Nutrients and defense infection as opportunities for the reduction of foliar symptom expression in Esca complex of grapevine

The esca complex is the most important, widespread and destructive trunk disease in the grape growing areas of Europe, and is important to viticulture worldwide. The appearance of the typical foliar symptoms was, in the last decade, described as the Grapevine Leaf Stripe Disease (GLSD). Several studies agreed on the involvement of toxins produced by tracheomycotic fungi such as Phaeomoniella chlamydospora and Phaeacremonium spp. in determining the appearance of the typical leaf necrosis symptoms. Nevertheless, the occurrence and development of foliar symptoms is a complex process associated with different factors resulting in a decrease in chlorophyll, activation of defense responses and changes in metabolic patterns in leaves. No correlation was found between the severity of wood deterioration and the severity of leaf symptoms; leaf symptoms were found on vines that had only wood discoloration without any decay. Other studies stated that white rot in the cordons was the best predictor for the so called "chronic form" of esca. However, foliar symptom expression is strictly correlated with grape yield and quality reductions. Strategies aimed at reducing the incidence and/ or severity of foliar symptoms would also limit losses in quality. The use of fungicides failed to provide satisfactory results, except for fosetyl-Al formulations (applied against downy mildew), which reduced the incidence of symptomatic plants and cumulative vine mortality. Recent studies suggest that host physiology and defense mechanisms, which in turn are correlated with climate, nutrition and agronomic conditions, play important roles in symptom development. Three-year trials have demonstrated that foliar applications of a mixture of calcium chloride, magnesium nitrate and Fucales seaweed extract led to a significant reduction of foliar symptoms. Treated vines showed increased content of trans-resveratrol and flavonoids, and accumulation of calcium oxalate in crystal druses in the leaf mesophyll tissues. Ongoing trials with products based on copper, zinc and formulated with effective and/or innovative substances, such as hydracid of citric acid or hydroxyapatite crystals, also reduced foliar symptom expression and open new perspectives and opportunities for disease control. The potential of an approach that stimulates the host reaction in reducing interveinal leaf yellowing and necrosis is discussed.