A field experiment with two irrigation treatments – wet (W) and dry (D) – and two vine disease statuses – healthy (RB-) and infected (RB+) – was initiated in a commercial vineyard to understand the interaction between GRBV infection and deficit irrigation on disease development, vine productivity, and fruit quality. Irrigation treatments were imposed by varying the number of drip emitters per vine. W vines were irrigated at 100% of crop evapotranspiration (ETc), while D vines received water at 66% ETc. Within each irrigation treatment, RB- and RB+ vines (split-plot) were identified based on symptomology data from 2016. The identified vines were confirmed for GRBV positive and negative by PCR-based assays.
Measurements of vine water status (midday stem water potential; Ψstem) were made at regular intervals throughout the growing season beginning just after berry set until just before harvest. Disease severity was recorded every week after the first symptom appearance was observed on RB+ vines. At harvest, berry samples were collected for berry size and compositional analyses; and vine yield and yield components were determined.
There was no significant interaction between irrigation treatment and disease status on disease progression and severity. There was a significant interaction between irrigation treatment and vine disease status on Ψstem, but it depended on phenology. In other words, preveraison Ψstem was not affected by disease status but was significantly higher in RB+ vines postveraison. The higher Ψstem in RB+ vines resulted in larger berries and yield at harvest. Interestingly, RB+ also had greater clusters per vine and berries per vine in the W, but few of the differences in yield and yield components among treatments were significant.
Differences in juice composition among treatments were smaller than previously reported, but all treatment effects were more pronounced on berry secondary metabolites in skins and seeds. In juice, Brix was only slightly impacted by the experimental treatments, but there were no significant differences among treatments in juice pH or TA. In skins and seeds, significant differences among treatments were observed in both concentration and content of some secondary metabolites (anthocyanins and iron-reactive phenolics (IRPs)), but not others (tannins). Irrigation treatment had a greater effect than disease status on anthocyanins, while the converse was observed with respect to tannins and IRPs. Differences in secondary metabolism were most pronounced in seeds, which is likely related to the inhibition of ripening commonly observed in RB+ fruit.
Small differences among treatments in tannins coupled with the large differences in IRPs suggests that GRBV inhibits biosynthesis of flavonols (such as quercetin), which are the dominant class of non-iron-reactive phenolics, particularly in seeds. Since flavonols are implicated in wine mouthfeel, this may offer some explanation as to why GRBV-infected fruit produces lower quality wines. Taken together, these results suggest that keeping vines well-watered may mitigate some of the negative effects of GRBV infection, but ultimate changes in 2 secondary metabolism due to GRBV infection may necessitate using infected fruit for different wine programs (e.g. rosé and/or sparkling) or blending with lots from healthy vineyards.