Evaluating the Effects of Grapevine Red Blotch-Associated Virus on Symptom Development and Fruit Maturity

Red varieties of grapevines with leafroll-like symptoms that are not infected with leafroll-associated viruses have been found infected with grapevine red blotch-associated virus (GRBaV), a new virus first identified in 2011 and subsequently shown to be the causal agent of red blotch disease. Diseased vines have been identified in several counties in California and in other states. Effect of GRBaV infection on differences in berry composition over the ripening period have not been documented and foliar symptom development in red and white varieties has also not been characterized. A study was conducted in 2013 to clarify symptom development in foliage, fruit maturity and vine growth in Chardonnay, Cabernet Sauvignon, and Merlot. At each of the three sites, vines selected for the study were determined to be GRBaV positive or negative by qPCR assay as well as negative for all leafroll-associated viruses, vitiviruses and nepoviruses. To determine the effect of crop load on disease expression, crop was reduced at two sites by approximately 35{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} at the onset of veraison.

Foliar symptom expression in vine canopies increased with time and is greatest in older leaf tissue. The severity of foliar symptoms varied greatly across the three varieties; however, on all evaluation dates, vines positive for GRBaV had significantly greater percentage of symptomatic leaves in the basal and middle regions of canopies. In addition, symptomatic leaves in these regions had larger area (greater symptom severity) with red or chlorotic coloration in red and white varieties, respectively, than younger terminal leaves. At harvest, the severity of interveinal chlorotic blotch symptoms in Chardonnay was greater than the red blotch symptoms in Cabernet Sauvignon or Merlot. In Cabernet Sauvignon and Merlot, initial leaf symptoms in GRBaV positive vines were predominately leaves on which only red veins were present. In Merlot, the development of leaves with red interveinal tissue followed. In Cabernet Sauvignon, blades with only red veins remained the primary leaf symptom. In addition, reducing the crop at veraison in Cabernet Sauvignon may have resulted in an increase in virus symptom expression late in the growing season.

This project has allowed us to associate the presence of GRBaV infection with a consistent delay in fruit maturity. For all three cultivars, fruit maturity was delayed in vines PCR positive for GRBaV. Brix was significantly lower on all sample dates and titratable acidity significantly greater on half the sample dates yet always elevated on other dates. At harvest, juice samples in all varieties had significantly higher malic acid in GRBaV positive vines as compared to GRBaV negative vines. Reducing crop load in GRBaV positive vines in Chardonnay and Cabernet Sauvignon did not significantly improve juice chemistry at harvest when compared to infected vines with full crop loads. Juice from fruit on Cabernet Sauvignon positive vines in which crop was reduced indicated a very slight improvement in ripening parameters although differences were not statistically significant. Red blotch disease did not affect vine yield. Berry weights in vines infected with GRBaV are at least as great as virus negative vines.

Effects of Pre- and Post-Harvest Practices on the Replenishment of the Nitrogen Reserve Pool in the Permanent Structures of Grapevines

The objective of this study was to quantify the amount of N remobilized and/or taken up from the soil or N fertilizer and put into the N reserve pool (within the trunk and root system) of field-grown grapevines.  This study also determined the effectiveness of a post-harvest N fertilization application on the dynamics of the reserve N pool.  Nitrogen within whole vines was quantified using destructive harvests.  Several treatments were imposed to assess their effect on the replenishment of N reserves independent of remobilization.  They included: 1.) The application of N during the growing season, 2.) The application of N post-harvest and  3.) Fifty percent of the leaves in the canopy removed after harvest to mimic the effects of mechanically harvesting a vineyard.

Petiole NO3-N at bloom for the non-fertilized treatment averaged less than 100 ppm (dry weight), a value many consider to indicate a N deficiency.  The application of a nitrogen fertilizer one month after budbreak (albeit at only ½ the total amount applied) significantly increased NO3-N, NH4-N and total N of the petioles compared to the no N treatment.  The NO3-N and NH4– petiole values measured at bloom were in excess of 2500 and 1800 ppm, respectively.  These values would be considered excessive by some.

A N budget for vines in the fertilized (‘+N’) and non-fertilized (‘–N’) treatments was determined at fruit maturity and at the end of the season (after leaf fall).  While it can be surmised from the previous paragraph that the vines in this vineyard may have been N deficient, vines from the ‘–N’ treatment still accumulated 58.9 g N/vine (78 kg N/ha; 69 lbs N/acre) in the leaves, stems, fruiting canes and clusters at fruit maturity with 82.2 g N/vine (109 kg/ha; 97 lbs N/acre) in the trunk and roots.  The amount of N in the leaves, stems, fruiting canes and clusters at fruit maturity for the ‘+N’ treatment was 72.9 g N/vine (96.5 kg/ha; 86 lbs/acre) while that in the trunk and roots was 103 g N/vine (136 kg/ha; 121 lbs/acre). N fertilizer recovery efficiency (REN) can be determined by comparing the uptake of N in plants with that of a non-fertilized treatment.  Vines in the ‘+N’ treatment accumulated 35.1 g N/vine more than that of the non-fertilized control at fruit maturity.  Since 34 g N/vine was applied to the fertilized vines the REN would be ~100{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}.

The REN was also calculated for data collected at the end of the season, subsequent to leaf fall.  The non-fertilized control would be another ‘–N’ treatment cohort of vines while the fertilized treatment would be the ‘PH +N’ treatment.  The amount of N taken up from the soil by the ‘PH +N’ treatment was 7.1 g/vine greater than that of the ‘–N’ treatment.  Since the ‘PH +N’ treatment was fertilized with 25.5 g N/vine, the REN would be ~ 28{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} ((7.1/25.5) * 100).  Therefore, under the conditions of this study, a fertilizer application one month after budbreak and at berry set was more efficient than the post-harvest application of N.