Tony K. Wolf

Spatial and Temporal Distribution of North American Grapevine Yellows Disease and of Potential Vectors of the Causal Phytoplasmas in Virginia

Abstract North American grapevine yellows is a destructive, phloem-limited disease of winegrapes in the mid-Atlantic and northeastern regions of the United States caused by at least two phytoplasmas (aster yellows and X-disease Candidatus phytoplasma spp.). Because insects have been identified as vectors of grapevine yellows in several countries, one or more insect vectors are suspected of transmitting the phytoplasmas in North American vineyards. Adding to evidence in support of insect transmission of North American grapevine yellows were observations that diseased vines often occurred in clusters frequently near vineyard edges that bordered woodland vegetation, suggesting movement of insect vectors into vineyards. The spatial distribution of North American grapevine yellows-affected vines was annually assessed in two northern Virginia vineyards over an 8-yr period. Spatial analysis indicated that diseased vines were significantly clustered, or their distribution was nonrandom, in both vineyards. Clustering was observed more frequently in the larger of the two vineyards. In addition, we surveyed the abundance of leafhoppers and planthoppers in and around three North American grapevine yellows-affected Chardonnay vineyards. The relative abundance of captured candidate vector species in and around North American grapevine yellows-infected vineyards differed among sampling methods. Agallia constricta was the most commonly captured leafhopper in sweep sampling in all years. Several species known to transmit phytoplasma were also captured, including Scaphoideus titanus, a grapevine yellows vector in Europe, Graminella nigrifrons, and Deltocephalus flavicosta. Three leafhopper species, S. titanus, Osbornellus auronitens, and Jikradia olitorius, exhibited seasonal movement into the vineyard that may account for the observed clustering of diseased vines.

Cover Crop, Rootstock, and Root Restriction Regulate Vegetative Growth of Cabernet Sauvignon in a Humid Environment

Cover crops, rootstocks, and root restriction were evaluated as means to regulate vegetative growth of Cabernet Sauvignon grapevines in a humid environment. Treatments were arranged as a strip-split-split plot with row-middle and under-trellis cover crop (UTCC) compared to row-middle only cover crop combined with 85 cm weed-free strips in the vine row as main plots. Rootstocks Riparia Gloire (Riparia), 420A, and 101–14 were subplots, while sub-subplots comprised two treatments: vines were either planted in root-restrictive (RR) fabric bags (0.015 m3) at vineyard establishment or were planted without root restriction. Root restriction and UTCC were independently effective in suppressing vegetative development as measured by rate and seasonal duration of shoot growth, lateral shoot development, trunk circumference, and dormant pruning weights. Riparia was the most effective rootstock in limiting vegetative development among the three evaluated; vines grafted to Riparia had ~25% lower cane pruning weights than did vines grafted to 420A or 101–14. Under-trellis cover crop reduced cane pruning weights by 47% relative to vines grown on herbicide strips. Canopy architecture was generally improved by both UTCC and by root restriction, but generally unaffected by rootstock. Root restriction reduced the discrimination against 13C assimilation in both berries and leaf laminae tissue as measured by δ13C, while under-trellis floor management did not affect this measure of chronic water stress. The principal direct effect of the UTCC and the root-restriction treatments was a sustained reduction in stem (xylem) water potential (ψstem). Stomatal conductance and net assimilation rate were depressed by increasing water deficit, particularly for root-restricted vines. Results suggest practical measures can be used to create a more favorable vine balance under conditions of variable rainfall, such as exist in the eastern United States.

Cover Crop and Root Pruning Impacts on Vegetative Growth, Crop Yield Components, and Grape Composition of Cabernet Sauvignon

Complete vineyard floor cover cropping (inter- and intrarow) and vine root pruning were evaluated as tools to restrict vegetative growth of Vitis vinifera cv. Cabernet Sauvignon grapevines. Treatments were arranged in a split-plot, randomized, complete block design with cover crop schemes as main plots and annual vine root pruning (RP), or not (NRP), as subplots. Five perennial grasses as complete floor cover crops were compared to a more conventional under-trellis herbicide strip combined with KY-31 fescue interrows. KY-31 fescue and orchardgrass each reduced shoot growth rate by >30% in 2006 and >20% in 2007, below that of the herbicide-strip control vines. Root pruning independently reduced shoot growth rates. The combination of cover crop and RP decreased dormant pruning weights more than did the additive effects of either factor applied alone. Pruning weights in 2010 were reduced 8% below the control by RP, by 15% by cover crop, but by 38% when both treatments were applied. Leaf petiole N concentration at bloom was ~11% lower in RP vines in two of three years evaluated, but did not differ among vines grown with different cover crops. Stem water potential (Ψstem) was not affected by treatments. Cover cropping did not reduce crop yield with the exception of reduced yield due to KY-31 fescue in 2006. Berry weights were slightly reduced by a RP × year interaction from 2007 to 2009 and by a year effect in 2011 compared to 2010. While complete vineyard floor cover cropping and root pruning were effective tools to reduce vine size and vigor, effects on canopy architecture and primary fruit chemistry were minimal and more influenced by seasonal variation.

Under-Trellis Cover Crop and Rootstock Affect Growth, Yield Components, and Fruit Composition of Cabernet Sauvignon

We compared the effectiveness of an under-trellis cover crop treatment (CC, Festuca rubra) and an 85-cm-wide herbicide-treated strip treatment (HTS) and rootstock (101-14, 420-A, or riparia) for regulating vegetative growth and improving fruit composition in Cabernet Sauvignon over a six- to seven-year period of data collection. The cover crop reduced average vine pruning weight by 26% and increased fruit exposure by 35% compared to the HTS floor management scheme. Vines grafted to riparia rootstock had pruning weights that were lower than those of vines grafted to 101-14 in two years; however, effects of rootstock were insignificant when averaged over the seven-year experiment. There was a measurable crop yield penalty for the CC floor management: vines grown with CC averaged 610 kg/ha/year lower crop yields, and had reduced cluster weight, berries per cluster, and berry weight compared to vines in the HTS floor management. Riparia rootstock increased berry weight, cluster weight, and crop yield per vine compared to 101-14. Fruit from vines grown with CC averaged greater soluble solids compared to vines grown with HTS. Rootstock 420-A produced fruit with lower pH than fruit from 101-14 or riparia vines. Compared to 420-A, inconsistent increases in grape anthocyanins and phenolics were observed in riparia (both) and 101-14 (phenolics only). The use of an under-trellis cover crop favorably reduced vine size, especially during the earlier years of the experiment.

Leaf Removal Effects on Cabernet franc and Petit Verdot. I. Crop Yield Components and Primary Fruit Composition

As part of a broader study of the effects of fruit exposure on fruit composition, this study aimed to evaluate how the timing and extent of leaf thinning impacted crop yield components and primary fruit composition of Cabernet franc and Petit Verdot. Treatments consisted of three post-fruit set leaf/lateral shoot removal treatments: (i) no leaf removal (NO); (ii) leaf removal from opposite the basal primary cluster and the node directly above (MED); and (iii) leaf removal from the node directly above the distal primary cluster down to the cordon (HIGH). An additional treatment applied immediately prebloom (P-B) involved removal of leaves and lateral shoots from the basal six nodes of primary shoots. Post-fruit set leaf removal had modest or no effects on crop yield components, whereas prebloom leaf removal reduced most crop yield components in both cultivars relative to the NO treatment. In both cultivars, P-B reduced crop yield by an average of 49 to 50% over two years when compared to NO, and that reduction was primarily driven by reduced cluster weight. Prebloom leaf removal implemented on the same vines in two consecutive seasons caused further reductions in crop yield in both cultivars. Both HIGH and P-B tended to depress juice Brix in Petit Verdot but not in Cabernet franc, and HIGH reduced titratable acidity more consistently than did P-B. Fruit zone leaf thinning after fruit set may balance the competing goals of improving fruit exposure, while avoiding precipitous reductions in crop yield.

Leaf Removal Effects on Cabernet franc and Petit Verdot. II. Grape Carotenoids, Phenolics, and Wine Sensory Analysis

The rationale for fruit zone leaf thinning in a humid environment is often driven more by the need for disease management than by anticipated improvements in fruit composition. Fruit zone management is often applied uniformly over climatically diverse regions, with little regard for interaction with ambient temperature or radiation. We evaluated how the timing and magnitude of fruit zone leaf and lateral removal altered fruit composition and wine consumer preference of Cabernet franc and Petit Verdot, with the underlying premise that current fruit zone management recommendations (one to two leaf layers) may be overly conservative in the humid, eastern United States. Three post-fruit set leaf/lateral shoot removal treatments (no removal [NO], removal from opposite the basal primary cluster and the node directly above [MED], and removal from the node directly above the distal primary cluster down to the cordon [HIGH]) and one pre-bloom leaf/lateral shoot removal treatment (removal from the six primary basal nodes [P-B]) were evaluated. P-B more consistently increased total berry phenolics in Petit Verdot than in Cabernet franc, while total berry anthocyanins were unaffected. Carotenoids were quantified due to their importance as aroma precursors. When compared to NO and MED, HIGH and P-B treatments tended to increase carotenoid accumulation more in the preveraison period than in the postveraison period. HIGH and P-B treatments also tended to increase carotenoid degradation more in the postveraison period, particularly for zeaxanthin. Wines made with fruit from P-B plots were ranked higher in color intensity compared to wines made with fruit from MED plots. Fruit zone leaf removal modestly improved fruit composition and wine quality potential, and created a microclimate less conducive to fungal diseases.

Cabernet Sauvignon Responses to Prebloom and Post-Fruit Set Leaf Removal in Virginia

Summary Goals: To evaluate how variations in the extent of fruit-zone leaf thinning at prebloom or post-fruit set would affect crop yield components, cluster compactness, primary juice composition, and Botrytis bunch rot of Cabernet Sauvignon grown under Mid-Atlantic conditions in the United States. We anticipated that our results would guide leaf removal practice to target crop yield, primary juice composition, and disease management goals. Key Findings: Prebloom leaf removal reduced crop yield compared with no leaf removal, and by a greater extent when eight (59%) rather than four (25%) basal leaves per shoot were removed. Prebloom leaf removal treatments reduced crop yield by 20 to 55% compared with no leaf removal in the inaugural season, and by 50 to 77% in the third consecutive season of treatment implementation. Post-fruit set leaf removal of six basal leaves per shoot did not affect crop yield when compared to no leaf removal. Prebloom leaf removal reduced cluster compactness compared with no leaf removal, and by a greater extent when eight (47% reduction) rather than four (24% reduction) basal leaves per shoot were removed. Regardless of timing or extent, leaf removal resulted in 19 to 22% lower Botrytis bunch rot incidence compared with no leaf removal. Treatment effects on juice composition were generally unremarkable, besides lower total titratable acidities observed in leaf removal relative to no leaf removal plots. Impact and Significance: Our results illustrate that exposed grape clusters have lower bunch rot incidence and produce musts with similar pH and with lower titratable acidity than musts produced from shaded grapes. Prebloom leaf removal should be judiciously used, however, as severe crop reduction may result, particularly if prebloom leaf removal is seasonally repeated. Our results complement previous findings by (1) reporting on the effects of multiple extents of prebloom leaf removal over several consecutive growing seasons in Cabernet Sauvignon, a cultivar that is relatively resistant to bunch rots, and (2) by illustrating generally positive primary (herein) and secondary metabolite (our unpublished data) responses in harvested grapes.

Cover Crop and Root Pruning Effects on the Rooting Pattern of SO4 Rootstock Grafted to Cabernet Sauvignon

Root biomass, root intercepts, and root length density of SO4 rootstock grafted to Cabernet Sauvignon and exposed to under-trellis cover cropping (CC) and with or without root pruning (RP or NRP, respectively) were evaluated over three years. The CC treatments included tall fescue (Festuca arundinacea Shreb.) varieties KY-31, with and without RP, Elite II without RP, and an 0.9 m wide, under-trellis herbicide strip with KY-31 fescue inter-rows, with and without RP. Around 70% of grapevine root biomass was observed at ≤60 cm soil depth, regardless of treatment or year. KY-31 fescue/NRP vines had the most (96%) root biomass at ≤60 cm soil depth in 2008. Rooting depth distributions were fitted to the model Y = (1 − βd), where d is soil depth (cm), Y is the cumulative root fraction from the soil surface to depth d, and the unknown parameter β is a measure of soil vertical root distribution used as a response variable to test for treatment differences. All root distributions across treatments and years, except KY-31 fescue/NRP in 2008, generated β values greater than 0.970, similar to β values from other root distribution studies. The modest treatment impact on root biomass and distribution suggests that these mature grapevines adapted to CC and annual RP in this environment, and partially explains the minimal impact on soil water content and vine water potential previously reported from this experiment.