Fruit Set in Grapevines 101
By: Michela Centinari
As the growing season rapidly progresses in Pennsylvania, we are well past fruit set in vineyards, and grapevines are getting close, or are already going through, veraison.
In central Pennsylvania, as well as in other Mid-Atlantic and Northeastern regions, above average precipitation was recorded in June during the flowering period. Overcast and wet weather may result in reduced fruit set with potential negative effects to grapevine yield. Although somewhat late, this might be a good opportunity for a short review on fruit set and some of the major factors affecting it. I will not discuss flower formation and the flowering process, a complex process which involves several steps spread over two seasons. If you are looking for a comprehensive review on flowering and fruit set in grapevines please refer to:
May, P. 2004. Flowering and Fruitset in Grapevines. Published by Lythrum Press, Adelaide, Australia. (Link: http://www.amazon.com/Flowering-Fruitset-Grapevines-Peter-May/dp/0975126067)
What is fruit set?
Fruit set occurs when the flower forms a berry. A grape inflorescence contains hundreds of flowers (Figure 1). However, only a portion of those flowers will develop into berries. Flower abscission occurs naturally in horticultural crop plants. In grapevines, a large number of flowers drop primarily in the two weeks following full bloom. Fruit set is achieved once the “berry shatter” period is complete . Fruit set could be considered a “self-thinning” technique that enables the vine to regulate the crop, by adjusting it to the available resources without risking survival of the plant . Percentage (%) of fruit set is a quantitative measure of the proportion of flowers that develop into berries following bloom.‘Normal’ vs ‘poor’ fruit set
Normal fruit set is typically in the range of 30-50% (Figure 1c), with less than 30% indicative of ‘poor’ fruit set. However, this is not a universal rule. Fruit set is highly variable between cultivars and is inversely related to the number of flowers per inflorescence . Certain varieties, such as Cabernet Sauvignon, have ‘normal’ fruit set as low as 15-20% . Thus, they would never be in the ‘normal’ 30-50% set range. It is more accurate to say that ‘normal’ fruit set occurs when “the cluster framework is filled with berries that have reached full size” .
Poor fruit set can be the result of:
- “coulure,” a French term that indicates excessive shedding of ovaries and young berries which can lead to clusters with few berries;
- and/or a higher than average number of small size berries, i.e. seedless ‘chicken’ berries and live green ovaries  (Figure 2).
Reduced number of berry per cluster, due to ‘poor’ fruit set, can be partially compensated by an increased growth of the remaining berries .Low number of berries per cluster doesn’t equal ‘poor’ fruit set
There are situations where certain varieties are said to have ‘poor’ fruit set because of a relatively low number of berries per cluster. However, a low number of berries per cluster may be a consequence of a low flower number per inflorescence rather than a ‘poor’ set. For instance, Tempranillo in Australia typically has a low berry number per cluster but fruit set can be 59% or higher . The only accurate method to assess % fruit set is to count the number of flowers per inflorescence and the number of berries per cluster after fruit set.
What are some of the factors that affect fruit set in grapevines?
There is general agreement that optimum conditions for fruit set include high light intensity, warm temperature, and adequate soil moisture and nutrients . Fruit set is strongly affected by environmental conditions, as well as cultural practices or biotic stress (i.e., disease or insect pest damage) that affects vine balance and health. Many factors can affect the % of fruit set either directly or indirectly via their impact on photosynthesis and nutrient and water availability. Some cultivars like Merlot, Grenache, and Gewürztraminer are more susceptible to environmental conditions leading to ‘poor’ fruit set than others like Pinots, and Chardonnay .
Overcast, wet, cool or hot weather can result in ‘poor’ fruit set and loose clusters (Figure 3). An extended period of low (<15°C/59°F) or high (> 32°C/ 90°F) temperature could interfere with the development of pollen and pistil during the flowering process or the development of the ovule after fertilization . Rain during the flowering period can physically inhibit pollination and fertilization by a “dilution of the stigmatic fluid and thus interfere with the germination of pollen grains” . Rain can also result in the failure of flower caps to be shed which will reduce fertilization and thus fruit set . Radiation intensity has an indirect effect. If conditions are overcast leaf photosynthesis will not proceed at optimal rate which could limit carbohydrate supply to the inflorescence.Vine factors
Vine factors include carbohydrates supply, water supply and mineral nutrition. Deficiencies of any of the essential mineral nutrients can affect fruit set detrimentally. The most important micro-nutrients in modulating fruit set are boron and zinc. Zinc deficiency can be induced by excessive applications of phosphorus or high pH soils. Deficiency can be prevented by foliar sprays applied during bloom . Molybdenum deficiency may also cause ‘poor’ fruit set, although this seems to be confined to Merlot . Deficiency of luxury consumption of nitrogen can also lead to ‘poor’ fruit set.
Carbon and nitrogen status of vines play an important role in fruit set regulation. The influence of vine carbohydrates and nitrogen is complex and not completely understood with regard to fruit set. Unbalanced C:N status of the vine can results in poor flower development and fruit set . Overly vigorous (lower C:N ratio) or weak vines (higher C:N ratio) with insufficient or inefficient leaf area (e.g., due to herbicide damage, insect feeding, disease attack) tend to have reduced fruit set and loose clusters. Rapidly growing shoots are the strongest sinks for carbohydrate and mineral nutrients. If the vine is not in balance, the shoots will divert resources away from the inflorescences leading to potential ‘poor’ fruit set.
We cannot change or control weather conditions, but we can help the vines to achieve good fruit set by implementing management practices that maintain or improve vine balance (i.e., balance between vine vegetative and reproductive growth).
Dr. Skinkis, associate professor and viticulture Extension specialist at Oregon State University, recommends documenting ‘poor’ fruit set. It is important to keep record of vines or blocks with ‘poor’ fruit set for future investigations. It is also important to record seasonal weather data, the vineyard nutritional analysis, as well as pruning weight and yield data to determine if the vines are in balance .
 Dookoozlian, N. (2000) Grape Berry Growth and development. Pages 30-37, in: Raisin Production Manual. By P. Christensen; Publisher: University of California, Agricultural and Natural Resources Publications, Oakland, CA.
 Keller, M. (2010) The Science of Grapevines: Anatomy and Physiology. Publisher: Academic Press.
 Iland, P, Dry, P, Proffitt, T, Tyerman, S. (2011) The grapevine: from the science to the practice of growing vines for wine. Publisher: Patrick Iland Wine Promotions.
 May, P. (2004). Flowering and Fruitset in Grapevines. Publisher: Lythrum Press.
 Vasconcelos, MC, Greven, M, Winefield, CS, Trought, MCT, Raw, V. (2009). The Flowering Process of Vitis vinifera: A Review. American Journal of Enology and Viticulture 60:411-434.