Grape Cultivar Developmental Stages: August 5, 2020
Phenology
At the West Madison Agricultural Research Station (WMARS) in Madison, WI, as of August 5, all cultivars, except Petite Pearl, have reached veraison, shoot development E-L* stage 35. Petite Pearl had clusters from E-L stages 33 (“Berries still hard and green”) to stage 34 (“Berries begin to soften”). Some clusters of Petite Pearl that are fully exposed to the sun have begun to slightly color, although the berries remain hard.
Veraison is one of the two times in the growing season (the other is bloom) recommended for performing leaf petiole sampling for nutritional analysis. Veraison is considered by some to be the best time to assess the vines’ nutritional status, as this is when nutrient levels are the most stable. Check out the link here to read a recent Wisconsin Fruit posting for instructions on how to best conduct leaf petiole sampling to get the most reliable results.
Brianna: E-L 35 “Veraison” 
Crimson Pearl: E-L 35 “Veraison” 
Frontenac: E-L 35 “Veraison” 
Itasca: E-L 35 “Veraison” 
La Crescent: E-L 35 “Veraison” 
Marquette: E-L 35 “Veraison” 
Petite Pearl: E-L 33 “Berries still hard and green” to 34 “Berries begin to soften”
*E-L stands for the Eichhorn-Lorenz growth stages scale to describe grapevine development.
Disease Incidence
This week in one of the vineyards at WMARS, we noticed the presence of powdery mildew on La Crescent (see pictures below). Graying of the berries, canes, and leaves is symptomatic of the disease. One way to distinguish powdery mildew from downy mildew is the presence of more widespread greying on the upper surface of the leaves in powdery mildew, as opposed to gray patches on the underside in downy mildew. Powdery mildew can be difficult to see on the leaves, especially in the early stages, unless they are moved into direct sunlight.
Powdery mildew on entire cordon of La Crescent 
Powdery mildew on canes, leaves (upper surface), and berry clusters of La Crescent
Check out the Wisconsin Fruit website page for Grape Resources to find links to several disease management publications. Also, information on other recently sighted grape diseases around the state are listed in the Plant Disease Diagnostic Clinic update for July 31.
Growing Degree Day (GDD) Accumulations
Depicted in the table and on the graph are the GDD accumulations from April 1 through August 4 for this season and the past two seasons. Currently at WMARS, the GDD accumulation for the past two weeks of the 2020 season was noticeably cooler, resulting in a total heat experience for the same time period close to that of 2019. In contrast, the temperature experience at the Peninsular Agricultural Research Station (PARS) over the last two weeks continued to be relatively warm and similar to that for 2018, making the total heat accumulations for the two years comparable.
We use the NEWA website as our source for GDD data. You can visit their “About degree days” page to learn more about the concept of degree days and the formulas used in calculations.
Growing degree day accumulation as of August 4, 2020 (April 1 biofix date; base 50 °F BE*) at the WMARS and the Peninsular Agricultural Research Station (PARS).
| Location | 2020 | 2019 | 2018 |
| WMARS | 1709.2 | 1645.6 | 1846.5 |
| PARS | 1324.3 | 1129.0 | 1356.2 |
*BE = Baskerville-Emin calculation method
