Shatter During Fruitset

We're doing a retrospective analysis on our somewhat low yields in our vineyard this year. The one variety that exceeded last year's weight at harvest was the Chardonnay. Coincidentally, the Chardonnay was the first to bloom around June 10 and reached approximately 30% bloom by June 13.
Most of our varieties were at some stage of bloom between June 10 and close to the end of June. On June 16 the following varieites were at this stage:

• The Auxerrois is beginning to bloom
• The Chenin is not in bloom
• The Chardonnay at about 50% bloom
• The Cabernet Sauvignon at about 20% bloom
• The Cabernet Franc at about 20% bloom
• The Barbera at about 10% bloom

Those flowers which bloomed early encountered rain on June 14, June 22, July 2 and July 3. One thing that we did notice during fruitset was the very sparse appearance of the fertilized berries.

This condition is known as shatter in the grape world. During the early summer, we had no idea of the impact of shatter on our yields. I looked online and found a very good article from the University of Maryland Extension on Grape Fruit Set and Shatter (Coulure) written by Joseph A. Fiola, Ph.D. In the article, Dr. Fiola mentions the following factors that can lead to decreased fruit set:

• Pre-bloom cultural practices such as shoot trimming or leaf pulling
• Nutrient deficiencies
• Deficiencies of any of the essential mineral nutrients can affect fruit set detrimentally
• Pre-bloom or post-bloom exposure to hormonal herbicide drift/volatilization
• Disease, insect, or other biotic stress before bloom can decrease fruit set
• Weather/Environmental conditions, including excessive heat (> 32°C/ 90°F) or cold (<15°C/59°F) can adversely affect pollen development, pollination, and fertilization
• Water stress can adversely affect fruit set
• Differences in cultivars

Of all of the above factors that affected our fruit set, I believe that weather conditions, as noted, rain during the period of flowering and fruit set affected our lower than expected yields this year.
References:
1. Joseph A. Fiola, Grape Fruit Set and Shatter (Coulure), University of Maryland Extension.