White mould on house plant leaves results from the powdery mildew fungus. The fungus is most active in high-humidity environments. It causes white, fuzzy spots on leaves and stems. African violets and grape ivy are highly susceptible to powdery mildew.
Oidium species fungi are responsible for powdery mildew on house plants. Infestations arise from fungal organisms in decaying or dead plant debris. They also may reach indoor plants as windborne spores from outdoor infested plants.
Powdery mildew can cause distorted, yellowing or prematurely dropping leaves. It also can prevent infested flower buds from opening.
Powdery mildew thrives on crowded plants in dim light and high humidity. It can't generate spores in dry air. New leaves and stems are more likely to develop powdery mildew than mature ones.
At the first appearance of white mould, remove the affected leaves. Dispose of them in a sealed plastic bag to stop the fungus from spreading. Put more distance between your house plants to improve air circulation. Don't use a mister on infected plants, and reduce misting of healthy ones.