Phenology

The berries of all cultivars at West Madison Research Station (WMARS) are harvest-ripe (stage 38). 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. This week we harvested Marquette and Frontenac on September 13 and 14, respectively. The clusters of both of these cultivars, but especially Frontenac, were full and of very good quality, with very little disease or pest damage. Crimson Pearl and Petite Pearl are the only cultivars left to harvest and we are leaving those on the vine for approximately one more week to see how sugar and acid values may change. Although we received over an inch of precipitation in the early hours of September 13, this did not result in a dilution of the fruit ripening parameter values, nor did it cause any significant berry splitting.

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

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Figure 1. Ripening clusters of grape cultivars at the West Madison Agricultural Research Station, as of August 4.

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).

% Total soluble solids (proxy for sugars)

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Titratable acidity (tartaric acid equivalents

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pH (concentration of free hydrogen ions)

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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 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
	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
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 1 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).

As we near the end of the growing season, we can see that the rates of GDD accumulation at both WMARS and PARS over the last three years are greatest for 2021 and then 2020 and 2019, respectively. Even the warmest year at PARS (2021) is cooler than the coolest year at WMARS (2019). At WMARS, 2020 and 2019 ended with a more similar GDD accumulation total than 2021, while at PARS, the GDD totals of 2021 and 2020 are more similar than in 2019.

Table 2. Growing degree day accumulation as of August 17, 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	2665	2423	2312
PARS	2112	1980	1701

*BE = Baskerville-Emin calculation method

Figure 3. Accumulation of growing degree days (GDD) as of September 1 (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, grape phenology, Grapes, grapes developmental stages, Jarret Miles-Kroening.