Like many plants, gerbera daisies may succumb to fungus. A white mould on these plants may be powdery mildew or a fungus called sclerotinia sclerotiorum. Treating powdery mildew is not difficult. Once sclerotinia is on plants, however, it causes extensive plant damage, affecting the stems and roots, and these diseased plants cannot be saved.
To get rid of either powdery mildew or sclerotinia from your gerbera daisies, there are several things you must do.
Remove all gerbera daisies with thick mould growing on the stems or that appear to be rotting. These plants most likely are dying and can infect healthy plants. Place them in garbage bags for disposal.
Rake up loose debris around the gerbera daisies. Debris, especially organic debris such as decaying leaves, may harbour fungal spores, which can reinfect your daisies. Dispose of debris with your household trash.
Apply a fungicidal spray on all gerbera daisy foliage and stems. If the disease is sclerotinia sclerotiorum rather than powdery mildew, a fungicide will not save the plants already showing signs of disease, but it may prevent the mould from developing on healthy plants.
Water gerbera daisies in the morning---rather than in the evening---to allow the water to evaporate from around the plant during the day. If conditions around the plant are kept too moist for several hours, such as may occur with an evening watering, white mould or powdery mildew are more likely to return.