I have a couple of JBP as well and have noticed these yellow areas with brown spots.
Does it look like the spots in the image below? If so this is apparently "Brown Spot Needle Blight".
I found this image with some pretty good info at
http://www.ces.ncsu.edu/depts/pp/no...odin19/od19.htm
Go to the link above and about half way down the page you'll find a description of this disease and suggestions on how to treat it.
From the link above:
"
Fungicide applications of Bordeaux mixture, maneb or chlorothalonil are effective in preventing infection. These fungicides are registered for use on longleaf, but not on Japanese black."
I'm not sure what they mean by not being "registered" for JBP. Maybe a little more research will reveal an alternative for JBP.
I haven't treated my trees because the affected areas were very small and have disappeared.