My guess is that trash thrown by string trimmers and mowers could certainly
damage iris foliage and invite rot. String trimmers are especially nasty at
throwing soil particles, such as grains of sand, and small pieces of bark,
wood, and other debris. Any wound that these projectiles made on iris
foliage would probably be a good entry point for pathogens. I've had wounds
that bled from debris thrown by string trimmers and lawn mowers.
I would speculate that vigorous edging with a string trimmer (especially the
first few times when you are developing eye-hand coordination) could be the
equivalent of inoculating your irises with soil and other debris that
naturally contains pathogenic fungi and bacteria. Anyone else agree or
disagree? At the very least you could make your iris foliage look pitted
and spotted from all of the bruising produced by the flying debris.