Spatial and temporal distribution of ceratocystis fagacearum in roots and root grafts of oak wilt affected red oaks

The spatial and temporal distributions of the oak wilt fungus, Ceratocystis fagacearum, in the roots of symptomatic and adjacent asymptomatic northern red oaks (Quercus rubra) were investigated. Root systems of 12 pairs of such trees were exposed to 1 m (3.3 ft) depth using an air excavation tool. In only one case was the fungus found in more than two cardinal directions of symptomatic trees based on isolation attempts from 192 root segments excised from 1.2 m (3.5 ft) lengths of primary roots extending in four cardinal directions from the root collar. Many (162) self grafts occurred in all oaks. Few (13) inter-tree grafts were found in less than one-third of the tree pairs. The fungus was isolated from the roots involved in two of thirteen inter-tree grafts and 14 of 62 self grafts assayed. The sporadic and unpredictable distribution of the fungus in roots of wilting red oaks supports current control approaches that assume the fungus is present in roots extending out in all directions once crown symptoms are evident. Additionally, self grafts may serve as a conduit for circumferential movement of the fungus around the main stem, but outside the root collar, through the interconnected roots.