Deliciously resistant avocados
Although avocados are very much appreciated in Colombia, crops fall to a oomycete (fungus-like) microorganism which has turned into a headache for avocado producers. A study determined two native varieties which would help obtain better quality fruit.Bogotá D. C., 18 de julio de 2016 — Agencia de Noticias UN-
Researchers used two strains of Phytophthora cinnamomi to infect grafted avocado plants. Photos: Personal file
The oomycete fungus-like Phytophthora cinnamomi produces root rot in avocados and diminishes their vigor, color and shininess.
In the Colombian Provinces of Antioquia, Caldas, Cesar, Bolívar, Cundinamarca, Quindío, Risaralda, Tolima and Valle del Cauca, the withering produced by Phytophthora cinnamomi, causes losses which range between 30% and 50% of the trees in the greenhouse stage and during the two first years of the crop in the open field.
For Colombia, the fourth world avocado producer after Mexico, Indonesia and United States, root rot is one of the main diseases, in fact it is one of the potential threats for close to 13,500 avocado farmers in Colombia.
Despite the economic importance the measures taken to fight the disease are still very limited. For example chemical control is carried out using metalaxyl and mefanoxam (Ridomil® and Ridomil Gold® respectively), fosetyl-aluminum (Aliette®) and several other phosphoric compounds which help reduce rot duration to less than four months but this does not really eliminate the causing agent. Furthermore long term use of these substances creates resistance of the pathogen.
Genetic improvement is the most viable alternative to offset the devastating effect of the pathogen. Precisely a research project on native avocados carried out by the Colombian Corporation on Agricultural Research (CORPOICA, for its Spanish acronym) and the Universidad Nacional de Colombia (UNal) in Palmira discovered two promising materials: NATU–001, which comes from Tumaco (Province of Nariño) and CANO–008 of Norcasia (Province of Caldas) with good tolerance to Phytophthora cinnamomi.
Eberto Rodríguez Henao UNal-Palmira master’s in Agrarian Sciences thesis project task was searching for avocado accessions (samples of a particular population of plants) with resistance to the oomycete at the CORPOICA germoplasm bank.
The purpose of Rodríguez was to identify material with use potential as well as a rootstock (plant to where the other plant is grafted), a preventive handling practice in fruit trees when they are susceptible to disease or pests produced in their root system.
“Trees grafted with tolerant stumps showed great performance and productivity inclusively in pathogen-ridden soils. This alternative has not been explored in Colombia due to the lack of tolerant clones or a method to adequately propagate them,” said Rodríguez.
After assessing features such as trunk surface, color of the lenticels (trunk, stalk or branch pores) of the young sprouts, color and surface of the young leaf, insertion angle of the foliar petiole and the form of the leaf in 187 accessions, they chose only 21.
In evaluating the resistance of the chosen samples, for five months they used two strains of the oomycete with high pathogenicity. Young plants were purposely infected with Phytophthora cinnamomi, making a 5 cm. (1.9 in.) incision above the graft with the pathogen and covered it with Parafilm® grafting tape to allow moisture to help the pathogen develop.
Later plant pathogen progression was assessed by measuring the length and width of the disease. With this information they established the Area Under Disease Progress Curve (AUDPC), a variable used to identify the plant tissue level of tolerance or susceptibility (0 is tolerant and 10 susceptible).
The material which showed greatest tolerance was NATU–001, which had been collected on the Colombian Pacific coast at 25 meters above sea level (82 ft.) at an average temperature of 29° C (84.2° F), relative humidity of 83.86%, 2,792 millimeters (mm) of average rainfall per year and an AUDPC reading of 0.760.
The other tolerant accession was CANO–008, collected in the Province of Caldas at 551 meters above sea level (1808 ft.), at an average temperature of 25° C (77° F), relative humidity of 76.76%, 3,675 millimeters (mm) of average rainfall per year and an AUDPC reading of 1,057.
“NATU–001 and CANO–008 are both from areas of high humidity and open a new scenario for use of root rot tolerant native avocado rootstock, thanks to the adaptation plants were subjected to,” said Álvaro Caicedo Codirector of the research project.
Professor and Director of the Project Jaime Eduardo Muñoz Flórez said this is another example of how inter-institutional cooperation between CORPOICA and UNal strengthens agricultural science development in Colombia.(Por: Fin/HAAG/DMH/APBL