WHAT IS FRUIT SET?
Fruit set is the process in which flowers become fruit and potential fruit size is determined. It occurs after pollen is released from male flower parts (anthers), lands on receptive female flower parts (stigmas), produces a tube that grows to the ovule, and fertilises the egg within the ovule. The fertilised egg forms the seed, which induces the surrounding (pericarp) tissue to grow and form a berry. Yes, as strange as it may seem, the avocado fruit is botanically classified as a berry!
NUTRIENT RESOURCES NEEDED FOR FRUIT SET
Fruit set is a stressful time for avocado trees due to the high demand and competition for resources such as sugars, starch, water and nutrients.
It is a period when the tree is providing nutrients and resources for the mature fruit that is being harvested, supporting the needs of the fruit that is being set and for producing new shoots that will produce floral buds next year for year three’s harvest – three concurrent growth cycles!
There is a tremendous amount of competition between these three cycles for resources and the dominant activity will sink the majority of the resources.
Because an avocado tree only has so many resources to offer, the tree will adapt in order to preserve itself — dropping leaves, dropping fruit, minimizing new shoot growth and even inhibiting root growth.
The challenge is to balance the resource needs of all three growth cycles in order to ensure optimal tree health and performance as well as optimal fruit quality and yield.
If these needs are not met, in heavy crop or flowering years, the tree will produce fewer new shoots because the majority of the resources are dedicated to supporting the large amount of fruit or flowers.
Fewer new shoots one year means less avocado fruit the next year because avocado trees only flower, set fruit and bear fruit on peripheral new shoots. This is primarily the cause of alternate bearing. Our aim as growers however is to achieve consistent yields year after year and thereby improve cashflow and profitability.
BALANCING FRUIT SET FROM YEAR TO YEAR
To reduce irregular or alternate bearing, we need to ensure that avocado trees have the nutrients and carbohydrate reserves they need for consistent and balanced flowering and fruiting and shoot and root growth for three concurrent growth cycles.
Correct, adequate and balanced nutrition will also ensure that overwintered leaves remain healthy throughout the winter period as well as the flowering and fruit set period without abscising (falling off). This is important for the accumulation of starch through photosynthesis, which is required for both successful fruit set and spring vegetative flush.
Remember as with most things, an integrated approach is required.
Nutrition, pruning, disease and pest control, tree health, orchard sanitation and soil health and fertility all need to be well managed. Bud break to harvest is a two-year cycle and ensuring improved consistent production and profits requires good balanced management for the entire two years preceding harvest.
It is normal for a large portion of flowers and fruitlets to fail and fall, even under optimum conditions but there are also other factors that cause flower and fruitlet drop that we can manage.
FACTORS AFFECTING FRUIT SET
- Strong competition between fruitlets, vegetative and root growth for resource reserves and photosynthates.
- Stimulation of strong vegetative growth during the period critical to fruitlet retention will result in increased fruitlet drop and reduced yields. Remember that balanced nutrition is crucial.
- Carbohydrate deficiency will result in excessive fruitlet abscission (drop).
- Failure in seed development.
- Poor pollen germination and subsequent fertilisation.
- Embryo abortion.
- Seed coat collapse: A properly functioning seed coat is essential for normal fruit development. The seed coat is a very rich source of growth regulators and acts as intermediary between the embryo, the pericarp, and the whole tree.
- Genetic factors.
- Environmental stress: Environmental conditions before, during, and after flowering have an impact on fruit set. Extremes in both temperature and soil moisture as well as frost, excessive rainfall and high winds can all affect successful fruit set.
- Water-deficit stress during flowering and fruit set will cause excessive flower and fruitlet drop.
- Under or over fertilisation, imbalanced nutrient levels or nutrient deficiencies will cause excessive fruit/flower drop (abscission).
- Over pruning or pruning at the wrong time results in low flower or fruit numbers and excessive vegetative shoot growth disturbing the balance between fruit set and vegetative growth.
- Hormone imbalance: When a plant sets flowers, the direction it grows, the size of its fruits, when it drops them to the ground and virtually every other aspect of plant development is controlled by hormones.Fruit set is regulated by the balance, concentrations and interactions of primarily cytokinins, auxins and gibberellins. In avocados, cytokinins ensure carbohydrate availability to the fruit for development. The concentration and the form of nitrogen fertiliser however also influences cytokinin production or synthesis. The levels of the hormone abscisic acid (ABA) acts as a sensor of the intensity of nutrient shortages. It then modulates the levels of other hormones ACC (1-aminocyclopropane-1-carboxylic acid) and ethylene, that activate abscission. ABA and ACC are the self-regulatory hormones that adjusts fruit load to resource supply, and induce abscission as necessary. This again highlights the all-important balance between nutrients, hormones and carbohydrates.
Understanding the factors that affect flower and fruitlet drop enables us to put strategies in place to help us improve fruit set and yield and subsequent economic returns.
To improve fruit set, balanced soil and leaf fertility management must be used throughout the growing season.
Soil, leaf and water testing
To determine the quantities and rates of fertiliser application; soil, leaf and water tests as well as knowledge of the phenological cycle should be used.
Remember, irrigation water also contains nutrients and this must be considered when calculating fertiliser programmes. Water high in nitrates due to contamination will have a significant effect on fruit set and production.
Pruning management is important for maintaining a balance between vegetative growth and production as well as ensuring adequate light into the canopy for successful flowering and fruit set.
As well as thinning regrowth well in advance of flowering, pruning also minimises competition and directs resources into the shoots that will flower and carry fruit.
Plant growth regulators
Use of plant growth regulators to reduce vegetative growth during the fruitlet retention period can balance the competition between the vegetative flush and the fruitlets and improve fruit set.
Prior to and during flowering, foliar fertilisers can help to improve flowering and fruit set.
Do not apply fertilisers that will spike the nitrogen levels causing a vigorous spring flush as this will strongly compete for resources and can result in excessive fruitlet drop.
A common source of plant hormones is seaweed. Seaweed products contain auxins, gibberellins and cytokinins, causing plants to grow more leaves, as well as stimulating flowering, increasing lateral branching, improving fruit set, developing more roots and dividing more cells. Different products however will have different concentrations and ratios depending on the extraction and concentration process and the type of seaweed used and will stimulate different processes.
Ensure healthy canopies
Leaves are responsible for providing important resources through photosynthesis.
Defoliation and shading decrease photosynthetic activity and induce
fruitlet abscission due to a shortage of carbohydrates and a rise in hormones controlling abscission. If accumulation of starch and sugars is low during the day, night respiration will deplete a large part of the reserves and this can trigger abscission.
Healthy roots are important in order to produce cytokinins and support the efficient nutrient uptake during times of high demand, as well as supply adequate water to meet the increase in transpiration during this period.
Even when soil and tissue levels appear adequate, the supply of nutrients from the roots is often insufficient to satisfy the high demand during flowering and fruit set.
Supplemental fertiliser nutrients applied to the foliage can help to meet this demand.
If you are fortunate enough to have irrigation, now is the time that you will reap the rewards. Any water deficit stress during fruit set will result in elevated fruit drop.
Fruit drop can continue well into February and ensuring we do everything we can to minimize unnecessary fruit drop will ensure we line our pockets rather than the orchard floor!