Powdery Mildew -- Erysiphe spp., Microsphaera spp., Sphaerotheca spp., Podosphaera spp., Uncinula spp., Oidium spp.
Host: A wide range of ornamentals including begonia, dahlia, gerbera, hydrangea, kalanchoe, poinsettia, rose and zinnia.
Symptoms: This disease produces a mass of white mycelium on the plant surface that may become powdery-looking as conidia (spores) are produced. The growth may occur on the upper and lower leaf surfaces, as well as on stems and flower parts. In some cases, dead patches may develop on the upper leaf surface even though the mycelium is sparse. Sometimes the upper leaf surface may show chlorotic or necrotic spots opposite patches of white mycelium growing on the lower surface of the leaf.
Conditions Favoring Disease: Conidia of powdery mildew fungi can germinate during extended periods of high relative humidity. In greenhouses, these diseases are favored by a relative humidity of 70–85%. Optimum temperatures vary among the different powdery mildew fungi. Disease thrives during warm conditions.
How Pathogen Survives/Disperses: In moist latitudes, the fungus overwinters as mycelium in diseased leaves and other plant parts.
Photo and write up provided by Syngenta Professional Products