Your cammelia buts may hae been frost burned prior to opening fully or may have been infected with a fungal blight. If you still have some take a sample to to your county Extension office to see if they can identify the cause and if a disease recommend a spray to prevent the problem next year. Usually such problems tend to be bad one year and not so the next as weather and other factors affect them a lot. It should not be necessary to spray the plants every year.
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