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18 Apr 15 Peter (Australia - cool/mountain climate)

I am originally from NJ, USA. I now live in Costa Rica. When I lived in NJ Rosemary loved the summer months in that partially shady area I kept it, but once I brought inside before frost, it almost always got the powdery white on it as you describe. It is called powdery mildew and it can be easily controlled by mixing a teaspoon of baking soda in a pint of water...once dissolved, spray it on the plant every few days until the problem goes away. Mildew loves acid environments and the bicarb makes it alkaline, killing the mildew but not harming the Rosemary.

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