Rosemary With Powdery Mildew?
Answered by: Inge Poot
Question from: Barbara Richardson
Posted on: December 8, 1998

Every winter I try to keep rosemary indoors. This year I received a particularly robust garden-grown specimen from a friend. It lived on my airy porch for several weeks, and I brought it indoors in mid-October. It has done what every rosemary I’ve ever grown inside does: It gets a fuzzy white coat! I don’t think it’s the white epidermal layers that Inge Poot describes in one of her answers, because the plant is still growing, sending up lush shoots of healthy-looking leaves above the white coated ones. I water regularly, and the plant is under a full-spectrum grow bulb. My house is pretty dry this time of year, and temperatures are moderate (55 to 70 degrees Fahrenheit or 13 to 21 degrees Celsius). Could this be powdery or downy mildew?

Since you say your house is dry, the change in conditions from the cool moist outside to the dry, warm inside could shock the plant so much that it becomes susceptible to fungal infections and succumbs to some mildew whose spores are either present from other slightly infected plants in the house or brought in from the outside either on the plant or by air currents passing over infected plant debris.

Since mildew resists eradication efforts by shedding liquids from its heavy waxy covering, anything you use on it must be combined with a good spreader sticker. In a dry house, the least toxic and simple remedy is to simply turn the plant upside down in a bucket of dishwash detergent and give it a one minute swishing! (Cover the soil with taped down crumpled newspapers to stop it from falling out and also to stop any detergent from getting into the soil accidentally). The detergent will dissolve the waxy covering of the mildew -and incidentally make it look less white - and the dry air will do the rest by drying out the fungus. Repeat this once per day for about three days - longer if a few moist days get in between- and you should be rid of the infection. The rosemary may lose its new shoots because they too will dry out too much, but they will regrow.

You might like to use a magnifying glass to see if you can discern any structure in the white covering. If it appears to be composed of filaments then it is most likely mildew, a fungus. If it is more like little bits of broken up sheets, then it is most likely the broken up waxy covering of the leaves that spalded off when the roots could not keep up with the higher rate of evaporation indoors and caused the leaves to shrink a bit. You are obviously a diligent waterer and the roots of your plant did not die. As soon as the plant recovered from the shock of the different conditions it grew more roots and then more leaves where the new leaves were adapted to the new conditions and therefore did not shrivel.

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