Phosphorous Deficiency in Grapevines

Phosphorous deficiency is an occasional problem for grapevines. Under natural conditions, almost all vines have a beneficial fungus, mycorrhizae, associated with their roots, which helps them to acquire P, but sometimes the soil itself prevents P uptake.

The distinct symptom of phosphorus deficiency is the appearance of discoloured, redden leaves.

Ben South has found this patch of vineyard over 'ironstone'. In acid soils P forms very insoluble compounds with iron, and it binds strongly to the surface of iron oxides.

Most soils used for viticulture in McLaren Vale have low native concentrations of phosphorus and therefore phosphorus fertiliser inputs are normally required. In this case the 'ironstone' has bound up all of the soluble phosphorous in the soil.

Phosphorus is taken up by vines from the soil water in its soluble phosphate form which would also be quickly bound up to the 'ironstone'.

EL 32 Bunch Closure

Light Brown Apple Moth damage to grapevines before bunch closure.

Bunch closure (EL 32) in tight bunched varieties is a critical time in the lifecycle of botrytis. The greater the amount of ‘trash’ or damaged berries from light brown apple moth (above) at bunch closure the greater the risk of botrytis.

Botrytis first infects berries as latent or ‘unseen’ infection, during flowering when the flower cap falls. A second type of infection begins to develop as berries press together. Botrytis fungal threads (mycelia) begin to grow from fungus on dead and dying plant tissues (such as ‘trash’ flower debris or damaged berries). This growth from dead material infects berries after berry softening and progressively increases in wet or humid weather.

Alternatively varieties grown with an open bunch configuration develop less rot in wet conditions. They have better airflow and lower canopy humidity after rain. A good canopy structure is a key part in botrytis control. High quality Shiraz that is picked in early March has a low risk of developing a significant level of botrytis before it is picked - without chemical application!

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? 

JP

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). 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 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.

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 burst and capfall. Development of flower parts begins shortly after bud burst 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 capfall to observe flower clusters for fruit set estimates. 

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)

Poor fruitset caused by high levels Sodium Chloride.

Water stress. Water stress prior to capfall can cause a lack of shoot growth and nutrient uptake prior to flowering.

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.

Hen & Chicken fruit set in Pinot Noir.

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. Some varieties are prone to very poor berry set and hen and chicken.

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.