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By: Dr. Maria Smith, Diane Kinney, and Dr. Imed Dami – HCS-OSU

Budbreak came far too early this year and along with it, problems with frost. Since the chilly days of early May, however, it’s felt impossible to keep up with the pace of growth this season. Let’s look at where we’re at as we head into the summer.

Weather:

As discussed in April, exceptionally warm (nearly 6 °F above average!) and dry conditions throughout March lead to accelerated budbreak days to weeks ahead of average (Fig 1). The months of April and May, however, were much cooler (May was approximately 4 °F below average) and punctuated by several freeze events that damaged new shoots across the state (See: Impact of April Frost/Freeze on Grapes in Ohio). June has generally rebounded to warm, above average temperatures (Fig 2). Throughout the first half of the year, we have remained quite dry here in Wooster and throughout much of Ohio, while the perimeter of the state has seen slightly more rainfall, particularly over the past several weeks (Fig 1 and 2).

Fig 1. Average monthly temperature (left) and precipitation (right) in Wooster, OH from January through May for 2019, 2020, 2021, and the 10-year average.

Fig 2. Average state-wide temperature and precipitation departures from the 30-year average from May 23 – June 21, 2021. Figure from https://climate.osu.edu

Phenology:

As of June 21, Wooster is at 1024 GDD, which is approximately 60 GDD ahead of the long-term average. Current phenology of vines in Wooster varies between full-bloom to fruit-set (Fig 3), and all varieties completed bloom (50%) by the week of June 14^th (Table 1). At this stage of growth, berries are rapidly growing and remain susceptible to major grape diseases through the next several weeks.

Fig 3. Phenological stages range between full bloom through fruit set on 16 July 2021 for varieties at Unit 2 in Wooster (photo credit: Diane Kinney)

Table 1. 2021 Bloom dates and corresponding GDD of selected varieties grown at the research vineyard in Wooster.

Cultural Practices at OARDC Wooster:

Pruning for the 2021 season started late this year towards the end of March.  Up to this point, we had a very mild winter with some very unseasonably warm weather (60 ^oF on the last day of February) with our lowest recorded temperature of -0.9 ^oF.   Therefore, “normal” pruning was conducted without adjusting for bud injury like we did in the past few years.

Budbreak across the vineyard occurred very rapidly within a 12-day period one to two weeks earlier than expected in mid-April.  May followed with a bit more rain and warm temperatures which encouraged rapid shoot growth.  Besides taking some time to plant replacement vines including Cabernet franc on different rootstocks we also replaced a couple of varieties that were not faring well with completely new varieties.  It has been a race against shoot growth to keep up with training and suckering younger vines in both VSP and high cordon training systems as well as tucking shoots in the VSP.  We have had very good fruitset across the vineyard after an early bloom.  Leaf removal will be coming up very soon followed by cluster thinning to desired crop loads.

We have applied 3 fungicide cover sprays to the vineyard to this point in mid-June and will begin watching for the Japanese Beetle’s arrival in the coming week. 

Timely management reminders – midseason:

Vegetation management: Tucking (VSP), combing (HWC), hedging/skirting, fruit-zone leaf removal

The goal of vegetation management in the midseason is ensuring that the canopy is open for air flow, sunlight exposure, and adequate fungicide coverage. Open canopies with optimal sunlight exposure of the fruit provides the best environment for disease management and fruit ripening later in the season.

Leaf removal has received the most questions recently, in regard to timing.

When is the best time to remove leaves surrounding the clusters? Leaf removal should be performed between fruit-set and pea-size berries. While leaf removal can be done earlier just prior to or during bloom (“early leaf removal” or ELR), the best timing for leaf removal for most varieties begins at fruit-set. When performed between fruit-set and pea-size stages, potential yield reductions are minimized (this contrasts with ELR) and the risk of berry sun is reduced.

Can leaves be removed later? Yes, there are still benefits to disease management and phenolic developments when leaves are removed later in the season, but again, berry sunburn is a concern and more caution for exposure intensity should be considered.

Leaf removal intensity may depend on vineyard location, variety, and training system. White varieties, very early ripening varieties, and those trained as HWC may only require defoliation on the shaded side of the canopy (usually East side with rows oriented North/South) instead of both sides of the canopy to avoid overexposure to sunlight and higher temperatures.    

Crop load management: Planning for cluster thinning, crop estimation

Some high-yielding varieties with high cluster counts (3+ clusters per shoot), large clusters, or very late ripening varieties benefit from moderate yield reductions to improve fruit maturity and winter hardiness. Examples include: Chambourcin, Cabernet franc, and Regent.

If there is a need to cluster thin, the optimal time to complete is between pea-size and bunch closure stages. This maximizes the impact of thinning on fruit ripening while minimizing berry size increases due to compensation from cluster removal. In some cases, retaining 2 clusters per shoot is sufficient to fully ripen the remaining crop, and in more severe over-cropping cases, retaining only 1 basal cluster may be necessary.

**For growers with new vines, removing clusters benefits shoot growth in the initial years of vine establishment. If clusters have not yet been removed, you should do so.

Crop estimation is very useful for managing vineyard economics and identifying potential vine imbalances in crop level. There are several methods for predicting yield, one of which is called “lag-phase method” and is based on midseason cluster weight at the lag-phase of berry development (See: Crop Estimation of Grapes).

Nutrition management: Watch for symptoms of deficiency, plan for veraison tissue analysis

Many vine nutrient deficiencies begin to appear in older leaves just after the nutrient-intensive periods of bloom and fruit-set. Since deficiency symptoms may take on a similar appearance, it is best to positively identify the deficiency and possible causes through tissue and soil analysis before attempting to correct. Monitoring vine and soil nutrient status on a regular basis aids in reducing acute onset of nutrient deficiencies. For more information on sampling vine tissue analysis, see: Grapevine Nutrient Management: Petiole Sampling and Analysis.

Welcome Megan!

The viticulture program would like to welcome our new hire, Megan Soehnlen.  Megan is a recent graduate of Walsh College and joined us in May as a Research Assistant.