Bacterial Hazelnut Blight

Bacterial hazelnut blight, caused by Xanthomonas arboricola pv. corylina, is among the most damaging bacterial diseases in hazelnut production. The pathogen spreads via water splash and infected pruning tools, moving readily under conditions common during establishment: cool, wet weather in spring and fall.

The infection cycle begins when bacteria enter the plant through natural openings or wounds. Young tissue is most vulnerable. Trees planted in poorly drained sites or areas with restricted airflow accumulate moisture on leaf and stem surfaces, extending the infection window. First- and second-year plantings face the highest mortality risk; established trees tolerate infections more readily but still suffer twig dieback and reduced yield where lesion pressure is high.

Characteristic signs include reddish-brown angular spots on leaves bounded by leaf veins, and black, sunken cankers on current-season wood. Under sustained wet conditions, lesions on small branches can girdle and kill entire shoots.

Management centers on reducing moisture contact with susceptible tissue. Copper-based bactericides applied in fall (after harvest, before leaf drop) and again at bud break provide the best protection window without requiring heavy seasonal spray programs. Timing matters: applications during dry weather improve coverage and reduce phytotoxicity risk. Pruning should be deferred until dry periods, and cuts made with disinfected tools between every tree.

Sourcing certified, pathogen-tested nursery stock is the single highest-leverage intervention for new plantings. No commercially dominant resistant varieties are currently documented for this pathogen, though extension breeding programs in Oregon and Washington are actively screening hazelnut selections for tolerance.