Question
I am located about 80 miles north of Atlanta, Georgia in the Blue Ridge Mountains at approximately 2400 feet elevation.  I sat out Knockout Roses last year and all went very well with them growing and blooming as expected and as others in the area did.  A neighbor friend of ours has Knockout Roses and this is the first noticed of the problem.  Our neighbor went away from their Georgia home for approximately 4 months and we always checked on their home to see that all is well while they are away.  In February we noticed one of their Knockout Rose bushes was laying over at about a 45 degree angle and was wilted extremely.  We propped the bush upright and added soil around the roots and base plus fully watered the plant.  That did no good and the bush died.  This was one bush within a group of 4 bushes.  The other 3 were not affected in any visible way.  Now in the month of May we have observed one of our bushes first wilting at the top  1/3 portion and slowly continuing down through the entire bush over a period of a week.  Further checking the bush as all hope of saving it had passed, we found the root at ground level or within one inch of the ground level had been completely severed.  We found no part of any tap root beneath that area.  No feeder roots were found.  We reset the bush, watered and pruned back to leaving each limb of the bush a length of about 6 inches.  Two days ago we see signs occurring in a second of the bushes.  We uprooted the bush and it has exactly the same signs as the first we had damaged.  We cut this one back, reset it and are hoping it will regrow its roots.  The only way I can describe the damaged root is that it looks as if it was broken off.  There is no evidence of anything eating through the root which is approximately 1 1/2 to 2 inches in diameter.  It just appears as though it was broken.  There were no jagged points nor edges, just a straight across break.  We dug through the soil in the area and found nothing we recognized as to be a cause with special attention towards grub worms or any other type worm, grub or insect that might injure the plant.  These two are within five feet of 3 other knockout roses and they show no adverse signs yet.  The first knockout roses is within about 3 feet of a hybrid rose and that bush is still very healthy.  In questioning some of our friends and neighbors that have knockout roses, several of them are experiencing the same occurrences and problems without any knowledge as to the cause.  
Any help in determining the problem will greatly be appreciated for we are currently at the mercy of whatever the problem is.

Answer
If there were white grubs in the soil then it would be Japanese beetles that are eating the roots. But I don't think so as the damage is too extreme for them. Squirrels, mice, voles and rabbits, all will eat the roots of plants. It isn't the Knock Out roses that is the problem as any rose will be eaten by these pests. The reason that they selected the new Knock Out roses is because the roots would be more tender than a rose that was in the ground for some time. If possible try and look for paw prints around the area. Also dig down and see if there are any tunnels near the roses. If you are aware of the small animals that are in your area naturally, that could give you a clue to what is doing the damage as it is an animal. Any wildlife society would have that information for you.