I don't know of any specific problems with pine straw and grass. It is possible that the grass needs more water, maybe the tree is soaking it up, but more likely the grass there is getting more shade than further out which is getting more sun. Turf grass needs sun, so my guess is that is the main problem. To remedy this, and if it will still look attractive, you could "limb up" the tree or thin it somewhat, you could extend your island and plant something else there that would tolerate some shade.
laurie

Thanks for the ideas. Those areas get full sun. Maybe they don't retain water well enough? If I fertilize, those areas green up for a little while, then it's back to yellow while the rest stays green.

P.S. to all--Since this seems more like a lawn care issue rather than design, and since I just realized there is a separate Lawn Care forum, i'll post there. Sorry for getting off topic.