Anyone have any experience with Chasmanthium latifolium under a black walnut? I planted it about four to six weeks ago and it was doing great but now it's beginning to show signs I've come to associate with walnut wither, i.e., yellowing leaves, brown and crispy at the edges from the bottom up.

There was some webbing at the crown, however, as well as some variegation along the length of a few leaves that I assume shouldn't be there and that aren't yellow (at least not yet). It's in a moist area--with all the rain and humidity in the past month, all the other plants haven't needed a drop from the hose, but this grass never seems to get enough water and is often drier than anything else in the area.

Am I simply not watering enough? Is it an insect issue? I should mention that some nearby hostas (always seem to be moist when I check) which are juglone tolerant are exhibiting some yellowing, though not to anywhere near the same degree. I'm new to gardening so any help would be appreciated.