We've had a very nice dawn redwood growing for the past 5 years. It's grown several of feet every year and is now about 20 feet tall. Recently, however, the needles in lower branches have become yellowish-brown and shriveled. It seems to be random but restricted to more of the lower branches.

I've attached a photo, it's upside down. Imagine this along the main branch and on four or five other branches.

Any idea what it is and how to treat it? Is this fatal?