Help! Holes in rhizomes

Another mystery malady has struck.  My newly planted rhizomes seemed to be
doing well, but now I find a number of them with holes.  Here in Tennessee
we plant rhizomes with the top of the rhizome exposed, because of the
humidity and (usually but not this summer) frequent rains.  The holes are
round, ranging in diameter from 1/4" to 1/2", and from 1/4" to over an inch
deep.  Most of the holes seem to be healing up, but the deeper ones hold
water and look bad enough to do some serious damage.  

I have not seen these holes before this summer, and I have grown iris here
since 1988.  An acquaintance in the local garden club said she has them,
too.  I have not observed any unusual insects or larvae.  There have been a
lot of grasshoppers, but don't they just eat the foliage?

Have any of you had this problem?  The holes are not in the fans--just he
rhizome itself.

Julie Allen jwallen@blomand.net  Sparta, middle Tennessee, Zone 6
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