Improving crops

Breeding better bananas

  • October 16, 2020

Importance of banana

In the highlands of Eastern and Central Africa (ECA), 50% of its permanent cropped area is under highland cooking banana (Matooke and Mchare) cultivation. This represents around half of the total area under banana cultivation across Africa. ECA countries (Burundi, DR Congo, Kenya, Rwanda, Tanzania, and Uganda) produce annually 21 million tonnes of bananas with a value of US$4.3 billion.

 

Bananas are an indispensable part of life in this region providing up to one fifth of total calorie consumption per capita. Uganda has the highest consumption rates of banana in the world. The average daily per capita energy consumption from banana in ECA is 147 kcal: 15-fold the global average and 6-fold the African average.

 

Matooke is the primary staple crop for over 13 million Ugandans (≈35% of the population), but less critical in Tanzania where 3.3 million (about 7% of the population) consume it as a staple. In Tanzania 20-30% of the population in selected rural regions depend on Mchare. Mchare can be extremely profitable, fetching farmgate prices 2-4x higher than Matooke. Household consumption accounts for the majority of the production, but they are also an important source of household income. Fresh market demand remains the largest market opportunity followed by processed banana goods, a growing value chain segment.

 

Yield gap and pests and diseases

The estimated yield potential for Matooke is 70 t/ha/year, while current production figures indicate yields of just 5-30 t/ha/year. The yield potential for Mchare is 60 t/ha/year while current production is 11 t/ha/year. Causes for the yield gap include the lack of clean planting material, inferior agronomy, and susceptibility to pest and disease stresses.

 

Fusarium wilt, caused by Fusarium oxysporum f.sp. cubense (Foc), is a soil-borne disease with a number of pathotypes or races. Infection is often fatal and kills infected plants, but with susceptibility depending on Foc race and banana type/cultivar. Foc race 1 is present across Africa, while Tropical race 4 (TR4), which is severely destructive to Cavendish dessert banana types, has recently become established in Mozambique. It was previously not present in Africa.

 

Nematodes are microscopic worms with a number of species infecting banana, depending on locality/ geography and climate/altitude. Weevils are insect pests. Nematodes attack the roots and rhizomes, and weevils the rhizomes, both causing premature plant toppling and loss of banana bunches. Black Sigatoka (or Black Leaf Streak, BLS: M. fijiensis) results in leaf necrosis causing severe yield losses. Bacterial wilt is caused by the bacterium Xanthomonas campestris pv. musacearum and kills banana plants.

 

Breeding

Breeding bananas is a long-term commitment, given its perennial nature and numerous issues relating to sterility. Over the last five years, IITA has established a formidable network of researchers and breeding programs across six continents through a project called Breeding better bananas. This highly interactive network has resulted in opening the exchange and sharing of hybrids, expertise and information across institutes and breeding programs.

Banana subgroups: East African Highland Cooking banana: Matooke (left) and Mchare (right)

 

The project is coordinated by IITA, is multidisciplinary, and involves research institutions in Tanzania (HORTI-Tengeru, ARI-Maruku, IITA) and Uganda (NARO and IITA), as well as international research collaborators in Australia (University of Queensland), Brazil (EMBRAPA), France (Bioversity International), India (NRCB), Malaysia (Faculty of Science, University of Malaya), South Africa (Stellenbosch University), Sweden (SLU), and USA (Boyce Thompson Institute, Cornell University)

 

Scanning of a digital code for varietal identification and automatic display (inset) on fieldbook of data to be collected as part of the Banana Tracking System.

 

Local banana breeding capacity is being strengthened and national banana breeding pipelines are being developed. Foremost, farmers and consumers are being sensitized to the cultivation and use of improved, disease resistant cooking type bananas which enhance yield and income generation.

Achievements

In East Africa, the project has strengthened the Matooke banana breeding program in Uganda and underpinned an infant banana breeding program in Tanzania on Mchare, both based on a specific product profile. Banana breeding efficiency has been significantly improved, as shown by the 3-fold increase in seed production and better understanding and knowledge of banana floristry and pollination. Using novel video techniques, the floral development of seed-fertile and seed-sterile cultivars were shown to differ and stigmas found to be more receptive before the flower opens, all of which enable more targeted pollination timings. Significant improvements in embryo rescue and germination rates were thus achieved. This has resulted in an accelerated production of progeny and the evaluation process, amounting to crosses and consequent selections for evaluation.

 

Over 200 Matooke hybrids have been selected for field evaluation, with thousands more in the pipeline. Of these 122 were planted in the first joint evaluation of Matooke hybrids independently developed by NARO and IITA. Heterobeltiosis for bunch weight up to 249% versus its matooke grandmother and 136% against its primary tetraploid hybrid parent was achieved. Broad sense heritability (across three cropping cycles) for yield potential and bunch weight were high (0.84 and 0.76, respectively).

 

Furthermore, a consumer evaluation system has been established that feeds back into the pipeline to ensure a robust mechanism to deliver new cultivars to farmers with preferred end-user traits, such as taste and cooking traits. This allows the project to dispose of unsuitable progeny early in the development cycle.

 

Seed set data from Matooke cultivars have also shown that seeds are mostly extracted from just a few cultivars. Currently, 541 Matooke hybrids from 3x-2x crosses and 13,598 Matooke hybrids from 4x-2x crosses are in EETs. Results also show that triploid hybrids next to tetraploids and diploids are generated from 3x-2x crosses, which requires further investigation, as this will help avoid the 4x-2x crosses cycle, and shorten the breeding schedule. Some 1,572 hybrid Mchare are under early field evaluation, representing the first-ever Mchare hybrids while 79 doubled chromosome Mchare plants were generated for 8 cultivars, of which 60 lines have been planted in the field. Also, 22 improved Fusarium resistant EMBRAPA diploids from Brazil were received for potential use in the breeding program. All material used is genotyped during the multiple stages of breeding to assure identity. The infrastructure was drastically improved with all fields irrigated and both the Matooke and Mchare breeding site supported by a molecular and pathology lab as well as in vitro labs and nurseries.

Mapping populations: Hybrids were developed segregating for traits of interest for gene discovery. Far left: fruit variation. Middle and right: variation in corm response to banana weevil.

 

The best Matooke hyrbids were called NARITA giving credit to the joint collaboration and achievement by the Uganda banana breeding program NARO and IITA. The NARITAs have been assessed for various pest and disease resistance both in the screenhouse and the field, with good resistance being demonstrated against Foc Race 1, weevils, Sigatoka, and R. similis.

 

Field and farmer testing of NARITA hybrids, including sensory evaluation, across 5 sites in Tanzania and Uganda over 2 cycles resulted in the selection of 4 hybrids planned to be released in Tanzania by end-2020. The most important desired visual traits are large bunch size, fruits, and hand size, moderate suckering, plant height, and resemblance to Matooke. In general, traits preferred by women reflect those of men. Across regional field-testing sites, NARITA 23 was ranked the best by farmers, followed by NARITA 2 and NARITA 12. Some NARITAs outperformed and were preferred by farmers than the local cultivars in some locations but not all.

 

The project has set the stage for developing molecular tools to speed selection in banana breeding. Preliminary assessments, including from Genome-Wide Association Study (GWAS), show that Chr3 is particularly important, including for TR4 resistance and probably weevil resistance, as well as for fruit filling, an important trait for breeding. Hundreds of lines of various populations have been phenotyped and genotyped to generate SNP markers. Numerous QTLs associated with resistance to pests and diseases (Fusarium, weevils, bacterial wilt) have been identified, as well as other important fruits traits, mapped from various genetic backgrounds and located on different chromosomes. Models for genomic prediction and the fruit filling QTL have been evaluated, which will be validated and deployed in Matooke breeding.

 

Knowledge on pest and disease populations and dynamics in the region has been improved and protocols for more rapid and efficient screening developed and implemented, and consistently finetuned. The accurate characterization of pathogens and pests has continued and used for evaluating hybrids and breeding materials for resistance. A total 220 isolates of Foc Race 1, 369 isolates of Pseudocercospora fijiensis, and 12 populations of weevil have now been collected, stored, and used to assess genetic diversity among populations. Monoxenic populations of Radopholus similis have been established in Kenya, Tanzania, and Uganda for routine use in screening purposes. Pratylenchus goodeyi and P. coffeae appear more prevalent than previously known and should be considered for resistance screening.

 

Screening of banana hybrids for Fusarium race 1 resistance (green plants are resistant and yellow-brown plants susceptible).

An open source system, Banana breeding Tracking Tool (BTracT), which accurately captures and efficiently tracks data throughout the breeding process was developed and successfully implemented and is now fully operational. BTracT is fully interfaced with the banana breeding database, MusaBase, which provides a global repository for all breeding data as it is collected in real time. FieldBook data collection, linked to BTracT, accurately records data at all stages of the breeding program, synchronizing it automatically with MusaBase to archive all information and data and make it publicly available.

 

To create awareness and visibility for the project and on the importance of banana in the Great Lakes region and why improving banana breeding is key to ensure food security of the local communities, two videos were produced and published to reach a wider audience and the general public: “Journey to a better Mchare: Improving Tanzania cooking banana” (https://youtu.be/BzhNKTLN73c) and ”Breeding Better Bananas – Improved Matooke for East and Central Africa” (https://youtu.be/yXbeDBg9SuI).

 

A range of training events have been undertaken during the project, including regular trainings in relation to MusaBase and data entry, breeding training, scientific writing in addition to exchange visits between partners and breeding programs to assess technical aspects as well as foster links and relations. Specific workshops for on-farm evaluation of NARITA, genomics, science writing, breeding techniques, and seed set have provided training opportunities and built capacity in partner institutions outside of the project and for associated postgraduate students.

A project website (http://breedingbetterbananas.org/) was developed that links directly with MusaBase (https://musabase.org/) and other relevant domains; a Twitter account (@BBetterBanana) was created in May 2018 has reached 210,687 users with a total of 649,671 impressions.
In total, 25 postgraduate and 1 graduate student have been associated with the project, with 6 females and 20 males.

 

Authors: Rony Swennen, IITA-Tanzania (Arusha), Danny Coyne, IITA-Kenya (Nairobi), Allan Brown, IITA-Tanzania (Arusha), Brigitte Uwimana, IITA-Uganda, Trushar Shah, IITA-Kenya, and Delphine Amah, IITA-NIgeria (Ibadan)

Leave a Reply

Your email address will not be published. Fields marked with * are required