Efficient and sustainable cultivation of avocado with the Ecoculture programme

By Angel Ruiz.

A sustainable avocado cultivation has been set as a goal for the company Ecoculture Biosciences. In order to define our sustainability concept we have established the following points and guidelines: 

• Improve land use for more intensive production
• Reduce losses caused by environmental stress 
• Reduce environmental impact by using less conventional fertilizers
• Increase the harvest by improving quality and reducing the number of discarded fruits
• Increase of fruit shelf life and less deterioration of fruits and vegetables

The problems associated with this crop can be summarized in the following factors, which are closely related to sustainability:

• Fruit drop related to situations of abiotic stress
• Flower drop and fruit set drop
• Drop of developing fruit
• Alternate bearing
• Depletion of carbohydrate reserves in the roots
• Bud differentiation (vegetative vs. generative)
• Apical dominance (excessive vegetative growth)
• Losses due to various stressful events throughout the development cycle
• Ca absorption
• Chilling injury
• Diseases resulting from abiotic stress factors; Anthracnose (Colletotrichum gloeosporioides) and Stem-end Rot. (Botryodiplodia theobromae)

Factors influencing flowering:

Flowering is the main process that determines the amount of harvest. In this sense, there are several factors that influence the flowering process. 

1. Photoperiodic induction influenced strongly by the length of the day. 
2. Temperature perceived by the whole plant. The low temperature (Vernalisation) determines the dependence of plants on exposure to a low temperature to flower.
3. Irradiation
4. Nutritional factors that make reference to the availability of different macro and micro elements
5. Water availability

The disturbance of these factors generates abiotic stress situations with a direct impact on production. A great number of assimilated profiles and phytohormones participate in the process of floral induction. The levels of this type of compounds will also be affected by the environmental conditions that occur during this period.

Avocado does not need a bud rest period to start the reproductive phase and both young and mature shoots are able to flower. The young shoots produce only apical inflorescences. Mature ones produce flowers and axillary buds. Referring specifically to the avocado “Hass”, the  buds are determined to flower after 4 weeks of low temperature (10/7 ºC day/night).

Avocado flowers have a dual reproduction system known as dicogamic synchronisation (Nirody, 1921). When a flower opens for the first time it is functionally female. After remaining open for several hours it closes to reopen the next day, and when it reopens it already has the capacity to release pollen, being functionally male. Low temperature delays the flower opening, therefore increasing the possibility of self-fertilisation. Likewise, low temperatures during the critical stages of flowering (pollination, germination, development of the pollen tube or fertilisation of the ovule) are determining factors for fruit set (Lovatt, 1990). 

The initial formation of the inflorescence meristems occurs in autumn (several months before flowering or anthesis), that is why it is so necessary to avoid stressful disturbances as far as possible at that time. Considering that the avocado can produce around one million flowers per tree (Bergh, 1985) the fruit set percentage has been estimated from 0.02% to 0.1%; however, only 1 or 2 fruits per inflorescence reach maturity (Chandler, 1958, Bergh, 1967).

The fruit set potencial varies depending on the type of inflorescence whether it is young or adult, as well as the cultivar. The initial fruit set is relatively high but the abscission (fruit fall) at the beginning of development is very high.

Factors influencing flower drop:

1. Competition between generative and vegetative growth.
2. Incorrect irrigation and fertilisation programs (excess of nitrates, ammonium, etc.) that stimulate vegetative growth and increase fruit fall
3. Abiotic stress (Lovatt, 1990), which increases the production of ethylene and oxidative compounds. The low temperatures decrease significantly the viability of the ovule so that early flowering can decrease the yield. The optimum temperature range for fruit set is between 20-25 ºC. By contrary, the abscission accelerates at temperatures above 28 ºC (Sedgley, 1977). 
4. Water deficiency in the fruit caused by excessive flower transpiration

FACTORS INFLUENCING ALTERNATE BEARING

Alternate bearing is a phenomenon whereby fruit trees alternate abundant harvests with years of low yields. This pattern causes great economic impact, since the ideal situation is an equal production each season. But due to the alternation, during the years of high yield, the fruits are small in size and therefore, their commercial value is lower, while during the years of poor production, the fruits are few but large, reason why its marketing is not optimal either.

The factors influencing the alternate bearing  can be summarised in five: 

1. Depletion of carbohydrate reserves in the roots
2. Differentiation of buds (vegetative vs. generative)
3. Apical dominance (excessive vegetative growth)  
4. Losses due to different stressful situations throughout the development cycle
5. Extension of permanence of ripe fruit on the tree

The stress due to low temperatures in autumn inhibits the growth of vegetative shoots (decreasing Gb levels) and the available carbohydrates go mostly to the roots increasing the reserves. In this way, higher levels of ABA and cytokinins are produced, which are responsible for initiating the formation of inflorescences. Cytokinins increase the number of generative buds and ABA supports the generative action of the cytokinins. On the other hand, the increase of NH4+ accumulation is linked to low light stress which happens in autumn (Lovatt et al. ;1988). This is determinant in the flower set process. 

The strategy designed by Ecoculture minimises the avocado crops losses. Using the complete treatment program which takes into consideration all the previously exposed factors, we respond to situations that cause important losses for avocado producers.

AVOCADO CULTIVATION PROGRAMME

At the beginning of sprouting, two applications of NHDeltaCa and XStress should be made in order to stimulate the tree to produce new buds, obtain more generative buds and achieve greater reproductive growth.

At the beginning of flowering an application of Rainbow Wave will increase pollen fertility and encourage generative growth instead of vegetative growth.

At the end of flowering, a foliar application of CalFlux will boost and improve the fruit set.

During the period of fruit set, a foliar application of CalFlux and XStress will reduce the fruit drop as well as the negative effects produced by stress.

During fruit growth, 3 applications of NHDeltaCa and XStress will favour reproductive growth and reduce the negative effects produced by all abiotic stress factors.

During this phase of fruit growth, 2 to 4 applications of Rainbow Wave will favour generative growth by boosting energy to the fruits and roots, increasing the size of fruits and ºBrix.

Finally, in pre-harvest period, 1 application of XStress will reduce fruit drop and increase post-harvest and fruit quality, reducing problems such as chilling injury or internal fruit tissue collapses.

In addition, in order to optimise the use of irrigation water and favour its horizontal movement, infiltration and retention, the use of Slick at 1 L/Ha on a monthly basis provides a substantial improvement that reduces water stress problems.

The Ecoculture programme has been tested in recent years improving the production. 

• Area: 2,8 Ha’s
• Variety: Hass
• Location: Orgiva (Granada)- Spain
• Farmer: Román Gerardo Castillo