Please help with tree problem
This past spring, I planted 11 Monterrey Oak trees on my property outside Austin. I have been watering pretty consistently, with a schedule of once every three days. Each time, I leave a light trickle from the hose onto the base of the tree for about 15-20 minutes. Overall, my 11 trees are doing very well, with pretty consistent leaf growth and generally look better than I would have expected with the drought we've been having.
However, one tree has developed a problem that seems like rot at the base of the tree (extending from the ground up about a foot), with significant discoloration. The picture below shows the problem. There is a layer (about 3") of pine mulch around the base of the tree, but I left a little bare ground at the very base. This particular tree was a 15-gallon, about 12' tall tree planted in the first week of March. Other than this "rot" at the base, the tree appears to be healthy, with fresh leaves appearing pretty regularly.
Any advice appreciated.