Answer: What you describe sounds like powdery mildew, a fungal disease. A dry whitish powder coats leaves, shoot tips and often flowers and is especially visible in summer.
Other symptoms include stunted and distorted growth, and reduced flowering. On rhododendrons a pale beige felt appears on the undersides of the leaves with a corresponding yellow or reddish-purple area above.
Powdery mildews are caused by a range of closely related fungal species. As each has a limited host range the powdery mildew affecting one plant is unlikely to be responsible for mildew on another species. However some mildews can spread to cultivated plants from closely related weed hosts, so weed control is an important part of disease limitation.
The following measures will reduce susceptibility to the disease:
Keep plants well watered, so they are not dry at the roots.
Mulch to preserve soil moisture.
Improve air-flow around plants to reduce humidity. For woody plants such as rhododendrons this can be achieved by pruning to establish an open branch structure.
Avoid high nitrogen fertilizers, as these encourage soft sappy growth that is more easily colonized by fungi.
Ensure plants are in their ideal position. For example, a sun-lover will struggle in shade and be at greater risk of infection.
To control, prune out infected areas as soon as seen. Collect and burn or otherwise dispose of all infected debris and prunings. You can also use Funginex Rose Disease Control at 1 Tbsp./gal. water. Thoroughly cover both leaf surfaces.
Best wishes with your rhodie!
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