Trunk Surgery as a Tool to Reduce Foliar Symptoms in Diseases of the Esca Complex and Its Influence on Vine Wood Microbiota

Abstract

In the last few years, trunk surgery has gained increasing attention as a method to reduce foliar symptoms typical of some of the Esca complex diseases. The technique relies on the mechanical removal of decayed wood by a chainsaw. A study on a 14-year-old Cabernet Sauvignon vineyard was carried out to validate the efficacy of trunk surgery and explore possible explanations behind it. Three levels of treatment were applied to three of the most characteristic symptoms associated with some diseases of the Esca complex, such as leaf stripe symptoms (LS), wilted shoots (WS) and apoplexy (APP). The most promising results were obtained by complete trunk surgery, where the larger decay removal allowed lower symptom re-expression. According to the wood types analyzed (decay, medium and sound wood), different changes in microbiota were observed. Alpha-diversity generally decreased for bacteria and increased for fungi. More specifically, main changes were observed for Fomitiporia mediterranea abundance that decreased considerably after trunk surgery. A possible explanation for LS symptom reduction after trunk surgery could be the microbiota shifting caused by the technique itself affecting a microbic-shared biochemical pathway involved in symptom expression.

White Rot Fungi (Hymenochaetales) and Esca of Grapevine: Insights from Recent Microbiome Studies

“…Among the numerous points in favor of culture-independent methods, NGS studies allow insights over unculturable and rare taxa, largely overcoming the cultural methods bias, and provide reasonable estimates for microorganisms' relative abundances [75]. Over the last three years, four NGS-based studies investigated the microbial ecology of esca proper-affected plants [76][77][78][79]. We hereby report some of the main findings and later discuss how they improve the state-of-the-art, over the involvement of white rot fungi in esca symptoms in leaves.…”

“…In Italy, Pacetti et al [79] studied the microbiome of plants before and after trunk surgery treatment, a technique that consists in removing white rot by mechanical means. The authors reported leaf symptoms remission after fully removing white rot from previously symptomatic plants.…”

“…These interesting results led the authors to state that 'the Fmed activity in wood colonization had a relevant role in foliar symptom expression in esca proper-affected vines'. However, the authors also suggest that some wood degradation mechanisms could be in common with agents of different wood symptoms and degradation [79].…”

“…As suggested by Bruez et al [78], competition-induced metabolites may be good candidates for explaining leaf symptoms. Hypothetically, environmental perturbations in the plant-fungi interaction dynamics (with a possible involvement of bacteria [77,79,82]) may trigger the release of such chemicals that reach the canopy and lead to tiger stripes. This scenario would explain, to some extent, the reasons behind annual symptoms discontinuity and the influence of abiotic factors such as climatic conditions (e.g., spring rain and/or heatwaves, [83][84][85][86][87]), as well as the difficulties in replicating tiger stripes by artificial inoculations.…”

“…Today, we know that this chemical affects the plants physiology and microbial ecology, successfully preventing leaf symptoms' manifestation [15,78]. Similarly, trunk surgery, an ancient practice recently rediscovered, is capable of strongly reducing the expression of leaf symptoms for several years after treatment [18,79]. The common ground between these two different treatments lies in their drastic effect on Fmed (the former) and white rot (the latter).…”