Disentangling the cascading effects of Grapevine Red Blotch Virus infection on vine physiology

Grapevine Red Blotch Virus is a major grapevine pathogen and is associated with reduced carbon assimilation and delayed berry ripening in Vitis vinifera L. Recent work suggests that the virus alters leaf carbon metabolism prior to emergence of visible symptoms. Therefore, diurnal and seasonal measurements were conducted to quantify changes in leaf carbon balance and to elucidate the chronology of symptom progression in leaves and fruit. Healthy and infected vines were assayed in a commercial vineyard during which leaf water relations, photosynthesis, and nonstructural carbohydrates were measured. Additionally, sugar and anthocyanin accumulation in the fruit were monitored at the end of the season to characterize the impact of the virus on ripening. Virus infection significantly reduced carbon assimilation pre- and postveraison, but the impact was more pronounced postveraison and during the afternoon when vine water status was the lowest. Similarly, virus infection significantly increased leaf starch concentration pre- and postveraison, but increased leaf starch in infected vines was detected two weeks prior to veraison. Virus infection had the greatest impact on obstructing leaf carbon export postveraison, especially during the afternoon. The virus had no impact on chlorophyll fluorescence, indicating there was no sustained photosystem impairment and suggesting that changes in chlorophyll fluorescence were a transient response to reduced carbon assimilation and export. This study provides evidence that reduced carbon export constitutes a feedback inhibition response to accumulation of leaf starch prior to the appearance of visible symptoms or impacts to ripening, which may aid earlier detection of the virus.