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- Author or Editor: Tony K. Wolf x
Thermal analysis (TA) was used to evaluate dormant bud cold hardiness of nine Vitis cultivars weekly during the 1993–94 dormant period. TA hardiness estimates were expressed as either mean low-temperature exotherm temperature (MLTE) or temperatures lethal to 10% (LT10), 50% (LT50), or 90% (LT90) of dormant bud sample. A destructive freeze on 19 Jan. 1994 presented an opportunity to compare dormant bud field survival with laboratory estimates of bud hardiness that had been derived from TA. Vineyard air temperatures of –24C caused primary bud kill that ranged from a mean of 15% with `Concord' to 100% with `Viognier'. With the exception of `Viognier' and one of two `Cabernet Sauvignon' clones, field mortality levels were accurately bracketed by TA estimates of LT10, MLTE, and LT90 values, which had been obtained in the week preceding the freeze. `Viognier' bud hardiness was overestimated by ≈1.5C, and the hardiness of `Cabernet Sauvignon clone UCD#6' was underestimated by <1C. The discrepancy with `Viognier' may have been related to prior destruction of primary buds by bud necrosis and the misinterpretation of secondary bud exotherms as due to primary buds.
Selective leaf removal in the proximity of grape clusters is a useful practice to manage fruit diseases and otherwise improve fruit composition. The current recommendation in the eastern United States is to create a fruit zone with one to two leaf layers and to focus removal on the “morning sun” side of the canopy. We evaluated a more intense and an earlier application of fruit-zone leaf thinning relative to current recommendations to determine whether additional benefits could be obtained without a penalty of impaired berry pigmentation or other ill effects of abundant grape exposure. Fruit secondary metabolites and berry temperature were monitored in two different field experiments conducted with ‘Cabernet Sauvignon’ in the northern Shenandoah Valley American Viticultural Area (AVA) of Virginia. One experiment evaluated the effects of no leaf removal, prebloom removal of four basal leaves per shoot, and prebloom removal of eight basal leaves per shoot. The other experiment evaluated the effects of no leaf removal and postfruit set removal of six basal leaves per shoot. On average, exposed grapes heated to ≥30 °C for a 126% longer period (53 hours) than shaded grapes in the postveraison period (from color development through harvest). However, postveraison grape temperatures did not remain above provisional, critical temperature thresholds of either 30 or 35 °C for as long as they did in studies conducted in sunnier, more arid climates. There were minimal differences in berry temperature between east- and west-exposed grapes in the northeast/southwest-oriented rows of the experimental vineyard. Regardless of implementation stage, leaf removal consistently increased total grape phenolics measured spectrophotometrically, and either increased or had no impact on anthocyanins relative to no leaf removal. Grape phenolics and anthocyanins were unaffected by canopy side. Berry total phenolics were increased and anthocyanins were at least maintained in fruit zones void of leaf layers—a canopy attribute that reduces bunch rot in humid regions.
Examination of `Riesling' grape (Vitis vinifera L.) in Virginia suggested that a high incidence of bud necrosis (BN) in some vineyards was associated with canopy shade and rapid shoot growth. BN appeared to originate as an abortion and dehydration of the primary, and occasionally secondary, buds of the developing dormant bud. BN frequency was lowest among the basal four nodes of a given shoot or cane, and increased in frequency through node 20. Experiments were conducted in 1991 and 1992 to evaluate the specific involvement of shoot growth rate and canopy shade on `Riesling' BN. Shoot growth rate (SGR), measured in a 17-day period around bloom, had a significant, positive relationship with BN in one of two vineyards. BN was positively associated with cane diameter and average internode length. Applying the growth retardant paclobutrazol significantly reduced SGR and BN incidence up to 80% among nodes 6 to 15 in two separate vineyards. Artificial shade (64% or 92% reduction in photosynthetic photon flux), suspended over vine canopies in the 3-week period before véraison, did not affect BN. Shoots of canopies that had been thinned before bloom to 10 shoots/m of canopy expressed slightly lower BN levels than shoots sampled from canopies that had been thinned to 20 shoots per meter. `Riesling' BN appeared more influenced by shoot vigor than shade under Virginia growing conditions. Chemical name β-[(4-chlorophenyl)methyl]-α-(1,1-dimethyl-ethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).
Thermal and differential thermal analysis (DTA) are used to detect exotherms that result from the freezing of supercooled tissues (4). They provide a convenient and rapid means of assessing the hardiness of tissues that supercool, such as the floral primordia of Prunus spp. (3) and the compound buds of Vitis spp. (2). The inability to process a large number of tissue samples simultaneously, however, has been a major limitation of DTA. Ashworth et al. (3) described a computer-assisted data-logging system for recording thermal analysis data generated when Prunus flower buds were frozen. Multiple cooper-constantan thermocouples were used to increase the number of buds monitored on a given channel of a multichannel data-logger. Copper-constantan thermocouples, however, were not adequate with our instrumentation to discriminate exotherms generated by the freezing of individual shoot primordia of compound grapevine buds. Furthermore, anatomical barriers to ice propagation may be negated if thermocouples are inserted into buds to increase exotherm detection (5).
Differential thermal analysis was evaluated as a means of determining the cold hardiness of excised dormant buds of Vitis vinifera L. cv. Chardonnay grapevines. The manner in which buds were excised and cooled affected the freezing characteristics of bud primordia. Buds excised with 1 to 2 mm of subjacent nodal tissue exhibited both high temperature exotherms (HTEs) and low temperature exotherms (LTEs). HTEs apparently resulted from freezing of supercooled moisture in bud scales and/or in the subjacent nodal tissue and occurred at inconsistent temperatures. Cooling similarly excised buds on a water-saturated substrate caused HTEs to occur at −4° to −8°C and did not affect the occurrence of LTEs, which were consistently associated with primordia death. Median LTEs associated with primary bud death were 1.5° to 2.0° warmer than LT50s derived from temperature/survival freezing tests of similar buds. Buds killed by freezing did not supercool appreciably when refrozen. Bud cold hardiness increased when single-node cuttings were exposed to a step-wise cooling cycle; however, the ability to detect exotherms decreased under these conditions. The decreased detection of exotherms was due to increased bud death and, presumably, a decrease of freezable (and thus detectable) moisture in the supercooled primordia of viable buds. DTA provides a useful and reliable means of determining grapevine bud cold hardiness when conducted in a standardized fashion.
Phytotoxicity of horticultural oil, applied shortly before antifungal sulfur, was evaluated for 23 grape cultivars. Oil application significantly reduced accumulation of soluble solids in berries of 9 of 23 cultivars, but there was no relationship with visible foliar injury. Treatment of leaves of Vitis labrusca `Catawba' with 1.5% JMS Stylet-Oil reduced leaf net photosynthesis (Pn) by 50% to 60% and of Vitis vinifera `Chardonnay' by 20% to 30% 1 day after application. Pn was reduced only when the lower (abaxial) leaf surface was treated; treatment of only the adaxial leaf surface had little effect. The Pn depression in `Catawba' persisted 3 to 4 weeks, whereas reductions in `Chardonnay' persisted less than 2 weeks. The Pn-depressing effect of oil was not significantly ameliorated by real or simulated rainfall, and washing the lower leaf surfaces with water and detergent also had only limited effect. There was no significant difference in Pn depression from oil applications made in the middle of the day (stomata open) compared to application in the evening (stomata closed), or from oil applied at higher versus lower application pressure. The greater sensitivity of `Catawba' than `Chardonnay' to Pn depression by oil may be related to the amount of oil retained by the leaves; the pubescent lower leaf surfaces of `Catawba' retained more than twice as much spray emulsion as did the more glabrous leaves of `Chardonnay'. Visible injury was mild in both cultivars, with small water-soaked lesions developing more commonly on `Chardonnay' than on `Catawba' leaves. Spray oil retention data for additional cultivars suggested that differences in retention can explain a portion of the differences in horticultural oil phytotoxicity.
Thermal analysis was used to determine if muscadine grape (Vitis rotundifolia Michx.) buds supercooled and to determine the seasonal cold hardiness of several grape cultivars. Buds of the muscadine cultivars `Carlos' and `Summit', sampled from vines grown at Clarksville, Ark., produced low-temperature exotherms consistent with the number of buds tested. Apparent hardiness of the buds increased from 5 Nov. 1993 through 7 Jan. 1994. Mean low-temperature exotherms (MLTE) were lowest on 7 Jan. and were –21.5C for `Carlos' and –23.4C for `Summit'. Mars (V. labrusca L.) buds, sampled at Clarksville and Winchester, Va., were included in the study, and increased in hardiness during the same period. MLTE temperatures for `Mars' from Arkansas were similar to those of the muscadine cultivars on 7 Jan.; however, `Mars' attained lower MLTE temperatures with vines grown in Virginia compared to those in Arkansas. Location differences may be due to cultural conditions, sample handling, environment or other reasons.
Prohexadione-calcium (prohexadione-Ca) was evaluated for its ability to suppress vegetative growth of grapevines (Vitis vinifera L.) under field conditions. Two or three applications of 250 mg·L-1 prohexadione-Ca reduced primary shoot growth of `Cabernet Sauvignon', but had little effect on other canopy characteristics or cane pruning weights. The reduction of shoot growth was not persistent and shoot hedging was ultimately needed to avoid canopy shading. Similarly, three applications of either 125, 250, or 375 mg·L-1 prohexadione-Ca reduced `Cabernet franc' shoot growth, but again did not eliminate the need for shoot hedging. Cane pruning weights of `Cabernet franc' were unaffected by treatment, and canopy characteristics were generally not improved. Two prebloom and one postbloom application of 250 mg·L-1 prohexadione-Ca were evaluated on `Cabernet franc' and `Chardonnay' in separate field experiments. The prebloom treatments retarded shoot growth of `Chardonnay', but had no effects on `Cabernet franc' shoot characteristics. To retard shoot growth, prohexadione-Ca had to be applied prior to bloom; however, prebloom applications had the potential for severe reductions in crop yield.
Thermal analysis was used to determine if muscadine grape (Vitis rotundifolia Michx.) buds supercool and to determine the seasonal cold hardiness of several grape cultivars grown in Arkansas. Buds of the muscadine cultivars Carlos and Summit, sampled from vines grown at Clarksville, Ark., produced low-temperature exotherms consistent with the number of buds tested. Apparent hardiness of the buds increased from 5 Nov. 1993 through 7 Jan. 1994. Mean low-temperature exotherms (MLTE) were lowest on 7 Jan. and were –21.5C for `Carlos' and –23.4C for `Summit'. `Mars' buds, sampled at Clarksville, Ark., and Winchester, Va., were included in the study and increased in hardiness during the same period. MLTE for `Mars' from Arkansas were similar to those of the muscadine cultivars on 7 Jan.; however, `Mars' attained lower MLTE temperatures with vines grown in Virginia than with those in Arkansas. Location differences in hardiness of `Mars' are conjectural.