Block spots on teak table top

[Jim Barstow]

I've got a teak table I use outside on a covered porch. It never gets any direct rain and virtually no direct sun. It does get fog occasionally. I treated it with 2 coats of teak oil last summer and it has held up beautifully until the last 2 months when it developed black spots in the grain. I'm not sure if it is mildew or some reaction of the oil. I can successfully sand it out and add a coat of oil but I don't want to have to go through that exercise every spring.

Any idea on:

1) what it is?
2) how can I prevent it?