one more quote from SLN in case this indeed turns out to be sunscald
PROTECTION AGAINST SUN-SCALD
During the late winter months (especially March) the warm sun will often cause sap to rise daily in the overwintering fruit tree. As temperatures cool toward nightfall, the sap returns to the roots. Sometimes, however, a particularly rapid drop in temperature will “catch” the sap in the trunk, freezing it and splitting the bark. This injury is known as “sunscald”, or “southwest injury”, and is often noticed only months later. To discourage the sap from rising, paint the trunk of the tree, from the base up to the first branches, with whitewash or white interior latex paint (see photo below). The goal is to lighten the bark color so that it will reflect late-winter sunlight and stay relatively cool. Do not use oil-base paints; they do not allow the bark to breathe. The paint job need not be neat and tidy; simply slop it on and it will usually last 1-2 years. Good coverage at the ground level will also discourage tree borers, or at least make their egg slits easier to spot.
Sometimes honeydew from aphids or sap from an injury will drip on the trunk and turn black. This black spot can absorb heat, contributing to sunscald in late winter. Use a small brush with soap and water to wash such deposits off the trunk.