No. 9 —July 6, 2020


Berry Cracking

Immature grapes growing on a vine, showing one berry that has cracked open laterally all the way across to show what looks like three tiny berries inside.
Figure 1. Berry cracking is often the result of an interaction with environmental conditions and a cultivar. Powdery mildew as well as calcium deficiency can also play a role in berry cracking. Photo credit: A. Sporleder submitted July 8, 2020.

I have received a lot of pictures the last couple of weeks of cracked berries (Figure 1). Grape growers are wondering what has happened and what is causing the cracking. In most all cases of early season berry cracking, the cracking of berries is the result of environmental factors interacting with the berries.

Three main environmental factors have a role in berry cracking. High berry temperatures result in reducing the strength of the berry skin, the berry expands, and the berry skin surface area shrinks. This results in berry cracking. Berry clusters that are shaded by the canopy often do not experience berry cracking as the canopy reduces berry temperature.

High relative humidity especially at night can result in berry cracking. During the nighttime, plant transpiration from the grapevine leaves is reduced compared to the daytime. At nighttime the grape berries continue to transpire, and this increases turgor pressure on the berry skins and results in berry cracking. During the day, the leaves are transpiring high amounts of water, and this lessons the turgor pressure on the grape berry skins.

Rain or heavy dew can induce berry cracking. Water can enter the berry by root uptake via transpiration. In vineyards that have experienced a drought period followed by a heavy rainfall event, this can result in increased turgor pressure on the berry skin, resulting in cracking. Water can also enter a berry by two other mechanisms. Water can enter a berry by moving across the berry skin or water can enter from the berry stem receptacle with the water moving in due to transpiration of the berry. At present time the mechanism causing the water to move into the berry from receptacle has not been elucidated.

Other factors have been identified that cause fruit cracking besides environmental conditions. Powdery mildew infections on the berry skins cause microfractures in the cuticle making the berries prone to cracking. Calcium deficiency has also been implicated in berry cracking. Calcium is important in maintaining the integrity of cell membranes. In addition, calcium plays a role in the membranes structure and regulates water permeability of the membrane. When calcium is deficient cell membranes deteriorate.

Good viticulture practices that begin with site selection will often alleviate berry cracking. Vineyard sites that shed water quickly and have well drained soils will lesson the impact of large rainfall events. Limiting canopy management, especially the removal of leaves that exposes fruit berries to intense sunlight and high temperatures can reduce berry cracking. Monitoring grapevine calcium levels at bloom can determine if a calcium deficiency exists. Calcium deficiencies can be corrected with foliar applications of calcium. Berry cracking can also be inherit to a cultivar such as Vignoles, which the berries often crack near harvest as a result of a rainfall event.

Birds Eye Rot Caused by Anthracnose Elsinoe ampelina

Three berries hand-picked show round spots of brown fruit rot on light green berries.
Figures 2. Birds Eye rot caused by Anthracnose Elsinoe ampelina. Photo credit: Robert Balek, MU Extension Horticulture Field Specialist, submitted July 9, 2020.

Anthracnose can be a fruit rot (Figure 2), shoot disease, leaf disease and infect any green tissue on the grapevine. In warm wet growing season’s Anthracnose is often more prevalent. If your grapevines or berries are infected with Anthracnose it is important to control the disease. The primary infections will produce fruiting structures that will produce spores during warm wet periods resulting in secondary infections.

Anthracnose of grapes for some unknown reason has been classified as a southern disease in the USA. Even though infections can take place during wet periods when the air temperature is 36 degrees F. As air temperatures increase the disease development increases. At 90 degrees F and wet conditions the time period from infection until symptoms appear is only four days.

At this period in the growing season, Anthracnose can be prevented with Captan as well as other fungicides such as Strobilurins. If your vineyard is severely infected with Anthracnose this season, you will need to consider a management plan for next year. Next season, prior to budburst (delayed dormant application) consider applying liquid lime sulfur or Sulforix for early season control. Also, during the dormant pruning season prune out any cane wood that is infected with Anthracnose. Remove the infected cane wood from the vineyard and destroy the infected cane wood by burning or burying.

ISU/UMN Joint Research and Winemaking Webinar Series

Cellar Sanitation

Prior to the beginning of the grape harvest season, it is necessary to clean and sanitize the winery cellar to maintain wine quality, production consistency, and the long-term winery reputation. Cory Marx (UC Davis) and Luke Holcombe (Scott Laboratories) will present this topic during the first one-hour webinar on Aug. 4, 2020, at 3 p.m. CST.

Proper and Practical Use of SO2

In the second one-hour webinar the importance of sulfur dioxide and good SO2 management in the winery, will be presented and discussed by Dr. Gavin Sacks (Cornell University) and Katie Cook (Scott Laboratories) on Aug. 18, 2020, at 3 p.m. CST.

These two free webinars are co-organized by Dr. Aude Watrelot, assistant professor of Enology at Iowa State University, and Drew Horton, enology specialist at the University of Minnesota’s Grape Breeding & Enology Project.

Two 1-hour webinars scheduled as follows:

  • Aug. 4, 2020: Winery Cleaning and Sanitizing
    • 20-minute presentation of the practical aspects of cleaning and sanitizing in a win-ery by Luke Holcombe from Scott laboratories.
    • 20-minute presentation of a recent work carried out at UC Davis by Cory Marx under the supervision of Dr. Anita Oberholster. This presentation will focus on a method for optimizing the use of chemical agents for cleaning and sanitation.
    • 15-minute Questions and Answers moderated by Dr. Aude Watrelot and Drew Horton.
  • Aug. 18, 2020: Practical Management of Sulfur Dioxide
    • 20-minute presentation on the definition of sulfur dioxide, the forms of sulfites; differences between free, bound, and total SO2; the importance of SO2 in winemaking; and a new method to measure SO2 by Dr. Gavin Sacks from Cornell University.
    • 20-min presentation on the practical aspects of the management of sulfur dioxide in a winery by Katie Cook from Scott laboratories.
    • 15-minute Questions and Answers moderated by Dr. Aude Watrelot and Drew Horton.

For further details or any questions, check out the Wine Industry Events in Dr. Watrelot’s website at or contact them at and

Cumulative Growing Degree Days (Base 50) for the Seven Grape Growing Regions of Missouri from April 1 to July 4, 2020

Region Location by County Growing Degree Days1
2020 2019 30-year Average
Augusta St. Charles 1439 1542 1546
Hermann Gasconade 1377 1515 1477
Ozark Highland Phelps 1500 1624 1587
Ozark Mountain Lawrence 1484 1587 1564
Southeast Ste. Genevieve 1474 1598 1608
Boone 1483 1555 1511
Western Ray 1427 1432 1464

1 Growing degree days at base 50 from April 1 to July 4, 2020. Data compiled from Useful and Useable at Click on link below to determine growing degree days in your area.

To determine the number of growing degree days accumulated in your area since April 1, use this tool.

Weather Outlook

Weather Outlook for Weekend

  • Above normal air temperatures
  • Heat index 95 to 105 degrees F

July 8 to 13

  • Areas of Missouri could see 0.5 to 1 inch of rain

July 13 to 17

  • Above normal temperatures forecast
  • Below normal precipitation forecast statewide

July 15 to 20

  • Excessive heat and humidity with heat indexes in the 100s