Rosemary Dusty - Knowledgebase Question

Columbia, Mi
Avatar for vaughnfarm
Question by vaughnfarm
January 29, 2011
My rosemary has dusty almost powder like substance on it. Will it kill the plant? Is it safe to use? I brought it indoors last fall as we live in Missouri. Thank you. Betty

Answer from NGA
January 29, 2011
I can't tell from your description whether your rosemary plant has a fungus or if what you are seeing is normal. The new leaves on a rosemary plant have white hairs which can look like a white substance. As the leaves mature, their appearance will change. If you examine mature leaves, you will see tiny white hairs on the undersides of the leaves This should be very similar to the substance you've observed on the new leaves. The only other explanation is that the your plant is suffering from powdery mildew. This fungal disease can attack a plant that is stressed. You'll find it all over the plant, not just on the new growth. Powdery mildew is a coating on the leaves and you can scrape it off with your fingernail. If your plant has developed powdery mildew the leaves should not be used in the kitchen. Depending upon the amount of infection, you may be able to save your plant. Start by pruning off all the affected stems and leaves. This will open up the plant to better air circulation. Rosemary needs direct sunshine or supplemental light during the winter months to help it maintain its health. Try increasing the amount of light it gets and give it lots of elbow room to grow (overcrowded plants can develop fungal diseases). I hope the new growth remains healthy so you can use the leaves in cooking.

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