Dead, curled leaves dangling by fungal “threads” are typical of a disease called thread blight. This disease is more common in eastern Kentucky, where it has been observed on apple, cherry, and viburnum. Other potential hosts include cotoneaster, dogwood, gooseberry, and rose. Unlike fire blight, which kills branch tips, thread blight begins on interior portions of trees (Figure 1).
Thread Blight Facts
- Caused by the fungus Corticium stevensii (formerly Ceratobasidium stevensii).
- Infected leaves wilt, turn brown, and remain attached to branches by a network of fungal strands (rhizomorphs).
- Silvery-tan rhizomorphs (aggregation of thread-like fungal structures) and tan to brown sclerotia (fungal overwintering structure) develop on the surface of branches (Figure 2) and fruit.
- Disease is favored by moist, shady conditions.
- Generally not a problem in well-managed apple orchards where a fungicide program is followed.
To prevent thread blight
- Selectively prune branches to improve air circulation and sunlight penetration within trees.
- Orchardists should follow a fungicide spray program. Thread blight can be managed with fungicides beginning in mid-June when the fungus becomes active; studies have shown that Merivon, Pristine, and Topsin-M can help reduce disease incidence and severity when used as preventatives.
- Avoid planting apple and susceptible landscape plants in low lying, shaded locations.
When thread blight is present
- Once established in an orchard, thread blight can be difficult to eliminate due to long term survival of the sclerotia.
- Where disease occurrences are minimal, prune and destroy infected branches.
By Cheryl Kaiser, Plant Pathology Extension Support and Nicole Gauthier, Plant Pathology Extension Specialist