Phenology

Crimson Pearl and Petite Pearl were the last cultivars harvested at the West Madison Research Station (WMARS) on September 20.  See Table 1 for veraison (beginning of berry color change) and harvest dates, as well as the final fruit quality parameter readings for each cultivar. While we do not have cluster pictures at harvest time for Crimson Pearl and Petite Pearl, their clusters were full and of very good integrity, similar to the other cultivars. Canes are almost fully matured (lignified, turned woody) (pre-E-L* Stage 41), save for Brianna (typically is the last cultivar to do this) whose canes are only about 25 to 50% lignified. Leaves have begun to yellow and senesce, but not many have fallen.

*E-L stands for the Eichhorn-Lorenz growth stages scale to describe grapevine development.

Berry Ripening Parameters

Below are graphs showing the progression of the berry ripening parameters at WMARS: % total soluble solids (TSS), titratable acidity (TA) concentrations, and pH. All cultivars reported here are grown on the high cordon training system.

TSS is a measure of the density (mass/volume) of all the soluble solids in grape berry juice and is determined easily using a refractometer as %TSS. Sugars (mainly glucose and fructose) comprise 95-99% of the TSS in grape juice and so %TSS is good stand-in measure for sugars. It is common for people to use %TSS and °Brix interchangeably when referring to sugar levels in fruit. Other constituents of TSS include organic acids, phenolic compounds, nitrogenous compounds, and structural polysaccharides. Hydrometers utilize various scales, including ºBrix, and measure the sucrose (a disaccharide composed of one unit of glucose and one unit of fructose) concentration in aqueous solution (1 ºBrix = 1 g sucrose / 100 g solution = 1%).

The concentration and strength of acids in grapes is accounted for in two ways: TA and pH. TA measures both the dissociated (free hydrogen ions) and un-dissociated acids (such as in organic acids). The main organic acids in grapes are tartaric and malic acids, with citric acid a distant third. TA is measured by titration of the sample with a base (an alkaline solution), such as sodium hydroxide, and is expressed as a concentration (g/L) on the basis of tartaric acid equivalents. The values determined are a good indication of the sensory perception of tartness. The pH of substance is a measurement of the concentration of dissociated free hydrogen ions, expressed on a unitless scale from 1 (acidic) to 14 (basic).

The highest %TSS levels occurred in Itasca, La Crescent, and Marquette, while the lowest levels were in Brianna, Crimson Pearl, and Petite Pearl. As expected, Brianna and Itasca yielded the lowest TA levels. Among the red cultivars, Crimson Pearl and Petite Pearl had the lowest acidity levels. La Crescent, Frontenac, and Marquette stood out with the lowest pH values, while Brianna, Itasca, Crimson Pearl and Petite Pearl had the highest values.

% Total soluble solids (proxy for sugars)

Titratable acidity (tartaric acid equivalents)

pH (concentration of free hydrogen ions)

Figure 2. Percent total soluble solids (%TSS), titratable acidity (TA), and pH of the berry juice from the cold climate interspecific hybrid grape cultivars at the West Madison Agricultural Research Station. Frontenac values from September 8 have been omitted due to a technical error.

Table 1. Veraison and harvest dates for the 2021 season and harvest fruit quality parameters of total soluble solids (TSS), titratable acidity (TA), and pH of the cold climate interspecific hybrid grape cultivars at the West Madison Agricultural Research Station in Verona.

Wine color	Cultivar	Date reached Veraison at WMARS	Harvest date	Harvest TSS (%)	Harvest TA (g/L)	Harvest pH
Red	Crimson Pearl	7/27	9/20	20.5	8.08	3.48
	Frontenac	7/27	9/14	22.4	15.83	3.26
	Marquette	7/24	9/13	24.1	12.34	3.32
	Petite Pearl	8/1	9/20	20.9	8.45	3.41
White	Brianna	7/30	9/1	17.4	6.20	3.52
	Itasca	7/20	9/1	23.2	9.11	3.50
	La Crescent	7/30	9/1	22.4	14.34	3.16

Growing Degree-Day (GDD) Accumulations

Depicted in Figure 3 and listed in Table 2 are the GDD accumulations from April 1 to September 29 for this year and the past two seasons at WMARS and the Peninsular Agricultural Research Station (PARS). Degree-days were calculated using a base 50 °F, starting on April 1 as a biofix date. We use the NEWA website and the “BE” (Baskerville-Emin) calculation. This method uses a sine wave instead of a simple average temperature calculation, which is thought to provide a more accurate estimation of degree-days. You can visit the NEWA “About degree days” page to learn more about the concept of degree days and the formulas used in calculations. (http://newa.cornell.edu/index.php?page=about-degree-days).

By charting the GDD accumulation for this and the previous two seasons, an interesting trend is visible: in the latter part of September for all three years, WMARS experiences continued warmth, while at PARS there is a plateauing or slowing of heat accumulation. This phenomenon extends the time needed to fully ripen the berries, as PARS also always experiences a lower total of GDD accumulation each season than at WMARS. It is also interesting to note that for this season the GDD accumulation at WMARS was the highest of the three seasons, while at PARS this year relatively cooler temperatures later this month decreased the GDD accumulation rate such that the season total is currently nearly similar to that of 2020.

Table 2. Growing degree day accumulation as of September 29, 2021 (April 1 biofix date; base 50 °F BE*) at the West Madison Agricultural Research Station (WMARS) and the Peninsular Agricultural Research Station (PARS).

Location	2021	2020	2019
WMARS	2887	2588	2548
PARS	2127	2059	1804

*BE = Baskerville-Emin calculation method

Figure 3. Accumulation of growing degree days (GDD) as of September 29 (April 1 biofix date) at both the West Madison Agricultural Research Station (WMARS) and the Peninsular Agricultural Research Station (PARS) for 2019, 2020 and 2021.

This article was posted in Grapes and tagged Amaya Atucha, Beth Ann Workmaster, cold climate grapes, Degree Days, grape phenology, Grapes, grapes developmental stages, Jarret Miles-Kroening.