Hazelnut (
Corylus avellana
) is cultivated on 118 ha and ranks eighth in Slovenian fruit growing production, representing 2.8% of the total area of fruit plantations in the country. However, decline ...of some of the trees appeared in 2012 in two plantations located in eastern Slovenia. Together these orchards cover 5 ha, with around 1600 trees planted 12 to 15 years ago. By October 2018, ~12% of these trees had died, and an additional 12% showed decay symptoms. The dead and dying trees were scattered throughout both orchards, with no apparent pattern. The most affected cultivar was ‘Istrska dolgoplodna leska’. Using molecular diagnostic methods, we showed infection of symptomatic trees with three unrelated phytoplasmas: ‘
Candidatus
Phytoplasma fragariae’, of the 16SrXII-E phytoplasma subgroup, and phytoplasma of the 16SrV and 16SrIX groups. In 2018, the presence of ‘
Ca
. P. fragariae’ and/or phytoplasma of 16SrV group were confirmed in decayed hazelnut trees in eastern, north-eastern, central, south-eastern and western Slovenia. ‘
Ca
. P. fragariae’ has also been detected in a forest in south-western Slovenia, for
Acer campestre
,
Carpinus betulus
,
Crataegus laevigata
,
Fraxinus ornus
and
Quercus petraea
. All infected forest trees showed unusual dense proliferation of sprouts from roots and/or trunks. Molecular characterisations of partial
16S rRNA
,
secY
,
map
and ribosomal protein genetic locus of hazelnut 16SrV phytoplasma isolates show that they are identical to isolates that can cause grapevine flavescence dorée disease. Here, the results of our recent study and the open questions on this burning issue for hazelnut production are presented.
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•WHF-infested kernels turn from bright colour to brown, dark brown and even black.•Infested kernels are less aromatic, crispy and oily, but more bitter than healthy ones.•Phenolic ...contents of the kernels decrease due to WHF infestation.•Hydrolysable tannins and TPC closely correlate with kernels' sensory properties.•The response to the infestation is cultivar and infestation-time related.
Walnut husk fly (WHF) is the main pest of common walnut. Its negative impact on external fruits’ properties is frequently reported, while inner kernel quality loss remains unknown. Brown (Bw), dark brown (Db) and black (Bl) kernels, infested by WHF were compared with non-infested bright kernels (B) in ten walnut cultivars in order to see how much sensory attributes of the kernels and their phenolic contents are affected due to WHF infestation. The changes of phenolic content and intensity ratings of sensory traits significantly depended on the WHF-infestation time and the cultivar. Infested kernels were less aromatic, crispy and oily, having a 1.2-fold (Db) to 5.5-fold (Bl) lower total phenolic content (TPC) than bright kernels. Besides TPC, ten compounds belonging to phenolic acids (PA), flavanols (FL) and hydrolysable tannins (HT) as the predominant group were quantified in the kernels of all colour classes. HT and TPC showed the strongest impact on the kernels’ sensory properties. Correlation between sensory attributes and phenolics increased with kernels’ darkening. In Db kernels, TPC defined bitterness, HT were a key source of aroma, oily taste and overall walnut flavour, whilst PA and FL closely correlated with the perception of crispness. An effictive protection, particularly against early-season WHF infestation, which resulted in dark brown and black kernels, is essential in order to avoid the loss of inner quality of walnut kernels.
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