The future of avocado diversity is not promising as more areas are joining the certification scheme for export purposes

07/07/2021

Mexico is one of the countries with a wide diversity of avocado types, as there are at least 20 different species related to the common avocado (Persea americana). To classify avocado diversity, the concept of races has been used, and three are recognized in the country: Mexican, West Indian, and Guatemalan. These were classified by Bergh and Ellstrand as botanical varieties, being Persea americana var. drymifolia the Mexican race, P. americana var. americana the Antillean race, and P. americana var. guatemalensis the Guatemalan race.

This diversity has been recognized since pre-Hispanic times: the Florentine Codex mentions three types of avocado: aoacatl, tlacacolaoacatl, and quiloaoacatl, which, according to their descriptions, could correspond to the three aforementioned races, each with distinctive morphological characteristics. There is a large number of hybrids with mixed traits, which causes problems for their placement in one breed or another. Morphological studies show a closer relationship between the West Indian and Guatemalan breeds.

Avocado races are distributed according to altitude and the presence of frost-free zones. Thus, in general, the Mexican race is found above 2000 m altitude (temperate zone), the Guatemalan race between 1000 and 2000 m (subtropical zone), and the Antillean race below 1000 m (tropical zone). Hybrids, on the other hand, have an intermediate adaptation. 

An example of this is the avocado called 'Hass', an advanced hybrid of the Guatemalan and Mexican races, which has been estimated to have between 10 and 15% of genes from the latter race, which gives it a better adaptation to more temperate zones. The type of vegetation where the Mexican race predominates is pine-oak forests, while the Guatemalan race is found in mesophilic mountain forests, and the Antillean race in tropical evergreen forest, although only a few specimens of the latter have been found in this type of natural vegetation.

Avocado consumption ranges from harvesting in the rainforests to processed products. The avocado is deeply rooted in the Mexican diet. Mexico produces around 800 thousand tons per year and is the world's leading producer with a little more than a third of the global total (approximately 2 million tons per year in recent years). Its cultivation is very important as a source of employment and income in the rural sector since it ranks seventh in the value of national agricultural production and is strategic due to its high socioeconomic impact on the rural community of 28 producing entities.

The loss of forests in the state of Michoacán is due, in part, to the expansion of Hass avocado cultivation.

Part of the avocado diversity is used to form family orchards, from which seeds are selected for commercial nurseries. In plots where the forest is cleared in mountainous areas, farmers generally leave trees that are useful to them, including avocado trees. This is very common in Los Altos de Chiapas, while in other states, such as Michoacán, Puebla, and Mexico, there are "criollo" avocado trees, whose fruits are for self-consumption and whose leaves are highly valued as a condiment. Some families also plant seeds in their orchards or backyards of fruits they like, thus contributing to the emergence of new varieties. The use of avocado and chinini (Persea schiedeana) to provide shade in the coffee plantations of Mexico is common, although the fruits are also eaten and the trunks are used as wood.

Thanks to their diversity, genetic improvement programs have focused on two main objectives: to obtain new cultivated varieties and to select rootstocks. In Mexico, only two institutions are carrying out genetic improvement: the National Institute of Forestry, Agricultural and Livestock Research (INIFAP) and the Salvador Sánchez Colín-CICATAMEX Foundation, S.C., in collaboration with the Autonomous University of Chapingo (UACH).

In Nayarit, INIFAP selects rootstocks tolerant to drought and to a disease called "tristeza del aguacatero", caused by the algae (formerly classified as a fungus) Phytophthora cinnamomi. In the case of CICATAMEX and UACH, they choose rootstocks resistant to P. cinnamomi and the genetic base used is mainly from germplasm of the Mexican race from all over the country. They are also using germplasm from the Mexican and Guatemalan races to make crosses, in order to look for better quality cultivars that are produced out of season, as well as avocados for the industry.

Although there is no institutional program to monitor the diversity of avocado and related species in the country, it is possible to point out some trends that have been occurring over the last 30 years in several areas with respect to the Criollo avocado. In the most important producing areas of Michoacán, the production of Criollo avocados has been reduced by grafting them with the Hass variety, which is used in 90% of the orchards in Mexico. 

This has resulted in a great loss of germplasm. The expansion of the Hass variety is increasing every day in the country due to its commercial value, which has also contributed to the loss of forests, mainly of pine-oak in the state of Michoacán, and perhaps to the illegal change in land use.

The quetzal, an inhabitant of the El Triunfo Biosphere Reserve, Chiapas, is a dispersal agent for wild avocado.

In production areas, native trees are being eliminated because they are hosts for "bone" and branch borers. The presence of these pests jeopardizes the phytosanitary certification required to export to the United States, which is why major campaigns have been undertaken to eliminate them. These actions contribute to the erosion of germplasm in these areas, which is abundant for the Mexican breed. The introduction of P. cinnamomi has also reduced the native varieties and natural populations of avocado.

Deforestation is another factor that has contributed to the reduction of avocado diversity. This crop and several related species grow in forests and jungles; in the case of Los Altos de Chiapas it is known that in the forests there are Persea americana var. guatemalensis, Persea steyermarkii, P. nubigena and P. schiedeana; in fact, the genus Persea is considered as an indicator of the mountain mesophyll forest. For example, it is estimated that in Los Altos de Chiapas there are currently between 3700 and 5250 hectares of this type of forest, which contrasts with the 40,000 hectares obtained by the National Forest Inventory of Mexico in 2000.

Since the 1950s there have been efforts to establish and maintain germplasm banks. In the 1970s, explorations and collections were made by the then INIA (National Institute of Agricultural Research), which were deposited in Celaya, Guanajuato, forming a germplasm bank that still exists today. In the 1990s, researchers from CICATAMEX and the UACH made collections in Mexico and abroad, forming what is currently the most diverse germplasm bank in the country.

The Western Regional University Center (CRUO) of the UACH also makes collections in Veracruz, while the INIFAP, at the end of the nineties, carried out explorations in the country collecting mainly individuals of the Mexican race with a wide diversity and as seed propagation material in 97% of what was collected.

The Mexican variety (var. drymifolia) is characterized by its small fruits and thin skin. It is the richest of all breeds as it has 30% oil content.

As part of the actions of the Avocado Network of the National System of Phytogenetic Resources for Food and Agriculture, it was decided to create the National Avocado Germplasm Depository, located in two campuses in Coatepec Harinas, State of Mexico, and in Celaya, Guanajuato, which house around 800 accessions, plus those that will be collected, including not only Persea americana, but many species of the genus and other related species.

The ex situ conservation of avocado has faced phytosanitary problems, technical problems in some cases, and lack of institutional interest, which have resulted in the loss of accessions. All efforts to conserve diversity, especially ex situ, require institutional commitment and sources of funding.

Some form of in situ conservation is being carried out in some parts of the country. There are protected areas that harbor Persea species, including the Sierra de Manantlán Biosphere Reserve, where Persea hintonii grows. Other reserves, where other Persea species are also found, include Pico de Orizaba and Los Tuxtlas (Veracruz), Benito Juárez (Oaxaca), El Triunfo, Palenque, Lagunas de Montebello and Lacantún (Chiapas).

However, Lorea-Hernández reported that only the Los Tuxtlas and El Triunfo reserves include mountain mesophyll forest that harbors Persea americana, so it is important to create other protected areas in the country that include this type of vegetation, or to increase the existing one in Los Altos de Chiapas.

Another option is to promote forest farms with diversity, as El Colegio de la Frontera Sur is doing in Chiapas, where there are already sample plots for the indigenous people to adopt a conservationist model and take advantage of it. Another form of conservation is seen in coffee plantations, which in many cases have a wide range of richness that includes avocado and Persea schiedeana, and also in home gardens.

The future of avocado diversity is not promising. These types dominated the majority of the country's orchards until the mid-1960s. Currently, 90% of commercial production is based on the cultivated Hass variety, and considering that more areas are joining the certification scheme for export purposes, the expectations facing native avocados are not encouraging.

Furthermore, the forests and jungles where Persea species grow have been drastically reduced and genetic erosion has increased. A coordinated effort and investment is required to maintain this diversity, since in vivo collections in field germplasm banks are costly, so inventory and planning studies must be carried out to carry out explorations and collections in order to conserve diversity.

By Barrientos Priego, A. F. 2010. El aguacate. CONABIO. Biodiversitas, 88:1-7