Black Powdery stuff? Check the undersides of the leaves above the ones with the black powdery stuff on them for sucking insects.

Has the plant been given a shower (natural or otherwise) recently?

Murraya Koenigii hates repotting.Water it well once and leave it alone till the soil dries a little. It likes sun and high humidity. My plants drop leaves in the winter when the humity is low. I prune it well in the spring.
It might also be infested with some pest so check the leaves and if you do find a pest, use an appropriate insecticide but then do not use the leaves till next year.
Hlily

Murraya koenigii likes humidity and indoors is very susceptible to spider mites, which can appear like black powder. Shake an affected leaf over a sheet of white paper; if you see little dots crawling across the paper they're spider mites.

If that's what you have, they are hard to get rid of. Rinse the foliage thoroughly, put a humidifier nearby, and if they return spray plant and soil with a neem oil solution.

Hi

I live in Fremont and this summer I planted a curry leaf tree in the ground. I want to know if the plant will survive the winter and if not what I should be doing to help the plant survive the cold/frost.

Thanks

I had a similar problem with my curry leaf plant. First, I failed to bring it inside early enough in the autumn and it lost all its leaves. Then the leaves grew back over the winter -- initially growing fast, but then they were yellowish with the yucky black speckle dusty stuff. So a couple of weeks ago since it is springtime and things are warming up both indoors and out, I decided to trim it right down to woody stems, and try to propagate the stems, and see if the mama plant leafs out again nicely. No results yet on either, though.