Poor Fruit Set in Grapevines - Q&A

DJ's Team.

We have a problem for you. We have had very poor fruit set this year in our Cabernet Sauvignon. We are not sure why. Do you have any thoughts? 


This has been common this year.

An individual grape inflorescence 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). 

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 is often called "hen and chicken" where large and small berries exist within a bunch.

There can be loss of the entire inflorescence, termed inflorescence necrosis and loss of individual flowers within an inflorescence, or flower necrosis. Some flowers may wither before flowering, and still others may abort prior to capfall. Finally, there can be flowers that set and form small shot berries that never ripen and may abort before harvest.

When poor fruit set is observed, it can usually be associated with factors that influence the development of flower parts between bud burst and capfall. 

PIC: Hen and Chicken set in Pinot Noir. (c) James Hook

Eutypa lata.

Where present Eutypa has a significant effect on reducing yield by reducing berry set. 

Vine nutrition.

Micronutrient deficiencies of molybdenum (Mo), 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. Molybdenum deficiency associated with poor fruit set ('hen and chickens'). Merlot grapevines, in particular, have a critical need for it.

Salt uptake from either irrigation water or from the soil can also affect fruit set (below).

PIC: Sodium Chloride toxicity in grapevines. (c) James Hook

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-capfall 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.


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, capfall, 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 capfall, the progression of capfall may be delayed and result in reduced set. Finally, rain during capfall 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 autumn frost, winter damage, hail, or other methods of vine defoliation (herbicide, insect feeding, etc). During autumn, the vine is redirecting nutrients from its leaves to store as reserves in the trunk and roots. If a severe autumn 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.

Some varieties are prone to very poor berry set and hen and chicken. Malbec is an example.

PIC: Poor set in cv Malbec in Langhorne Creek - Nov 2014 (c) James Hook

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 flowering.

A good source of information -

May, P. 2004. Flowering and Fruitset in Grapevines. Adelaide: Lythrum Press.


Jock said…

Thanks for the note re poor berry set. The two most serious events where the cold conditions leading to the 2000 vintage (that was cold and rain of Spring 1999). The cold cause the abortion of the berry and the rain causes a dilution of the vine fluids.

I’m suggesting we will see crop levels similar to vintage 2007

1. Dry mid row sub soils
2. Lighter crops
3. Smaller berry size

Lets hope we don’t get 50mm in February – it was the split (or berry explosion) that got us.

Jock Harvey

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