Close to 48 million tonnes of banana are produced every year in Asia, making the fruit one of the most important crops in the region. The fruit is part of the daily diet of Asians both as fresh fruit and processed delicacies, and plays an important role in the livelihoods of millions of banana growers who supply the local and export markets. The region, however, faces many challenges. Banana bunchy top disease has caused significant damage to the banana industry in many Asian countries over the last 20 years, and the recent outbreaks of tropical race 4 (TR4), a highly virulent race of Fusarium wilt, are extremely alarming. But there is also good news. Asia lies in the center of origin of the crop, and is home to a rich diversity of wild and cultivated bananas. This genepool is a valuable source of genetic variability that has been the basis for crop evolution and is of vital importance for direct use by farmers or for breeding new varieties.
On the first 2 days of the symposium, researchers reported on how this diversity can be better explored and used to address major banana constraints. New tools for characterization of germplasm were described, and the application of these tools to unravel genetic relationships between species and subspecies as well as genome groups and subgroups were discussed. During the session, it was announced that the Musa genome sequence will be available in two years, and several presentations on the structure of the Musa genome were given. With new advances in biotechnology, useful genes can now be identified and molecular approaches will help make conventional breeding programs more efficient.
In the second session of the symposium, scientists reported on the status of Fusarium wilt in Asia and discussed actions for raising awareness and preparedness in regions where TR4 has not been reported yet. New methods to study pathogen diversity and a diagnostic tool specific for TR4 were described. Several authors presented their work on host-pathogen interactions and evaluation protocols, while others reported on field management practices, including the use of resistant cultivars. The newly elected chair of the ISHS section on banana and plantain, Stephan Weise, closed the oral presentations with a keynote on linking agricultural research networks to innovation platforms.
Opportunities for bridging the gap between genomics and genetic improvement in Musa were further discussed in a workshop on the last day of the symposium. The participants identified access to germplasm as a central requirement, including the need for further conservation efforts (particularly of wild types and improved diploids), a strategy to deal with intellectual property rights and proper characterisation of the germplasm in terms of important phenotypic traits. Germplasm enhancement was identified as another area that needs attention, including the development of improved diploids and the identification of sources of genetic determinants of important traits, such as resistance to biotic and abiotic stresses, agronomic performance and post-harvest quality. Better ways to increase seed production from crossings and improve the results of embryo rescue need further attention. ‘Ready-to-use’ molecular marker technologies, such as markers unique to each genotype or to traits of interest, are seen by breeders as a first step in bridging the gap between breeding and genomics. Great progress in resistance breeding can be expected if information about genes for useful traits and alleles in diverse germplasm can be combined with molecular information gained on host-pathogen interactions, as well as pathogen diversity.
Other genomic research areas identified by breeders are: whole-genome selection tools, increased knowledge of genetic determinants of important traits, polymorphisms for useful traits, gene discovery, participation in phenotyping/genotyping and prebreeding, and information on pathogen diversity and host-pathogen interactions. New sites, in both banana-growing and non-banana-growing regions, for screening pre-breeding materials and hybrids are also required. These include screenhouses, greenhouses and closed facilities (e.g. growth rooms or chambers). For molecular biologists to have their work more directly applied by breeders, they would need: access to breeding populations (e.g. segregating populations), easier access to germplasm (mainly wild types), priority for traits (as it is difficult to pyramid genes for different traits: e.g. disease resistance and increased yield) and more work to increase the diversity of parental lines by using a wider range of diploids that still need to be characterized and evaluated for important traits.
Issues that were prioritized during a pre-workshop survey were shortlisted by participants during the workshop, and four areas were identified for further discussion: germplasm collection and characterization, phenotyping, molecular markers for gene discovery and for studying diversity, and identification of parents for recombination and cultivar development. In a side discussion, the need to promote existing and new banana breeding programmes in Asia mentioned in previous ProMusa meetings, was reiterated as a priority.
During a parallel workshop on Fusarium wilt of banana, a number of priorities for action were identified. A better knowledge of the basic biology, population biology and epidemiology of the pathosystem is urgently needed. This includes understanding the influence of edaphic and other environmental conditions on the outbreak and development of epidemics, pathogen survival in the absence of the banana host, alternative hosts, modes of pathogen dispersal including the potential role of insect vectors (if any) and aerial dissemination, and the potential value of soil amendments (e.g. silicon). The interactions between host, pathogen and environment need to be better understood. Two research groups announced ongoing efforts towards transcriptome and genome sequencing of multiple races of Fusarium wilt pathogens. This work will be instrumental in facilitating opportunities to increase our knowledge of Fusarium-banana interactions. Host-pathogen screening should be expanded, including testing of closely related resistant and susceptible germplasm.
There is also a need to prospect for new, potentially resistant germplasm, and to characterize the pathogenic and genetic variability of Fusarium wilt races, clonal lineages and vegetative compatibility groups. The development of a standardized small-plant screening bioassay that can be adapted to local conditions as needed is considered an important starting point to accurately and reliably assess host response and pathogen virulence and facilitate comparisons across different geographical locations. Validation of such small-plant bioassays conducted under controlled conditions is needed to demonstrate that disease reactions are representative of field trials.
Continental action plans to limit the movement of Fusarium wilt pathogens and prevent the entry of TR4 into Africa and the Americas need to be developed or further refined, and supported by effective public awareness campaigns, reliable diagnostic methods, and strict quarantine policies and procedures. The availability of a new TR4 diagnostic tool would facilitate surveillance in regions where the pathogen has not yet been found and would lead to more efficient identification and faster containment of uncharacterized Fusarium wilt outbreaks. Field evaluation of important cultivars from Latin America and Africa (e.g. plantain and highland bananas) in countries where TR4 is already present (e.g. Asia) should contribute to increased preparedness in TR4-free countries.
More research is needed to identify reliable and efficacious options for inoculum reduction in soil, e.g. by using rotation crops, fumigation, solarization and biological control. Area-wide comprehensive IPM management programmes need to be better integrated, and more communication is needed between local companies and farmers. The community also needs to look into cost reductions for disease-free planting material, taking into account that tissue culture is not always an option. Workshop members also discussed the establishment of an international collection of Fusarium pathogens associated with Musa and the continued recognition of using strict biosafety protocols for the movement of pathogens.
In total, 58 talks and more than 50 posters were presented during the symposium, which attracted over 180 participants from China and the rest of the world. The delegates also visited the Fruit Tree Research Institute of the Guangdong Academy for Agricultural Sciences (GDAAS) and a banana production area in Zhongsan where Cavendish (AAA) and Fen Jiao (ABB) cultivars are grown.
The symposium was hosted by the GDAAS and co-organized by them, Bioversity International, the International Society for Horticultural Science (ISHS) and ProMusa. Additional sponsors were the Science and Technology Department of the Guangdong Provincial Government (China) and the Agricultural Department of the Guangxi Provincial Government (China). The Technical Centre for Agricultural and Rural Cooperation (the Netherlands) supported several participants from African, Caribbean and Pacific countries, and the National Fund for Scientific Research (Belgium) supported the travel of a keynote speaker.