The fungus that causes Oak Wilt can spread in one of two ways.
Overland Sap-Feeding Insects
First, the fungus can be spread overland through various sap-feeding insects. Infected trees in the red oak family have the potential of producing spore mats the year following infection (the white oak family of trees do not produce spore mats). Certain insects are attracted to the fruity aroma produced by these mats.
As insects contact the spore mat, fungal spores can stick to their bodies; when these insects move to a freshly wounded oak tree to feed on the sap, the fungal spores can rub off and infect the new tree.
Root Grafts
The second way that oak wilt spreads is through grafted (connected) root systems of individual trees. It is not at all uncommon for oak trees of the same species (i.e. red oak to red oak or northern pin oak to northern pin oak) to have a “joint” root system with an adjacent tree(s). If one tree is infected with oak wilt, the fungus can move through that tree’s root system and into the healthy tree’s root system, resulting in another infected tree.
Root graft transmission is the most common mode of infection. Over 90% of all new oak infections are transmitted in this manner. A root graft is formed when the roots of the same species meet and fuse together. The disease is then able to move from the infected tree into the uninfected tree.