an inch) inverted mushrooms. Each of them produces several million spores that travel on the wind and, when they come to the surface of the camellia petals, start an infection that results in flower blight. Eventually, the fungus reaches the base of the flower where a hard, black structure called a sclerotium forms. When the camellia flowers fall to the ground, the sclerotia also fall. They remain dormant in the soil until the next blooming period when they activate again, producing new apothecia (and repeatedly doing so for three to five years). The apothecia release new spores, causing new infections, and the fungus cycle continues. Mild temperatures, which we have in San Diego, and wet weather, which we have seen less of recently, favor the development of this disease.

Identifying and Treating Camellia Petal Blight

Interestingly, Ciborinia camelliae only affects camellias. The blight will not spread to other plants in your garden. Fall-flowering camellias are less likely to be affected, as the fungi are not active at that time. Other camellia diseases, such as Botrytis blight can look similar. 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 (versus appearing first near petal margins), and symptoms that occur only on petals.

Because the camellia flower opens slowly and gradually, the effectiveness of spraying fungicides is limited and no biological control agents are currently available. Sanitation is the recommended method of control although, as Tom Nuccio pointed out, unless all of your neighbors pick up their fallen camellia flowers, keeping spores from reaching your camellias may be a lost cause. However, since the fungus can arrive at your garden in nursery soil, removing the top soil of new camellias and replacing it with clean soil may help a bit. In the long term, genetic improvements of camellias may be the only way to control this annoying disease.

Resources for Camellia Growers

Of course, camellia blight is not the only disease that troubles camellia growers. Root rot, aphids, sunburn, salt burn, and viruses are not uncommon. Fortunately, there are excellent resources on the Internet to help you if you notice problems.

The American Camellia Society maintains an extensive website, part of which is devoted to the diseases and insects that affect camellias.

The University of California’s Integrated Pest Management Program is another excellent resource. Their camellia section discusses everything from blight to mealybugs, and from nematodes to bud drop. If you won a beautiful camellia at the meeting raffle, or if you already have camellias in your garden, these two sites can help you care for your plants.

Susan Starr is SDHS Newsletter editor and a former science librarian.

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