Apple Powdery Mildew (Podosphaera leucotricha) (image : Jonathan Billinger)
A fungal disease of many species, including many fruits. Most annoying during long wet periods. An easy one to identify, as it really does look like a white powder. As with most fungal diseases, keeping an open tree canopy that allows for good air circulation can help. This airflow will help foliage dry off quickly and keep the disease in check. In constantly damp conditions, some intervention may be necessary if mildew crops up. It can be managed with many fungicides, including biologicals. With either of these strategies, especially biological products, you need to be diligent and timely with applications as most need to be on the plant before the disease strikes. Allowing for good airflow in the orchard and gardens will go a long way in keeping this under control.