Grapevine responses to red blotch disease – a structural-functional perspective of symptomatology development and fruit quality

Abstract

Context and purpose of the study – Red Blotch disease caused by Grapevine red blotch-associated virus (GRBaV) is a severe  concern to grape growers and winemakers in major grape-growing regions worldwide. One key aspect of all viruses, including Red Blotch, is their intimate association with cell components and anomalous structures following infection. Therefore, the objective of this study was to analyze symptomatology, vine function, fruit quality and ultrastructure of various tissues and document the relationship of ultrastructural cytopathology with the GRBaV infection in Pinot Noir and Merlot employing various microscopy techniques. Such knowledge is fundamental to understanding the progression and the mechanisms by which the virus causes the infection, and designing strategies to control its spread in vineyards.

Material and methods – During the growing season, vine samples were collected from vineyards with a history of Red Blotch (Pinot Noir and Merlot) located in the states of Washington and Oregon. Starting at flowering, shoots (leaf and stem tissues) were sampled for microscopy analysis. These samples were used to determine the structure and functionality of the vascular strands (xylem and phloem) using callose-specific dye, aniline blue, and various microscopy techniques. At harvest, fruits were sampled to compare primary and secondary metabolites between healthy and infected vines.

Results – The infected vines exhibited typical red blotches in leaves with pinkish-red-colored veins without rolling off the margins at the onset of ripening. The infected vines developed clusters of hens and chickens and altered seed morphology.  Conversely, the healthy seeds were pyriform with a distinct beak. The infection significantly altered the primary and secondary metabolites desired for making wine. Since post-veraison berry development and ripening rely on phloem influx, the altered metabolism was indicative of a disruption of the phloem pathway either in the source leaf or in the berries. While the infected vines maintained the primary leaf anatomical organization, the chloroplast underwent significant ultrastructural changes ranging from the complete dismantling of the chloroplasts to massive accumulation of starch and plastoglobuli development in addition to tannins in the cytoplasm. The study demonstrated that structural integrity is a key to maintaining the normal metabolism of the grapevine, providing new insights into implementing innovative approaches for Red Blotch disease management.