by Kathryn Carter, Fruit Specialist, OMAFRA
We are starting to see bloom in grape cultivars, so it is a good time to review the various factors (weather, vine nutrition, vine vigour and balance) impacting fruit set.
Each grape flower cluster (inflorescence) contains hundreds of flowers, however on average on 50% of flowers within an inflorescence set fruit and become berries. If too many of the fruit set, it can result in compact tight clusters that are prone to fungal infections. Poor fruit set (less than 30 percent fruit set) can result in clusters with few berries, and/or clusters with small green (shot berries) or significant variability in berry size within a cluster (hens and chicks). Shot berries may never ripen and may drop before harvest.
Poor fruit set can be a result of the entire inflorescence being lost, or the loss of individual flowers within an inflorescence. Loss of inflorescence can occur prior to bloom, or flowers may set and form small shot sized berries.
2019 has been a cool and wet season, and weather conditions before and during bloom can have a significant impact on fruit set. Cool, cloudy or wet conditions can have a negative impact on flower development, resulting in fewer berries. Cool or hot temperatures (below 18 ° C or above 38 °C) during bloom can slow the growth of the pollen tube, resulting in reduced fruit set. Rain during bloom prevents the calyptra (caps for flower petals) from detaching from the flower, resulting in a reduction in pollination. Some cultivars (ie. Merlot, Grenache, and Gewurztraminer) are more susceptible to weather resulting in poor fruit set than others (ie. Pinot and Chardonnay). Although weather has a significant impact on fruit set, unfortunately, there is nothing that grape growers can do change the weather conditions.
Vine Nutrition and Vine Health
Vine nutrition from last season, has an impact on the number of flower clusters in a bud, and on a shoot. Vines with a nutrient deficiency may be more prone to poor fruit set. Prior to bloom, grape vines use their nutrient reserves from last year to support the vine. Water stressed vines from last year, are also more prone to having issues with fruit bud development and fruit set this year.
Research suggests that boron and zinc are the most important nutrients for fruit set in grapes as they impact early season shoot growth. In addition, boron can have an impact on pollen tube generation which is needed for fertilization.
The ratio of carbon to nitrogen may also have an impact on fruit set. Vines with an unbalanced C: N status (excessive vigour or weak vines) may be more prone to poor flower development and fruit set. Some studies have shown that nitrogen levels may be the reason for early necrosis of the cluster.
To ensure healthy vines with adequate nutrition. Collect petiole samples (every other year) to monitor nutrient levels. If boron levels are low, consider applying 1 to 2 sprays of boron before bloom to vines that have low boron levels. Some studies suggest that the application of Boron in the fall, has greater impact on fruit set the following year.
Events that damage the vine canopy (early fall frost, winter damage, hail) or defoliation (due to insect feeding or herbicide injury), can have a negative impact on fruit set as they reduce carbon assimilation and storage.
Vine vigour and balance
Shoot tips compete with blooms for nutrients and resources. An excessively vigourous vine will put more resources into the shoot tips than fruit clusters, resulting in poor flower development and poor fruit set. Weak vines will have fewer sources of stored carbon and nitrogen, resulting in a shoots pulling more nutrients from the flowers, leading to poor fruit set. Early leaf pulling around the cluster at bloom will reduce the fruit set, and the compactness of the clusters.
Some cultivars and clones of Vitis vinifera often have poor fruit set, to which the cause is unknown.
There are many factors that impact fruit set in wine grapes including weather, vine nutrition and health, vine vigour/balance and cultivar. If you observe issues with fruit set in your vineyard, document your observations, as this might help to determine the cause of the problem. It is best to evaluate fruit set 10 to 12 days after full bloom to evaluate flower clusters for fruit set estimates.
Causes of Poor Fruit set in Grapes. Patty Skinkis, Oregon State University. https://articles.extension.org/pages/33106/causes-of-poor-fruit-set-in-grapes
Fruit set in grapevines 101 by Michela Centinari, PSU https://psuwineandgrapes.wordpress.com/2015/08/07/fruit-set-in-grapevines-101/
Soil Management, Fertilizer Use, Crop Nutrition and Cover Crops for Fruit Production
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