By Stewart Lindsay, Queensland Department of Agriculture and Fisheries
The outbreak of Panama disease TR4 in the North Queensland banana industry in March 2015 has generated a lot of interest among banana growers and marketers about the potential for new banana varieties with desirable pest and disease resistance.
While banana varieties with resistant genetics offer the best option for effective control of the different strains of Panama disease, there are many other exotic and endemic pests and diseases, as well as production and marketing attributes that need to be considered in the evaluation and selection of new varieties.
The Australian banana industry has invested in the importation and testing of international banana selections and hybrids at different times since the late 1980’s in the pursuit of improved pest and disease and production characteristics.
The rigorous quarantine procedures required to protect the Australian industry from exotic pests and diseases means that the importation and testing process is relatively slow and expensive.
As a result, it is important to have a clear strategy for the selection and importation of new varieties to ensure the most efficient use of the R&D investment.
There has also been the limited use of nonconventional breeding techniques for bananas in Australia, particularly the use of somaclonal selection and genetic modification, as alternative approaches that try to achieve resistance in existing varieties that are accepted in the market.
Currently the Australian banana industry is actively investing R&D levies in both importing new varieties from overseas for testing as well as the development of resistant forms of existing varieties using somaclonal selection.
BANANA BREEDING TECHNIQUES
The development of new banana varieties is a slow and costly process and high levels of infertility make it difficult to do conventional breeding from some commercially important varieties like Cavendish.
Banana breeding techniques fall into three broad categories:
- Conventional cross-breeding techniques— fertile male and female lines are crossed to produce seedling plants which are often tetraploid (4 sets of chromosomes) but manipulation can also produce triploid offspring (3 sets of chromosomes). Most commercial banana varieties, like Ladyfinger, Ducasse or Cavendish are triploids.
- Somaclonal selection—uses natural or induced mutations in existing cultivars to produce plants with desirable attributes such as pest or disease resistance, improved plant
stature and increased yield. This method was pioneered by the Taiwan Banana Research Institute (TBRI) in developing Cavendish varieties with varying levels of resistance to
Panama disease TR4.
- Genetic modification—involves the manipulation of the banana DNA by inserting identified genes for desirable characteristics from bananas or other organisms. Newer techniques include the manipulation of the plant’s own genes without the introduction of any external genetic material.
According to Bioversity International, in 2013 there were 14 active conventional banana cross-breeding programs, 10 active somaclonal breeding (natural and induced) programs and 6 banana genetic modification programs underway across the globe.
Not all these programs have relevance to the Australian banana industry, as they are breeding plantains and cooking bananas, while access to varieties from some of those programs with relevance are closed or restricted.
CURRENT INDUSTRY FUNDED VARIETY ACTIVITIES
Importing and testing banana varieties from overseas
Variety importation and testing has been supported since 2010 by BA10020 Banana Plant Protection Program and now BA16001 Improved Plant Protection for the Banana
Industry which are part of the Hort Innovation Banana Fund.
Access to varieties is negotiated under research agreements with the respective breeding programs and the plants are then introduced as tissue cultured plantlets and held under strict quarantine conditions at DAF Queensland facilities the Eco-Sciences Precinct in Brisbane and the quarantine tissue culture facility at the Maroochy Research Facility in Nambour.
The introduced plants are tested for the presence of specific viruses and phytoplasma that cause disease in bananas and are spread in infected planting material.
If the plants are found to be free of disease then they are multiplied as tissue culture plantlets and planted at field screening sites in NSW, North Queensland and the Northern Territory as part of a coordinated assessment program.
In the field screening site at Duranbah in NSW varieties are assessed for their resistance to Panama disease Race 1 strains and their production characteristics under subtropical conditions.
In North Queensland the varieties are grown at the DAF South Johnstone Research Facility and assessed for their resistance to Yellow Sigatoka and production characteristics under tropical conditions.
At the Coastal Plains Research Station in the Northern Territory the plants are assessed for their resistance to Panama disease TR4 with some assessment of production characteristics.
The non-Cavendish banana hybrids imported represent a range of both tetraploids and triploids from the FHIA program based in Honduras as well as the CIRAD program from the French West Indies.
These programs contain breeding lines with a range of pest and disease resistances but are unable to test against Panama disease TR4 in their own region.
The outputs of these programs are completely new varieties that may have the right combination of desired traits for pest and disease resistance and eating quality or production characteristics, which can only be confirmed by disease screening and production assessment.
Any of these varieties that tick all the boxes will then need to be introduced to the market as a brand new banana product.
Selections of Cavendish and Gros Michel have also been imported, with some from somaclonal breeding programs in Taiwan and Cuba looking to develop varieties resistance to Panama disease TR4 and Race 1 respectively.
Most of the Taiwanese Cavendish selections have some level of resistance to TR4 but often have longer crop cycles and smaller bunches than the current industry standard Williams Cavendish.
So far a resistance Cavendish with equivalent production characteristics to Williams has not been identified although there are some selections with good levels of resistance to TR4
Three new resistant Cavendish selections from Taiwan have cleared quarantine and are available for planting in the new field trial in mid-2018.
Developing resistant banana varieties using somaclonal selection (mutagenesis)
The second key activity under way in Australia is the use of somaclonal selection to develop resistant varieties.
As mentioned earlier, somaclonal selection relies on natural or induced mutations in existing varieties to produce progeny with desirable attributes such as pest or disease resistance, improved plant stature and increased yield.
The current somaclonal selection program in Australia is led by DAF Queensland and funded through the Hort Innovation project BA14014 Fusarium wilt Tropical Race 4 Research Program, with coinvestment from DAF Queensland.
The activity is based on the same approach taken to develop the variety DPM 25 as a Cavendish selection with acceptable resistance to Panama disease Subtropical Race 4 in the early 1990s.
The mutagenesis approach deliberately induces changes in the banana plant’s own genetic information with chemical or radiation treatment.
The changes induced by mutagenesis are uncontrolled with unpredictable outcomes and the technique relies on treating thousands of plants in the starting population to deliver only relatively few plants to the field for screening.
However, by combining this approach with selected parent material with an acceptable level of resistance we can improve the odds of producing one or more plants with a combination of disease resistance and desirable production characteristics.
The current strategy is to mutate 2–4 Cavendish selections with TR4 resistance and assess 500 of each these selections in field screening in the Northern Territory for TR4 resistance and improved production characteristics.
So far the Taiwanese selection GCTCV 119 has been treated and the resulting plants were planted in the field in the Northern Territory in June 2017.
Dwarf Nathan, an extra-dwarf Cavendish showing good resistance to TR4, has also been treated and the surviving plants are currently in the nursery in the Northern Territory with planting to occur before the end of 2017.
A non-Cavendish line, the highly resistant Goldfinger, has also been mutated in an attempt to improve fruit quality characteristics and will be grown and assessed initially at South Johnstone.
Any lines identified with improved fruit quality will then be screened against TR4 in the Northern Territory.
The somaclonal selection technique is relatively uncontrolled but currently offers the best opportunity for developing resistant selections that are familiar and acceptable to Australian
The mutagenesis approach is more efficient for the Australian situation than the detection of natural somaclonal variants from standard tissue culture multiplication due our limited area and resources for identifying improved selections.
ACCESSING NEW VARIETIES
Currently there is a high level of interest from growers to grow and assess new varieties under their own commercial conditions.
In many instances access to new varieties is restricted as part of the testing agreement with the breeding program and the varieties cannot be provided for testing on farm.
The project BA16001 Improved Plant Protection for the Banana Industry is currently planning for coordinated pre-commercialisation trials for some varieties allowed under the testing agreements on a small number of representative farms in North Queensland.
The long term investment required in breeding banana varieties means that commercial production of promising new varieties will most likely be licensed and managed to maximise the return on the investment to the breeding program as has occurred in other crops like apples, stonefruit and mangoes.
STEERING THE DIRECTION FOR VARIETY R&D
With the limited resources available and the high priority for identifying resistant varieties it is essential that the R&D investment decisions represent the best value for the banana ndustry.
A key part of this is the development of the Variety Committee as an industry group to provide advice and feedback to the Hort innovation Strategic Industry Advisory Panel (SIAP) and BA16001 project staff.
The Variety Committee consists of banana growers, supply chain businesses and researchers and focuses on the broad strategic issues associated with variety importation and development.