Oak Wilt: Ceratocystis fagacearum Spread, Diagnosis, and Management

Oak wilt occurs west of the Susquehanna River in Pennsylvania. Fungus Ceratocystis fagacearum attacks most oaks, especially red oak group (Quercus rubra; northern pin Q. ellipsoidalis; shumard Q. shumardii). American, Chinese, European chestnuts, tanbark oak, bush chinquapin also susceptible. White (Q. alba) and bur oaks (Q. macrocarpa) less susceptible. Susceptible trees die in weeks; resistant may decline 2–3 years or recover.

Symptoms

Leaves at top brown at tips/margins, wilt, fall while still some green. Progresses down tree. Twigs/branches die; brown streaks in outer sapwood. Cut twig ends: outermost annual ring may be completely brown. Fungal mat develops under bark, erupts in spring.

Spread

1. Sap-feeding and bark beetles feed on spring fungal mat, pick up sticky spores, spread short distances to adjacent oaks feeding on fresh bleeding wounds. Fresh wound required for invasion. Beetles not thought to spread long distances.
2. Fungus viable under firmly attached bark. Transport of infected logs moves fungus long distances.
3. Most important local spread: roots naturally grafted to infected tree.

Management

First: obtain positive diagnosis that oak wilt is the cause.

Break root grafts to nearby oaks before removing infected tree. Trench or fumigate midway between oaks within 50 ft of infected oak, to 3 ft depth. Fumigation best at soil temp ≥50°F.

After root grafts disrupted, remove infected trees. Bury, burn, or debark logs and stump. Do not stack or transport any wood with firmly attached bark—insects may leave and carry fungus.

Do not prune oaks in late spring or summer—creates fresh wounds attractive to fungus-carrying insects. Prune only November through mid-April.

Trees with <30% crown affected: temporary remission via fungicide injection. Fungicide doesn't kill fungus in roots; root grafts must still be disrupted. Oaks in high-risk areas not yet infected: protect by fungicide injection every 2 years.

Prepared by Gary W. Moorman, Professor of Plant Pathology.