Phenology

Over the past two weeks, all cultivars at WMARS reached veraison (E-L* Stage 35), meaning that the berries have begun to change color and softening continues (see Table 1 for dates). In a white wine grape cultivars like Brianna, Itasca, and La Crescent, berries begin to change from green to a more golden hue. Red wine grape cultivars, like Crimson Pearl, Frontenac, Marquette, and Petite Pearl, reach veraison when the berries begin to turn from green to pink and gradually darken and soften.

*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

Starting with this WFN, we will report the date of veraison for each cultivar and the current soluble solids and titratable acidity (TA) concentrations, until harvest.

Cultivar	Date reached Veraison at WMARS	Total soluble solids (%)	Titrable acidity (TA) (g/L tartaric acid equivalents)	pH
Brianna	7/30	10.2	24.4	2.8
Crimson Pearl	7/27	9.0	30.0	2.7
Frontenac	7/27	12.6	30.4	2.8
Itasca	7/20	15.0	20.7	3.4
La Crescent	7/30	13.9	29.4	2.8
Marquette	7/24	15.2	26.9	2.8
Petite Pearl	8/1	6.6	25.1	2.8

Table 1. Date of veraison and percent total soluble solids, titratable acidity concentration, and pH of juice from the cold climate interspecific hybrid grape cultivars at the West Madison Agricultural Research Station on August 4.

It is common for people to use total soluble solids (TSS) and °Brix interchangeably when referring to sugar levels in fruit. This is because sugars (mainly glucose and fructose) comprise 95-99% of the TSS in grape juice and so are good stand-in for sugars. Other constituents include organic acids, phenolic compounds, nitrogenous compounds, and structural polysaccharides. TSS is a measure of the density (mass/volume) of all the soluble solids in the juice and is determined using a refractometer. 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: titratable acidity (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 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 to 14.

Growing Degree-Day (GDD) Accumulations

Depicted in Figure 2 and listed in Table 2 are the GDD accumulations from April 1 to August 3 for this year and the past two seasons. 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).

At WMARS, the GDD accumulations of the current and past two years continues in the same relative pattern of the current having the greatest, 2020 the next greatest and 2019 the lowest. The differences between the totals is similar to that of two weeks ago. At PARS, a different scenario is occurring, where although the current and 2019 season accumulations are different as of August 3, they have a parallel trajectory. In contrast, the mid-June to early August temperature experience at PARS in 2020 was decidedly warmer, with the total accumulation in early August reaching near that of what has been recorded for the current season.

Table 2. Growing degree day accumulation as of August 3, 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	1810	1699	1622
PARS	1344	1314	1109

*BE = Baskerville-Emin calculation method

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