Seamus Ma' Cleriec helpfully suggested in message
om after I wondered:
My four yo rosemary bush has suddenly sprouted fuzz on every leaf!
Wha' happened?

Downy Mildew. You'll want to read
http://www.hort.uconn.edu/ipm/greenh.../downymlgh.htm

I've used Safer's fungicide on Rosemarinus sp. w/ good results.

sigh I /knew/ it wasn't a part of the plant. Thanks for the article.

The Ranger