|Every year my Rhododendron will bloom but what cause the leaves to turn a rusty brown color? Its almost as if they were hit with a chemical or something like that - but they weren't.|
|You didn't describe the symptoms very thoroughly, so I'll venture a couple of guesses. There's a fungus that attacks rhodie leaves called Botrytis. The symptoms include brown, water-saoked spots on leaves, which later produce a gray-brown mass of spores. If this sounds like what your rhodie has, remove the affected leaves and try to decrease humidity by increasing air circulation around the plant. Be sure to pick up any fallen leaves and remove spent flowers.
A different problem with similar symptoms is also a fungal leaf spot, caused by Phyllosticta. Spots are dark brown and frequently cover half the leaf area. Again, remove and destroy affected leaves and fallen leaves.
For a positive identification of your rhodie problem, take a sample into your local Cooperative Extension office.