Cause Unknown. Not a common problem but found sporadically on Ennis (1990s), Jefferson (2010s) and Wepster (2020s) trees over the last few decades. Affected trees were neither considered juvenile nor mature but tended to be 5 to 10 years old on average. Affected branches do not yield any specific fungi, bacteria or viruses when sent to OSU Plant Clinic. Trees are not consistently associated with cankers, root rots or insect borers. Growers do not know of any associations with management tactics such as chemicals or irrigation applications. Some guesses to the cause include over-cropping, winter injury, and/or drought. Unknown if symptoms are due to a specific cause or a general reaction hazelnuts have to multiple problems.
There has been some association with poor planting techniques where trees exhibit root girdling from small planting holes or uncut circling roots from when trees were grown in pots. One theory suggests that extensive sucker growth presses in, essentially girdling the main trunk.
Symptoms Whole trees seem to have a thinning canopy and dieback in upper branches. Affected branches have shorter shoot growth, fewer leaves and dead twigs. Leaves are twisted and/or have downward curling at the margins. Defoliation of affected branches is progressive during the growing season. Trees may show slight symptoms the first year but it progressively gets worse in succeeding years with whole branch dieback. Some trees may go on to collapse during summer heat stress. Sucker shoots left to grow up seem to be unaffected. Roots appear healthy on excavated trees.
Initially single trees or small groups occur in scattered areas of the orchard. Other nearby trees may show symptoms in succeeding years. Rarely are whole orchards or areas affected.
Although some aspects are similar, symptoms are not consistent with Filbert Stunt Syndrome (Dilly Disease). Above ground symptoms may be confused with Phytophthora root rot, flat headed borer injury or root girdling.
Cultural control None known or recommended. Some trees might be rejuvenated by retraining a sucker sprout.