Patty Skinkis, Oregon State University
An individual grape inflorescence (flower cluster) contains hundreds of flowers. However, not all of those flowers will set fruit and develop into berries. On average, 50 percent of flowers within an inflorescence set fruit and become berries (May, 2004). Any greater percentage of fruit set can lead to more compact, tight clusters that can be more prone to fungal infections, particularly in those regions that have higher risk of botrytis and other rots. Fruit set that is low (less than 30 percent fruit set) can lead to clusters with few berries, and/or clusters with significant berry variability. This phenomenon is often called “hens and chicks” where large and small clusters exist within a cluster.
Defining Poor Fruit Set
There are a number of different ways that poor fruit set can be defined. There can be loss of the entire inflorescence (flower cluster), termed inflorescence necrosis and loss of individual flowers within an inflorescence, or flower necrosis. Some flowers may abscise before bloom, and still others may abort prior to bloom. Finally, there can be flowers that set and form small shot berries that never ripen and may abscise before harvest. In some cases, these shot berries are retained. If you see a significant lack of fruit set, you should document and describe your observations, as this may be indicative of a potential causal factor.
Potential Causes of Poor Fruit Set
When poor fruit set is observed, it can usually be associated with factors that influence the development of flower parts between bud break and bloom. Development of flower parts begins shortly after bud break and takes approximately six to eight weeks. The conditions during bloom can be a critical factor in how many flowers per inflorescence set fruit. Because nearly 50 percent of the flowers may not set fruit in a cluster, it is normal to have some flower abscission. It is best to wait until approximately 10 to 12 days after full bloom to observe flower clusters for fruit set estimates.
- Vine nutrition. Research to date indicates that vine nutrition has an impact on bud fruitfulness (number of flower clusters in a bud and on a shoot) developed during the previous growing season as well as floral differentiation in the current season prior to bloom. Carbon (C) and nitrogen (N) status of vines have been implicated as potential causes of poor fruit set and inflorescence necrosis. Micronutrient deficiencies of boron (B) and zinc (Zn) can result in poor fruit set as they play a role in early season shoot growth, and in the case of boron, pollen tube generation which is required for fertilization. Water stress prior to bloom has also been associated with poor fruit set, and this can be related to the lack of shoot growth and nutrient uptake prior to bloom. The influence of vine C and N is more complex and not completely understood with regard to flowering and fruit set. However, too high or too low vine N can lead to poor fruit set or inflorescence necrosis. The relationship may not be solely in total concentrations of N in vine tissues, but rather the C:N status of the vine. This is directly related to vine vigor status.
- Vine vigor status. Vines with high vigor tend to have higher N in their tissues, making C:N lower. Conversely, weak vines have lower N and higher C, leading to a higher C:N ratio. In either case, having an unbalanced C:N status of the vine can lead to poor flower development and fruit set. This also relates to competing sinks in the vine: shoots vs. clusters. In overly vigorous vines, shoot tips can out-compete clusters for resources pre-bloom and can lead to reduced flower development and poor fruit set. Conversely, a weak vine will have fewer resources in stored carbon and nitrogen, leading to weak growth. The stronger sink in the weak vine (shoots) will pull resources from the flowers leading to poor fruit set. Therefore, it is best to achieve good fruit set by managing vines for vine balance between vegetative and reproductive growth. The goal is a moderately vigorous vine, not a weak or overly-vigorous vine.
- Weather. Overcast, cool, and wet weather can reduce fruit set; however, the mechanism differs between the times when the weather occurs for different processes (floral initiation, development, bloom, and fruit set). Cold and overcast weather prior to bloom can lead to problems in floral development. These environmental factors are likely linked to vine C and N status, particularly if growth is stunted during the early stages of the growing season. If the weather is cold at the time of bloom, the progression of bloom may be delayed and result in reduced set. Finally, rain during bloom can physically inhibit pollination and fertilization by dilution of the stigmatic surface which is to receive pollen from the flower’s anthers.
- Damaging Events. Anything that is drastically damaging to the vine’s canopy can lead to problems with poor fruit set. These events may include early fall frost, winter damage, hail, or other methods of vine defoliation (herbicide, insect feeding, etc). During fall, the vine is redirecting nutrients from its leaves to store as reserves in the trunk and roots. If a severe fall frost is experienced well before leaf-fall, there can be a significant disruption of this nutrient storage that will leave the vine in a weaker state the following spring. Similarly, any event that can significantly defoliate a vine late in the growing season or in early spring can lead to poor flower development and reduced fruit set by way of reduced carbon assimilation and storage.
- Plant material. A few cultivars and clones of Vitis vinifera can normally have poor fruit set. The underlying cause is not certain. In some rare instances, self-pollination incompatibility may be an issue. Wild and seedling Vitis muscadinia vines are often dioecious (have either only male or female flowers). Therefore, male vines would have flowers, but would lack fruit. Some of the older muscadine cultivars have only female flowers, requiring either a monoecious vine or a male vine to be planted nearby.
Documenting Poor Fruit Set
If you observe poor fruit set in your vineyards, it is best to keep a record of the situation. If you are not currently doing some estimate of fruit set, it is wise to begin the practice to develop a baseline of information for a given block. To begin observing fruit set, monitor clusters within 10 to 12 days after full bloom. Remember, nearly 50 percent of the flowers may not set fruit, so they can be found falling from the clusters before, during, or after fruit set. Consider taking some observational notes and photos at fruit set for rough estimates. Also, fruit set can be estimated through cluster weight data. Records of berries/cluster and berry weight are certainly good to have in your records, but this requires significant sampling across blocks and is very time consuming and not practical on a production scale. If you observe inflorescence necrosis and/or significant flower necrosis, make note of the block and flag the vines for future investigation. Record weather data from bud break to bloom. Consult your vineyard nutritional analysis records and pruning weights to determine any changes over time in vine vigor as indicated by yields and pruning weights.
While we cannot control weather conditions, we can do our best to manage vineyards for a healthy, balanced state. When this is achieved, even poor years will cause only a minor problem with flowering and fruit set. Where there are considerable problems with over- or under-vigorous vines and/or poor fruit set, the problems in vegetative and reproductive balance can be difficult to bring back into equilibrium and may take more than one season to achieve.
May, P. 2004. Flowering and Fruitset in Grapevines. Adelaide: Lythrum Press.
For information on bearing potential of rootstocks, see Grapevine Rootstocks for Oregon Vineyards, Oregon State University Extension.
Vasconcelos, M.C., M. Greven, C.S. Winefield, M. C.T. Trought, and V. Raw. 2009. The Flowering Process of Vitis vinifera: A Review. American Journal of Enology and Viticulture 60:411-434.
Reviewed by Sara Spayd, North Carolina State University and Jim Wolpert, UC Davis