- Winegrapes are an important crop for Oregon agriculture, ranking amongst the top ten agricultural commodities based on farmgate value. The most widely planted winegrape cultivar in the state is ‘Pinot noir’ (Vitis vinifera L.), and the majority of acreage is produced in the Willamette Valley. Production of quality ‘Pinot noir’ is expensive due to small vineyard size, dependence on manual labor, and low yield targets that are believed to enhance fruit quality. These industry production standards have traditionally been rooted in practices adopted from other wine regions and on higher-yielding cultivars. ‘Pinot noir’ has low physiological yields compared to other winegrape cultivars, which may not warrant such extensive crop removal. Additionally, the Willamette Valley has high rainfall and deep soils, resulting in vegetatively vigorous vines that cannot be managed through deficit irrigation practices to reduce vine vigor as in other winegrape production regions of Oregon and the West Coast. Vineyard floor management has been shown to reduce vine growth through competition for water or nutrient
resources in other high-rainfall regions and was evaluated as a means to alter vine balance in Oregon ‘Pinot noir’.
Vine balance is the optimum ratio between canopy size and fruit yield (vegetative versus reproductive tissue) required to fully ripen the crop. However, the optimum ratio in the current literature does not apply to ‘Pinot noir’ produced in Oregon’s Willamette Valley, as vine balance ratios fall below the range considered optimum. To understand how canopy size relative to fruit yield affects ‘Pinot noir’ productivity and fruit quality, a three-year trial was conducted to determine the impacts of vineyard floor management and cluster thinning on vine balance. To alter canopy growth, three vineyard floor management practices were used, including soil tillage and competitive perennial grass cover crop in the vineyard alleyways. Two crop levels were compared, including vines with a full crop maintained and removing nearly half of the clusters per vine. The perennial grass cover crop was successful at reducing véraison shoot leaf area by as much as 44% and dormant pruning weights by 52%, but the cover crop did not negatively affect vine water status, as determined by midday stem water potential. Vines with smaller canopies had lower tissue nitrogen (N) status compared to vines with larger canopies that resulted from complete tillage. Results suggest that vine N, rather than water limitation, was responsible for altering vine vegetative growth. Interestingly, as perennial grass cover crop reduced canopy size, yields were also reduced resulting in similar canopy size to yield ratios as higher vigor vines grown with different levels of tillage. While floor management primarily affected vine vegetative growth, crop level mainly affected fruit composition such as total soluble solids, pH, total anthocyanins, and total tannins in multiple years. There was no universal vine balance metric that could be used to predict fruit composition amongst the years.
As vine yield differed due to floor management treatments in the first trial, a subsequent two-year trial was conducted on the same vines, maintaining only the floor management treatments. Focus of this second trial was to understand how vine nutrient status may influence yield through floral primordia development in the first season and fruitfulness of the following season. Seasonal dynamic of carbohydrate and nitrogen status were monitored in various vine tissues over the course of the two years to understand the impact of these nutrients on fruitfulness in the bud, observed fruitfulness following bud break, and yield at harvest. Root, trunk, internode, and bud tissues were analyzed for total non-structural carbohydrate (TNC) and N concentrations, two main reserves involved in primordia development. The number and size of inflorescence primordia, also known as bud fruitfulness (FFL) and integrated fruitfulness index (IFI), was determined in each of the buds that composed of the latent, compound bud. High vigor vines had more and/or larger inflorescence primordia than low vigor vines. Low vigor vines had lower N concentrations in root and trunk tissues. Few correlations were found for bud FFL or IFI and internode, root, or trunk TNC concentrations measured throughout the growing season as results were inconsistent between years. In addition to higher bud FFL, high vigor vines showed a greater decline in root TNC concentration from bud break to bloom and a larger canopy and higher leaf blade N concentration by vérasion than low vigor vines in the first season. In the second season, however, there was a similar decrease in root TNC concentrations between the two vigor levels and no difference in bud FFL or IFI, likely suggesting the ability to supply TNC to canopy establishment directly or indirectly, through photoassimilation, influences inflorescence primordia development and/or growth. This finding is in opposition to many studies that suggest solar radiation is a prime driver of inflorescence primordia initiation and
growth as bud FFL and IFI decreased as the amount of light infiltrating the canopy increased in
In addition to vine size and growth, cane vigor was also evaluated for relationships with
bud fruitfulness and inflorescence primordia size. Unlike several studies, high vigor vines and
canes with greater weight and diameter, were associated with higher bud FFL and IFI averaged
along the cane. The presence of a woody lateral at a node, which was more common in the high
vigor vines, increased bud FFL and IFI at that node. Additionally, buds at each node position
along a dormant cane were evaluated to determine the role of node position on inflorescence
primordia number and size. Both bud FFL and IFI increased from a low at the base of the shoot
(node position one) to node position four. Node position had greater influence on FFL and IFI
than the vineyard floor management practices. Overall, the two basal nodes had the lowest FFL
and IFI and are not likely to be increased by cultural practices that increase cane or vine vigor.
The results of this work indicate that a perennial grass cover crop can be an effective
long-term management strategy for overly-vigorous vineyards of Oregon’s Willamette Valley. In
our study, this practice reduced canopy size and yield, which maintained vine balance and hence
sufficient canopy leaf area to adequately ripen fruit. Vines were not subject to water stress under
the deep soil conditions of the vineyard in this study. The cover crop likely restricted soil N
availability leading to lower yields, which needs to be monitored to ensure production standards
are met. Although growers may choose to reduce yields, maintaining minimum base yields is
still critical and of concern due to the high yield variability experienced in this region. Therefore,
understanding the conditions under which yield is affected can aid growers in management
decisions. Cane selection at pruning, may be a potential strategy to affect yield under those
conditions. Growers choosing to maximize potential yield, may opt to promote vegetative vigor,
or retain more vigorous canes at pruning. Seasonal events that reduce vine reserves, such as canopy loss due to frost, pests, or disease, will likely reduce fruitfulness through lowering carbohydrate reserves.