Two field experiments were established to investigate the effects of grapevine red blotch disease (GRBD) on flavor and flavor precursor formation in the grape berry and on resulting wine quality. The two objectives of the overall study were to 1) investigate the effect of GRBD on grape berry development with a specific focus on flavor and flavor precursor formation; and to 2) investigate the effect of GRBD on wine quality. Both experiments were located in the same vineyard located near the town of Jacksonville, OR. In both experiments, data vines were identified by visual disease symptoms (or lack thereof), and disease status was confirmed using PCR-based assays in Dr. Achala KC’s laboratory at SOREC.
To evaluate the response of flavor and flavor precursor compounds to GRBV infection during berry development (objective 1), clusters from GRBV+ and GRBV- vines were sampled weekly beginning from just before veraison through to commercial harvest. Vine water status, berry growth, and development were also monitored in those plots subjected to different irrigation treatments. Vine water status was monitored by measurements of midday stem water potential (Ψstem). Results showed that there was no significant interaction between irrigation treatment and disease status on Ψstem. However, there were significant effects of irrigation treatment and disease status on Ψstem independently. Berry size (fresh weight; FW) was consistently higher in GRBV+ vines, significant differences in TSS between GRBV+ and GRBV- vines were observed. There were no significant differences in berry pH between vines of different disease status over the entire course of berry development. Berry titratable acidity (TA; g L-1) were lower in GRBV+ fruit. These responses were only observed after veraison, but they are not as consistent. Flavor and flavor precursor analysis in the grapes is underway.
To evaluate the response of wine quality to GRBV infection (Objective 2), replicate wines were produced from field plots under the supervision of Dr. James Osborne using a standard protocol. Wines were analyzed for volatile aroma compounds using different techniques including headspace-GC-FID, solid-phase microextraction gas chromatography-mass spectrometry (SPME-GC-MS) and stir bar sorptive extraction-gas chromatography-mass spectrometry method (SBSE-GC-MS), stale-isotope compounds were used as internal standards for accurate analysis. Monomeric anthocyanin and total phenolic contents in wine were also analyzed. Results showed that the RB negative wines under irrigation condition have the highest level of monomeric anthocyanin than other three groups. Total phenolic content varies in wines with different irrigation conditions. Fermentation derived aroma compounds did not show any difference between RB+ and RB-, nor free form grape-derived aroma compounds. Since the free form of grape derived aroma compounds only exist in a small portion, and the majority of these compounds exist in the bound form, analysis is underway for the bound form of volatile flavor compounds in the wine.