Now that we are getting close to fruit set (and maybe even seeing a bit in early varieties planted in southern / lower altitude vineyards), it is a good time to review fruit set, what can impact it, and what this means in an applied sense.
Flowering (see left picture, below) and fruit set (see right picture, below – source: https://flgp.cce.cornell.edu/submission.php?id=52) are complex physiological processes. The intention of this post is not to review literature on fruit set. For further, in-depth reading, please read suggested literature at the bottom of this post. Information herein is derived from several of those sources.
Fruit set is the process of a flower forming a berry. In seeded grapes, this occurs when successful fertilization occurs and initiates seed development. Fruit set is the stage roughly defined by peppercorn-sized berries (i.e. immediately after bloom), although berries throughout a vineyard, and even within a cluster, are often at different transitional stages. There are hundreds of flowers in a grape inflorescence (group of flowers arranged on a stem). However, even in the most perfect fruit set conditions, a relatively low percentage of the flowers in a grape inflorescence actually form a berry; this supported by the relatively low berry numbers per cluster compared to the number of flowers in an inflorescence. Fruit set percent can be impacted by several factors; a few of these factors are variety, weather during bloom, vine nutritional status, and carbohydrate status at bloom.
Variety. Varieties differ in their inherent fruit set percentage, and also differ in their ability to tolerate environmental stresses that affect fruit set percentage. For example, typical fruit set in Cabernet Sauvignon, is lower than the “norm” of 30-50% across varieties.
Weather. In general, wet, cloudy, and rainy weather reduces fruit set percent while clear, warm weather improves fruit set percent. Extremes in temperature (<60 F and/or >100 F) adversely affect pollen tube growth and germination and, therefore, fruit set. While ranges vary, optimal temperature for both aforementioned processes is 80 F to 90 F.
Vine nutritional status. Excess or deficit levels of several mineral nutrients can result in direct or indirect fruit set reduction. For example low vine nitrogen status can reduce fruit set. However, high vine nitrogen status may also reduce fruit set, particularly as it relates to vigorous shoot growth serving as competitive sinks to flowering and setting clusters. Particularly key nutrients in fruit set are phosphorous, zinc, boron, and molybdenum, as these are important for several pollen processes. Thus, deficiencies in any of thes mineral nutrients may reduce fruit set as well.
Carbohydrate status. There have been several publications about the effect of vine carbohydrate status on fruit set. Bloom and fruit set are the primary early season stages in which the cluster is a strong carbon sink. Interference with carbon supply or allocation to the cluster at this stage can result in reduced fruit set. For example, when I removed several primary shoot leaves before bloom, I reduced berry number per cluster (see top two pictures, below). I did not measure fruit set per se. However, based on the findings of others, I would attribute the reduction in berry number per cluster to reduced fruit set as a function of reduced carbon allocation from source tissues (i.e. leaves). When I waited until a week after fruit set to remove leaves, I did not effect berry number per cluster or crop yield (see bottom picture, below). This illustrates the effect of leaf removal timing on berry number per cluster.
Take home for this season (for now anyways): Strictly speaking about temperature and weather patterns, we could be in for a good fruit set given our past, current, and predicted weather patterns in Georgia – the same appears to be true for parts of North Carolina and Virginia as well. Crop yield is a function of vine number per unit land mass, shoot number per vine, cluster number per shoot, and berry number per cluster. All else equal, crop yields could be average to above average if we do in fact experience optimal fruit set. Thus, if your operation estimates crop yield, consider your weather patterns throughout bloom and fruit set – if warm and dry, it may be safe to assume optimal fruit set and average to above-average crop yield. Of course, it is important to go out and observe clusters and execute your normal crop estimation protocols – whatever those may be (i.e. count berries and weigh clusters, or weigh berries at 30 days after bloom or lag phase). Estimating crop yield is difficult, and a good record book specific to each variety on your farm may be the best tool. If accurate, crop yield estimation may aid in budgeting tank space and grape sales.
If fruit set is optimal, then berry number per cluster will be relatively high. As this could mean tighter clusters and greater fungal disease severity, this is just a reminder that this is a VERY critical period to protect your clusters from powdery, downy, and black rot, but also the bunch rots (botrytis, bitter, ripe). And I would remind all to visit Dr. Mizuho Nita’s blog (https://grapepathology.blogspot.com/) and use his resources posted at the right sidebar of his blog – such as his fungicide template.
Best of luck to all for optimal weather through bloom and fruit set.
Further flowering / fruit set reading:
Flowering and Fruitset in Grapevines (book), by Peter May.
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 (book). Publisher: Academic Press.
Lebon, G., E. Duchene, O. Brun, C. Magne, and C. Clement. 2004. Flower abscission and inflorescence carbohydrates in sensitive and non-sensitive cultivars of grapevine. Sex. Reprod. 17: 71-79
Lebon, G. G. Wojnarowiez, B. Holzapfel, F. Fontaine, N. Vaillant-Gaveau, and C. Clement. 2008. Sugars and flowering in the grapevine (Vitis vinifera L.). J. Exp. Bot. 59: 2565- 2578.