Cause Unknown. Witches' brooms in hazelnut are an oddity that is seen every few years. Growers are curious about the problem but do not report that it affects production in any way.
Witches' broom symptoms were associated with hazelnuts infected with Candidatus Phytoplasma fragariae in Slovenia but trees with the same pathogen in the UK only had yellowing leaf symptoms.
Symptoms Normal sized limbs will develop a swelling from which a large number of small shoots arise. Shoots are slow growing with shortened internodes and smaller leaves. Brooms can persist for several years, become very large and weigh down affected branches. Similar in appearance to the fungal-induced witches' brooms of cherry or abiotic brooms of Douglas-fir.
Cultural control Remove limbs with the problem.
Reference Mehle, N., Ravnikar, M., Dermastia, M., Solar, A., Matko, B., and Mešl, M. 2018. First Report of 'Candidatus Phytoplasma fragariae' Infection of Corylus avellana (Hazelnut) in Slovenia. Plant Disease 102:2636.