From almost a century, the South American Leaf Blight (SALB) - a disease caused by the fungus Microcyclus ulei - economically frustrates the commercial planting of rubber trees in tropical rainforest regions, and specifically in the Brazilian Amazon. To solve this problem researchers from Embrapa have developed a new technology to develop double grafted rubber trees.

The research involved crosses between different species of rubber trees, clone selection and grafting to form rubber trees with high yield and resistance to SALB.

Selected seeds of Hevea brasiliensis are used to form the base of the plant. The trunk or panel comes from the graft of a clone of another tree of the same species, which had been proved to be highly productive. Finally, the canopy comes from a Leaf Blight resistant clone, created from interbreeding between the species H. guianensis, H. pauciflora and H. rigidifolia.

From the seed to the planting phase 25 months are required to produce a rubber tree. During this time, two grafting and the handling necessary for plant development take place.

"Even spending one more year to produce the plant and at higher cost, this rubber tree provides economic returns over time because it produces earlier and with quality", explains Everton Cordeiro researcher from Embrapa Western Amazonia in Manaus (AM) responsible for the research.

The rubber tree starts the latex production with 6 years, one year earlier than the conventional rubber trees. The production stabilizes between 9 and 10 years and the well-managed plantations can be exploited by 30 to 35 years.

40 years of research

It took 40 years of research to define the appropriate combinations to the rubber trees. That time included the research on controlled pollination for intersections, planting, assessment and selection of compatible plants with better performance.

A pioneer in research on rubber trees in Embrapa was the agronomist Vicente Haroldo de Moraes Figueiredo, who worked in the Amazon between 1974 and 2005. His main contributions were the crosses and the initial selection of clones used for the current double grafting technology.

The agronomist Vicente Haroldo de Figueiredo Moraes was a pioneer in the rubber tree research in Embrapa working in the Amazon from 1974 until 2005. His main contribution is the cross breeding and original selection of clones to the current rubber tree technology.

"It is a time consuming and laborious work. If we have today resiliente clones is because much effort was done over time", analyses Everton Cordeiro, the researcher that continues Moraes's work in refining the selection of productive clones resistant to SALB.

The latest experiment was started in 1999 when 11 clones of rubber tree canopies selected by Moraes were analyzed.

To evaluate the production of these trees in the period 2005 to 2010, Cordeiro have selected five clones with better results for dry rubber production, among which only three had standed out by production yield.

Until 2016, besides the current research Embrapa is working on a rubber cultivation technology transfer project that involves production of seedlings and diffusion of technical capabilities to 20 municipalities of Amazonas.

High variability of the fungus

The use of molecular biology tools in the study of M. ulei has brought a new approach to the research since the high variability of this microorganism causes instability in clone resistance of clones to SALB.

Between 2009 and 2012, the researcher Braz Tavares Hora Júnior produced a thesis at the Federal University of Viçosa (UFV) in partnership with the CIRAD-Michelin-Brazil consortium aiming to develop a genetic improvement project of rubber tree. The methodology involved the analysis of SSR microsatellite markers in samples of fungus populations collected from commercial fields in 8 Brazilian states.

Presently, Braz Junior integrates a research group on M. ulei and coordinates projects to understand the evolution of the fungus populations. Braz enlightens that this group is working on the sequencing the fungus genome reponsible for the SALB. This information will be important in predicting the efficiency of the durability of resistance to the pathogen. Researchers from Embrapa Western Amazon and Embrapa Acre also participate in these studies.

Another relevant work is been developed at the Centre de Coopération Internationale en Recherche Agronomique pour le Développement (CIRAD) in France, and attempts to elucidate the genetic basis for the H. brasiliensis resistance to SALB and the transcriptome of interaction.

Genetic improvement

With the objective to strengthen and expand the research on  the rubber tree, the project "Genetic improvement of rubber tree in Brazil" is mobilizing a broad network of researchers from several federal and state research institutions among them Embrapa.

One of the concerns that motivate this national effort is the fact that the Brazilian rubber tree culture is based in a few clones imported from Southeast Asia. To overcome this dependency, the project aims to produce, test, assess and select new cultivars for planting rubber trees in various regions of the country.

The project leader is the researcher Marcelo Fideles Braga of EMBRAPA Cerrados in Planaltina (DF). Also participates the research centers of Embrapa in the states of Acre, Amazonas, Mato Grosso, Pará, Tocantins, Goiás, São Paulo and the Federal District, beside the partnership of the Executive Committee of Planning of Cocoa Farming of Bahia (Ceplac-BA), Agronomic Institute of Paraná (Iapar-PR), Agricultural Research Company of Minas Gerais (Epamig), Federal University of Lavras (UFLA), Federal University of Uberlândia (UFU), School of Agriculture Luiz Queiroz, University of São Paulo (Esalq-USP) and State University of Campinas (Unicamp).