Discovered this on the trunk of my 5g Matthew while moving it (cold front coming through today and tomorrow) into the garage. It's about 1.5"x1.5" and right at the graft union. I'm hoping that it's just some surface frostbite finally showing up, but I'm worried it's something worse. All my maples were sprayed with phyton 27 (copper-based bactericide/fungicide) 10 days ago, and will be treated again this weekend, once things are back outside. Is there anything else I can/should do? Other than worry myself sick?