White Powder on Rosemary - Knowledgebase Question

Fitchburg, MA
Avatar for jjjj
Question by jjjj
February 18, 1998
I have beautiful rosemary plants in the fall, but when I bring them in for the winter they develop a white powder on their leaves. The leaves turn brown and die, and plants look awful in January! The problem clears up when I put them back outside in the summer. Is this mildew? How can I prevent this problem?

Answer from NGA
February 18, 1998
Yes, that's powdery mildew. Luckily, it usually doesn't kill the plant! Overwintering rosemary requires good air cirulation and adequate humidity. First, make sure the plants overwinter in a well ventilated area. Some gardeners use small fans to keep air flowing around the plants. Second, keep water droplets off the leaves and water only in the morning. If dry air is a problem, use a humidifier, mist the plant very lightly, or set the pot in a tray of water (the roots shouldn't contact the water) to raise the humidity around the plant.

To treat powdery mildew, try repeat applications of the following spray mix:

3 tablespoons of baking soda
1/2 teaspoon of liquid soap
2.5 Tablespoons hort oil
1 gallon of water

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