Avocado: Preventive treatments against fruit drop

History and Evolution of the Avocado

Avocados have been part of the Mesoamerican diet for approximately 8000 years. It is believed that avocados began to be cultivated for human consumption around 5000 years ago, with the Persea americana variety.

The word avocado comes from the Nahuatl language , where the word "ahuacatl" means testicle and refers to the fruit's shape. This explains why it was also called the "fruit of fertility," since the Aztecs believed it had special powers, including aphrodisiac qualities in both men and women, as well as giving strength to whoever ate it.

The honor of making the first technical description of the avocado went to Francisco Hernández in his " History of the Plants of New Spain " after having carried out the first exploration in Mexican territory from 1571 to 1576.

The first mention of the word avocado (English) was in 1696 by Henry Sloane , who also described it as the " crocodile pear " because the rough appearance of its skin allowed it to be compared to the skin of this reptile.

The conquistadors must have brought the avocado to Spain around 1600, as there are records of the tree in the Valencia Botanical Garden and of some plantations in the province of Málaga in 1770. Likewise, Indian traders and wealthy families brought avocado trees to Spain in the XNUMXth and XNUMXth centuries to provide shade for their gardens. Later, avocado trees began to spread worldwide. 

The avocado was described by the British horticulturist and botanist Phillip Miller and published in " The Gardeners Dictionary" in 1768 in its eighth edition, using the name Persea, designated by Theophrastus, which derives from the Greek in honor of Perseus, the demigod, to refer to a tree from the east, and americana to mention its origin in the American continent.

In 1924, the project began to be implemented in Málaga , at the Southern Agricultural Station, where the initial plan included the introduction of exotic fruit trees. It was with the incorporation of the Torrox experimental station that an initial avocado nursery was established around the 30s.

The popular Hass variety first appeared in 1926, and the first Hass tree died of old age and root rot around 2002. It was a purely lucky discovery. In those years, the most popular avocado variety was the Fuerte, and this was the variety that Rudolph Hass intended to cultivate from a seed, attempting to graft his plant with various types of avocados, but none yielded the expected results. Disappointed, he considered cutting down his tree, but his children convinced him not to, as they preferred the flavor of his fruit to that of the popular Fuerte. Thanks to them, we now have this variety of avocado with a creamier taste.

There are currently over 400 varieties of avocado. It is considered one of the most nutritious fruits , high in fiber , with more potassium than bananas, and rich in folic acid and vitamin E. It is also one of the fruits with the highest protein content, and its oils are good for the skin, as they contain antioxidants that prevent aging.

Requirements for avocado cultivation

Before establishing an avocado plantation, it is necessary to know its soil and climate requirements , and to verify that the characteristics of our plot are suitable to such requirements, this being an essential condition for the crop to be profitable.

The avocado originally evolved in Andisol soils , characterized by low bulk density, high macroporosity, high organic matter content, and high rainfall . Therefore, the avocado thrives in soils with high hydraulic conductivity, exhibiting shallow roots, lacking root hairs, and highly sensitive to oxygen deficiency.

Avocado crop humidity

Periods of heat and drought can cause fruit drop , especially in mountain varieties. Excessively humid sites are unsuitable due to the increased likelihood of certain soilborne diseases to which the species is highly susceptible.

Lighting in avocado cultivation

Avocado cultivation requires good, direct sunlight . Shaded branches are unproductive. Pruning is highly recommended to allow sunlight to reach the interior of the canopy. Only the main branches and trunk should be shaded.

Soil in avocado cultivation

It adapts to a wide variety of soils, from sandy to clayey, volcanic silt, lateritic, and calcareous, but grows best in well-drained, slightly acidic, loamy soils rich in organic matter . The West Indian variety tolerates calcareous and slightly saline soils. No variety tolerates heavy, poorly drained soils, and it should not be planted when the water table is less than 1 meter below the surface. The optimal pH range is considered to be between 5,5 and 7, although some cultivars grow well in soils with a pH of 7,2 to 8,3.

Water requirements for avocado cultivation

Irrigation is undoubtedly the most crucial factor in the success or failure of our avocado tree , especially in its early stages. It needs high but well-distributed rainfall over time and a slow release rate. It is susceptible to the fungi Phytophthora cinnamomi and Rosellinia necatrix , whose development is favored by excessive moisture, poor aeration, and inadequate drainage.

The amount of irrigation water depends heavily on climate, rainfall, soil type and composition, crop varieties, etc. For commercial production in Spain, average values ​​have been established for localized drip irrigation , with irrigation doses per month, etc., but these are always subject to significant variations due to the other soil and climate factors already mentioned. Localized irrigation is the best option because:

• It is slow emitting.
• It is easier to regulate the amount emitted by the dropper.
• Since irrigation is variable depending on age, we can better calculate irrigation doses.
• It is easy to program.
• The irrigation dose will increase over the years and it will be easier to establish frequencies and times.

Recommendations for avocado cultivation in the southern and eastern climate zones of Spain.

These data are for guidance only. According to the FAO, water volumes of 4000 m³ ^/ ha/year would be sufficient for avocado cultivation, while other publications recommend irrigation doses calculated according to climatic data from a weather station, which indicate irrigation needs of approximately 3500 m³ ^/ ha/year.

On the other hand, the quality of the water must also be taken into consideration , since the avocado does not tolerate irrigation with water with an electrical conductivity above 1,5 mmhos/cm.

Nutrition in avocado cultivation

It's important to note that the nitrogen, phosphorus, and potassium requirements for trees younger than 3 years old are 1:2:1; for slightly older trees just beginning to produce, the ratio is 1:1:1; and for mature trees in full production, the ratio is 2:1:2.

In avocado nutrition, we must primarily consider the needs for nitrogen, phosphorus, potassium, calcium, magnesium, zinc, boron, and iron, among other essential nutrients.

• Nitrogen: The requirements for this element are estimated at 100–200 UF per hectare per year, with the highest requirements occurring between May and October. Given the high mobility of this element and to avoid leaching losses that result in the contamination of aquifers, applications should be made in small quantities along with irrigation water.
• Phosphorus: The requirement for this element is estimated at 20–40 UF per hectare per year. Additions should be made during flowering and at the beginning of root development.
• Potassium: The requirement for this element is estimated at 100–200 UF per hectare per year. Maintaining an adequate level is important to ensure fruit growth, and it should be supplied between May and October.
• Football: The requirement for this element is estimated at 30-50 UF per hectare per year. The greatest absorption of Ca by avocado occurs in the early stages of fruit development.
• Magnesium: The needs of this element are estimated at 10-30 UF per ha per year.
• Zinc: Requirements are estimated at 10-30 UF per hectare per year. Soils with high pH levels are often deficient in this element.
• Boron: The requirement for this element is estimated at 3-5 UF per hectare per year. It is very important for fruit setting and is well assimilated by the inflorescences.
• IronThe peculiarity of this element lies in the fact that it solubilises in acidic media and precipitates in basic media, making it difficult to supply plants. Chelates, by sequestering the metallic iron ion, form stable, soluble molecules that are easily assimilated by the plant. Therefore, iron applications to plants should be carried out in the form of chelates.

The distribution of each element throughout the growing cycle will depend on the role it plays in the plant's development. Nitrogen requirements are highest during vegetative growth, while potassium and phosphorus are most important during flowering and fruiting. Calcium is essential during fruit development, and boron is crucial during flowering, given its influence on pollen viability.

To establish an appropriate fertilization plan, it is essential to conduct soil, foliar, and irrigation water analyses , as this allows us to use the correct products to address critical levels or nutritional deficiencies promptly. Fertilization should be carried out through monthly application programs, taking into account the crop's phenological stage, and paying particular attention to nitrogen application, since an excess of this element can cause flower and fruit drop.

Organic matter is a very important factor in complementing chemical fertilization, so sufficient quantities should be applied according to the age and size of the tree. Maintaining or restoring soil fertility is essential for the stability of the plantation.

To summarize, for proper avocado nutrition you should:

1. Maintain an oxygenated, fertile soil rich in organic matter.
2. Define the expected extractions from our crop.
3. Discount some of the nutrients supplied by water, taking into account the interactions between them, and adjust based on soil reserves.
4. Choosing the optimal application times.
5. Verify and correct with foliar analysis

Causes of fruit drop in avocado cultivation

Considering only the phenological stages between fruit set and initial fruit growth and the start of vegetative activity in shoots , there are four main causes that can directly or indirectly cause fruit drop in avocados:

1.Nature

Avocado cultivars can produce thousands of inflorescences , each of which, in turn, can be made up of more than 100 flowers , so that the total number of flowers per tree can be more than a million.

Avocado fruits that do not set fruit are divided into two groups: those from pollinated flowers that did not achieve fertilization , and those from pollinated and fertilized flowers that produce a normal, seeded embryo.

Under favorable conditions, avocado trees produce more fruit than the tree can bear to maturity . In these conditions, the plant adjusts its capacity to nourish the fruit by reducing the number of fruits it produces, causing a mass drop of newly set fruit during the first three to four weeks and again in the summer, when the fruit has reached between 10% and 40% of its final size.

Some studies show that during the first week after anthesis, 80% of the fallen fruits came from pollinated but unfertilized flowers. However, one month after anthesis, all the fallen fruits had been fertilized and showed normal embryo and endosperm development.

A final drop can occur in autumn, as a result of insufficient water supply and rising ambient temperatures.

2. Physiological

This cause is based on the fact that the plant suffers stressful conditions that hinder its normal physiological functioning. In other words, when faced with excessive temperatures above 35°C or a water deficit , the plant will respond physiologically by altering its normal functioning. One of these responses can be abnormal stomatal behavior , leading to excessive transpiration and, therefore, excessive water loss. Conversely, the plant may completely close its stomata, inhibiting photosynthesis and carbohydrate transport. In both cases, potential fruit drop would be related to these abiotic stresses: on the one hand, to excessive temperature or sunscald, and on the other hand, to a water deficit, whether due to excessive water loss through transpiration or insufficient irrigation.

There are three possible solutions to prevent this accident.

a) Plant avocado in an area with climatic characteristics suitable for its cultivation.

b) High-level spraying , using drip sprinklers, to wet the tree's leaf surface so that the water, upon evaporation through heat exchange with the leaf surface, cools the leaves. This method involves a larger initial investment and an extraordinary expenditure of water, although it has the advantage that it can incorporate fertilizer treatments and can also be used for preventative measures against low temperatures.

c) From Cultifort's technical and R&D department, we recommend a nutritional solution using one of our Special Products, CultisilK . This is a potassium silicate that also incorporates free amino acids. On the one hand, silicon creates a protective layer on the plant to buffer the thermal regime against excessive temperatures. On the other hand, potassium acts as a regulator of proper stomatal function , helping to maintain cell turgor and thus preventing excessive water loss through transpiration. Finally, the free amino acids are designed to reinforce chlorophyll synthesis and activate photosynthesis , preventing interruptions in the synthesis of assimilates and ensuring that competition between fruit and growing shoots is not exacerbated.

Another possible cause of stress is root asphyxiation . It's important to remember that avocado cultivation is extremely sensitive to excess soil moisture. Root asphyxiation occurs either due to poor irrigation management or excessive rainfall during a period of lower water demand. In any case, there is a period of approximately 90-100 days after flowering and fruit set when this crop is especially sensitive to excess moisture, potentially suffering from root asphyxiation stress that directly causes massive fruit drop . To minimize these situations, it is recommended to plant avocados in loose, well-drained soils with a macropore volume greater than 17% (24% is ideal), at a minimum depth, carefully planning water runoff from the plot to avoid accumulation in the lower areas, and implementing effective irrigation management , optimizing water application and soil oxygenation.

Regarding soil oxygenation, at Cultifort we have developed Oxifort , another of our Specialty Products. This product, when applied with irrigation water, slowly releases oxygen , improving aeration in the root zone, halting root asphyxiation, and inhibiting the growth of anaerobic microorganisms. It also improves soil structure, making it more porous and allowing air and water to circulate more freely through the pore space. This increases the effectiveness of fertilizers, especially nitrogen fertilizers, and promotes the growth of beneficial microorganisms that participate in the transformation and mobilization of soil nutrients.

3. Nutritious

From a nutritional point of view, fruit drop could originate from a limited supply of assimilates or from strong competition for nutrients between fruits and vigorous shoots that develop at the same time.

Implementing a proper fertilization program is essential, as this practice represents an expense for the farmer and can negatively impact the environment if not done correctly. The current approach to avocado fertilization management focuses on providing fertilizers at specific times and in specific quantities for each phenological stage , according to the nutritional requirements of the avocado to sustain a given level of production. This approach aims to increase yields without unnecessarily increasing production costs.

When the content of these elements is deficient, a series of symptoms often appear in the plant, and visual observation can help us identify which element is below its optimum level. However, determining deficiencies through visual symptoms is not a reliable tool for establishing a firm diagnosis . A multitude of causes (phytotoxic effects of pesticides, fungal, bacterial, and pest infestations, damage caused by adverse weather conditions, toxicities, etc.) can generate symptoms in the tree similar to those caused by deficiencies, leading to a misdiagnosis . Likewise, deficiencies of certain elements may not be reflected in the appearance of symptoms, or the characteristics of the plot itself (organic matter content, soil pH, etc.) as well as its management (excessive application of soil amendments or fertilizers, soil disturbance, etc.) may be the determining factors in the low availability of elements for the plant. However, excessive levels of essential elements can also negatively impact the crop, causing toxicity problems and other undesirable situations.

Improving Fruit Retention in Avocados: Tips for Easier Reading

Avocado flowers require more than just nutrient reserves to develop into fruit. However, massive flower and fruit drop can limit retention. Only about 1% of the fruit remain on the tree. Although factors such as extreme temperatures and insufficient pollination have been suggested, we still don't fully understand why some flowers are retained and others drop prematurely.

Connection between Nutritional Status and Fruit Set: There is a direct link between the starch content in flowers and their ability to develop into fruit. Flowers with higher starch content are more likely to produce fruit. However, most avocado flowers have a low starch content, reducing their chances of fruit development.

Importance of Sugars in Development: In the process of photosynthesis, the plant synthesizes glucose, essential for nutrient assimilation and efficient transport. During fruiting, the fruits accumulate most of the exported sugars.

Cultifort's Specific Products for Avocado: Cultifort offers products such as AMACOL and BVC EVOLUTION (formerly BVC 2021), rich in reducing sugars. AMACOL, a biostimulant with amino acids, provides energy and promotes absorption. BVC 2021 acts as a leading biostimulant, optimizing flowering, promoting fruit set, and improving vegetative development and final yield with amino acids, nitrogen, potassium, and seaweed.

The role of nitrogen in promoting vigorous shoot and fruit growth is widely recognized. However, excessive vigor can inhibit the development of flower buds in autumn, reduce fruit set in spring, and decrease the quality of the harvested fruit. A nitrogen deficiency will weaken the tree, causing a decrease in production, while an excess of nitrogen could reverse the tree's tendency toward greater vegetative than reproductive growth, leading to fruit drop.

Phosphorus is an important nutrient associated with the chemical energy transport processes during photosynthesis for carbohydrate production. These molecules are subsequently required during flowering and to increase oil accumulation in the fruit. Phosphorus also plays a crucial role in the formation of cell membranes that regulate the transport of water and other nutrients between the cells of shoots, flowers, and fruits. Phosphorus availability is known to be strongly associated with root development and is influenced by soil pH.

Potassium is a mineral element that contributes to the transport of water within the cells of the tree and its fruit, thus playing a vital role in the vigorous growth of roots, shoots, and fruit. Specifically, potassium facilitates the transport of carbohydrates to the growing roots and fruit, essential for adequate dry matter accumulation and high yields of marketable fruit. It is also important in the plant's defense mechanisms against frost and drought by regulating stomatal activity in the leaves. A deficiency of this nutrient can lead to impaired stomatal function, with the associated problems previously discussed regarding stomatal opening and closing. Cultifort offers various solutions in its product catalog to prevent potential potassium deficiencies, including Cultifort K and its pH-neutral version, Cultineutral K.

However, to meet the NPK needs of both newly planted trees and avocados in full production, Cultifort provides several alternatives:

• Cultifort Soluble 11-24-11 for newly planted trees.
• Macrofol Green Plus (18-18-18+2Mg+ME) for avocados that are going into production.
• Cultifort Special Avocado (16-8-20) for adult plantations in full production.

Calcium is the most demanded macronutrient by avocados after nitrogen, phosphorus , and potassium. It is a nutrient with limited mobility in the plant , lacking a specific absorption mechanism, and is transported via the xylem by simple transpiration. Therefore, a potassium deficiency that reduces transpiration could also cause a calcium deficiency. Calcium is required by developing fruit and apical meristems due to auxin synthesis in these growth zones. Consequently, as the fruit grows and ceases auxin synthesis , calcium becomes less important for them. In other words, the greatest calcium demand by avocados occurs during the fruit growth period , especially during the first few months after fruit set. Put another way, all the calcium contained in the fruit accumulates during the first five months of initial development. This is because calcium is only mobilized to actively growing cells, as occurs during the initial development of the seed. For this reason, the new shoots that develop in the spring compete strongly for this nutrient, reducing its availability for the new growing fruit.

To prevent a potential deficiency of this essential nutrient, we recommend applying Cultifort Calcium . This liquid formulation contains calcium complexed with lignosulfonic acid to enhance its absorption, along with carbohydrates and organic polyacids to boost its mobility within the plant. Cultifort Calcium does not contain nitrogen, so the calcium is distributed more evenly throughout the plant, strengthening cell walls in roots, stems, leaves, and developing fruit, preventing fruit drop (peduncle) and improving firmness.

Magnesium is a structural macronutrient. It forms the core of the chlorophyll molecule , and is therefore directly linked to the net rate of CO2 assimilation _, or photosynthesis . A deficiency of this nutrient causes chlorosis in the basal or older leaves of the tree's branches, reducing their photosynthetic potential and, consequently, the synthesis of photoassimilates. This can exacerbate competition between developing fruit and new shoots. Our solution for a magnesium deficiency is Cultifort Mg.

Sulfur is a multifunctional element, involved in the synthesis of sulfur - containing amino acids (methionine and cysteine) and in protein formation. It also plays a role in the synthesis of vitamins and phytohormones and is important in the formation of cell membranes (sulfolipids).

Boron is another multifunctional element, in this case, a micronutrient. Its role in pollen production and pollen tube germination is well-known, but it also participates in the transport of sugars, carbohydrates, and potassium, nitrogen metabolism, protein formation , and hormonal regulation. Boron accumulates most rapidly during the second fruiting season. This is important because boron is required in early spring to support pollen tube development, at a time when the growing older fruit is placing a high demand on it. Therefore, in years with a high fruit load, it is beneficial to increase the boron dose proportionally to the expected harvest. Cultiboro Plus is our recommendation to prevent deficiencies of this nutrient.

Iron is an enzyme activator, a micronutrient precursor to chlorophyll, and present in cytochromes, which are important in photosynthesis and plant respiration. Iron deficiency manifests as chlorosis in shoots or young leaves, leading to a loss of photosynthetic efficiency. Ferrofort is Cultifort's alternative for preventing iron deficiencies.

Manganese and zinc are also enzyme-activating micronutrients. Manganese is involved in carbohydrate and fatty acid metabolism, the Krebs cycle, and photosynthesis, while zinc is involved in auxin synthesis. A deficiency in either is associated with reduced photosynthesis and, in the case of zinc, with impaired growth. Cultifort's solution for preventing zinc and manganese deficiencies is Manzifort.

Due to competition for nutrients among the growing shoots, flower buds, fruit from the previous season, and newly set fruit, avocados need to produce sufficient carbohydrates and provide the necessary nutrients to meet the requirements of each phenological stage. If these are insufficient, the result is typically a natural drop of set fruit , reduced growth of vigorous and non-vigorous shoots needed to form flower buds in summer, a significant decrease in yield, or a reduction in fruit size, which is usually determined during the first three months after fruit set.

4. Pathogens

The pathogens responsible for possible fruit drop in avocados are related to root rots that weaken the plant.

In this case, we will differentiate between two pathogenic agents, two soil fungi that cause root rot in avocado: Phytophthora cinnamomi Rands. and Rosellinia necatrix Prill.

Phytophthora cinnamomi is classified as an Oomycete fungus.

The disease manifests in the field with a progressive decline of the tree , giving it a general wilted appearance. The leaves are smaller than normal, pale green to yellow, and often withered. Defoliation begins at the crown and progresses downwards. In advanced stages of infection, all the leaves are lost, and the branches dry out and are scorched by direct sunlight due to the lack of foliage. Fruit production declines , and in a very advanced stage of the disease, the tree dies. The appearance of aboveground symptoms is a direct result of the rotting of most of the small feeder roots (1 to 3 mm in diameter), which appear blackened, brittle, and dead, making them difficult to find in severely affected trees.

Infection by the pathogen is optimal at soil temperatures between 21 and 30°C, and infection is virtually non-existent above 33°C or below 9-12°C. The optimal pH for disease development is 6,5.

Preventing root rot involves producing and distributing healthy nursery plants and preventing the spread of the pathogen in established plantations. To obtain healthy nursery plants , completely contamination-free seeds must be used. In infected seeds, the disease can be controlled by immersion in hot water at 49°C for 30 minutes. Using a substrate free of the fungus, planting in an area unaffected by the fungus, and using irrigation water from sources other than infected areas are recommended preventative methods. Furthermore, locating and isolating potential sources of infection is essential to prevent the spread of the disease.

For chemical control of the disease, foliar applications or trunk injections of phosphorous acid buffered with potassium hydroxide or aluminum ethyl phosphite are quite effective and can recover diseased trees. To maintain a low level of Phytophthora cinnamomi inoculum in established orchards, soil solarization treatments are recommended, using a transparent plastic cover applied to the soil for at least six weeks during the summer. The use of avocado rootstocks tolerant to this pathogen (Duke7, Dusa, etc.) can also be beneficial for replanting dead trees and in new orchards suspected of being infested.

Rosellinia necatrix is ​​classified as an Ascomycete fungus.

The aboveground symptoms show a progressive weakening of the plant, accompanied by a loss of vigor . The leaves wilt and dry out , and the tree may eventually die. The fungal mycelium can be observed at the base of the trunk and on the surface, under the bark of the roots.

Rosellinia necatrix can survive in the soil for years, which is why control measures should be preventative rather than curative. Some studies recommend integrated pest management , including the use of tolerant rootstocks, removal and burning of infected plants, and physical measures such as soil solarization. Both biological and chemical control in the field are still under development, although the use of Trichoderma appears to be yielding good results. Similarly, the proliferation of antagonistic organisms could be an interesting alternative for preventing this fungus.

The incidence of these fungi does not cause a direct drop of fruit, but as a consequence of the symptoms produced and indirectly , the abscission of avocados could occur.

Cultifort's technical and R&D department recommends a series of alternatives aimed at preventing these diseases:

1. On the one hand, avoid planting in heavy or poorly drained soils, conditions that favor the development of these fungi.
2. Microvital – LIt is a biological soil activator of plant origin, rich in Magnesium, micronutrients, organic compounds and flavonoid molecules, aimed at improving the physical-chemical characteristics of the soil and the microbiota activation.
3. Foliquino. Formulated with aluminum lignosulfonate with systemic and phytofortifying action, which increases the synthesis of metabolites in the plant, such as polyphenols and phytoalexins. Its action is mainly focused on the prevention of Phytophthora cinnamomi in pre-flowering and flowering stages, as it also improves the tree's covering and balances the leaf surface and the number of flowers.
4. Cultisan Cu. Product that combines the multiple properties of chitosan with the antifungal power of copper. Recommended for the prevention of Phytophthora cinnamomi In post-flowering stages. Chitosan strengthens the roots and stimulates the tree's growth and defenses, creating a protective layer that prevents colonization by external agents.
5. Spiralis Long Life or your version ECO Long Life. Fruit of our R&D line of Natural Defensive BiotechnologyIt is an activator of systemic resistance, both acquired and induced. It promotes the accumulation of lignin and callose, strengthening cell walls, preventing or hindering their colonization by microorganisms, and also induces the synthesis of PR proteins and phytoalexins, providing the plant with defenses against a possible attack by pathogens.