Mouse damage to the roots of small fruit trees usually happen during the winter when food sources for these little rodents is in short supply. It can be much worse when there is snow cover. This type of damage oftern goes undetected untill the heat of late spring. Prior to the heat the trees behave like cut flowers, absorbing water through the woody interior of the tree trunk. When the heat hits the lack of roots to take up the water the tree requires becomes critcal and it appears that the tree dies suddenly from some mysterious cause.
Flatten smow cover
use wood not straw
wire around trunk and into soil
Remove vegetation in fall to eliminate cover