Two previously healthy 7 year old sweet cherry trees died this winter. Both fruited last year and had consistent 1/4 inch growth rings the last 4 years. They appear to have died from the bottom as the roots were severely compromised - trees were removed from the ground without tools (a couple of people pulling and twisting). Attached are cross section photos taken at graft level (5" diameter) and 12" above the graft (4.5" diameter) showing the upper part of the tree not compromised but what appears to be dark tissue at the graft level (the dark tissue is not spongy or soft). I could find no sign of borer or other insect damage above ground level. The trees were planted on a 15 degree slope in loamy soil so standing water was not possible. Varieties were Lapins and Emperor Francis both on CT500 Mahaleb root stock. Any opinion on possible causes? I am successfully growing a variety of other fruit trees (plum, asian pear, apple) in the same soil.