Oak wilt is a deadly oak tree disease caused by the fungus Bretziella fagacearum. The fungus clogs up the xylem tissue, blocking the flow of water and nutrients from the roots to the crown, which causes the leaves to wilt and die. Red group oaks (scarlet, pin, red, etc.) can die within a few weeks to six months while white group oaks (bur, swamp white, white, etc.) often take years to die. Infections spread in two ways: belowground through grafted roots or aboveground by beetles.