See:
Maple (Acer spp.) - Nectria Canker
Notes Nectria fungi invade trees through wounds and injuries including winter injury, drought damage, sunscald, leaf scars, cracks in the twig axil, or senescent low branches. Cankers appear as slightly sunken areas generally associated with wounds. Cankers are not obvious until striking, orange-red, fungus-fruiting structures (perithecia) develop in large numbers in the cankered area. Depending on the fungi involved, an elongate or elliptical canker may form year after year. Remove and destroy affected twigs and limbs. Avoid leaving pruning stubs when removing rootstock top growth. Make a clean cut with a sharp tool, which will heal promptly.