Levels of phytoalexins in vine leaves with different degrees of grapevine leaf stripe disease symptoms (Esca complex of diseases)
Grapevine leaf stripe disease (GLSD) is one of the most common diseases in the esca complex. Losses in grape yields and quality caused by GLSD are correlated with the severity of the leaf symptoms. The time course of phytoalexin levels was examined in vine leaves of two vineyards, in leaves of healthy, asymptomatic/diseased and symptomatic vines. Symptomatic leaves were further divided into four categories according to symptom severity: 1, 5% chlorosis; 2, 20% chlorosis; 3, 40% leaf surface covered with tiger stripes; or 4, 65% leaf surface covered with tiger stripes. Leaves were sampled at three growth stages: ‘berries beginning to touch’; ‘berries developing colour’; and ‘berries ripe for harvesting’. The phytoalexins trans-resveratrol, trans-ε-viniferin; trans-δ-viniferin; and trans-pterostilbene were detected and assayed. Patterns of each of the phytoalexins were similar at each growth stage to those found earlier for trans-resveratrol, with an increased phytoalexin levels with increasing leaf symptom severity on the leaf blade, especially at the stages ‘berries beginning to touch’ and ‘berries ripe for harvest’. A laboratory test was also carried out. Petioles of healthy grapevine leaves were immersed in culture filtrates of Phaeomoniella chlamydospora and then dipped in solutions of trans-resveratrol or trans-pterostilbene to assess the effects of these substances on the leaf blades. Adding these phytoalexins did not influence the effects of P. chlamydospora. These results indicated that phytoalexins increased in the leaves after GLSD symptoms appeared, and not before host response (phytoalexin synthesis) to control symptom development.
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