Alternative breeding strategies, based on colchicine-induced autotetraploids, have been proposed as a means of introducing disease resistance into banana breeding programs. This paper describes techniques for the in vitro induction of banana autotetraploids by the use of colchicine on cultured explants. The technique can be readily applied and large numbers of autotetraploids produced. The optimum treatment involved immersing shoot tips in a 0.5% w/v colchicine solution for 2 h under aseptic conditions. Dimethyl sulfoxide (DMSO) was applied with the colchicine treatments to increase cell permeability and so absorption of colchicine, resulting in the optimum treatment unchanged at 0.5% colchicine, but including the addition of 2% v/v DMSO. Of the shoot tips treated over 30% were induced to the autotetraploid level.Methods for in vitro selection of induced tetraploids from treated diploid plantlets were also developed. Tetraploid plants were more robust with thicker pseudostems, roots and broader leaves than diploids and they could be selected on these morphological characteristics. Mean stornatal lengths of diploid banana plants growing in vitro were significantly smaller (16.0 pm) than the tetraploids (26.9 pm) and were used as a more reliable indicator of ploidy than morphological criteria alone. A root tip squash technique using carbol fuchsin was developed for positive confirmation of ploidy change by chromosome counts. Although chimerism and reversion to the diploid form occurred, it was not considered a problem because of the large number of autotetraploids induced. Stable autotetraploids were recovered and established in the field and were characterised by their large, drooping leaves and thick pseudostems. They have retained these characteristics for more than 3 years in the field.

A field experiment was established in which an amendment of poultry manure and sawdust (200 t/ha) was incorporated into some plots but not others and then a permanent pasture or a sequence of biomass-producing crops was grown with and without tillage, with all biomass being returned to the soil. After 4 years, soil C levels were highest in amended plots, particularly those that had been cropped using minimum tillage, and lowest in nonamended and fallowed plots, regardless of how they had been tilled. When ginger was planted, symphylans caused severe damage to all treatments, indicating that cropping, tillage and organic matter management practices commonly used to improve soil health are not necessarily effective for all crops or soils. During the rotational phase of the experiment, the development of suppressiveness to three key pathogens of ginger was monitored using bioassays. Results for root-knot nematode (Meloidogyne javanica) indicated that for the first 2 years, amended soil was more suppressive than non-amended soil from the same cropping and tillage treatment, whereas under pasture, the amendment only enhanced suppressiveness in the first year. Suppressiveness was generally associated with higher C levels and enhanced biological activity (as measured by the rate of fluorescein diacetate (FDA) hydrolysis and numbers of free-living nematodes). Reduced tillage also enhanced suppressiveness, as gall ratings and egg counts in the second and third years were usually significantly lower in cropped soils under minimum rather than conventional tillage. Additionally, soil that was not disturbed during the process of setting up bioassays was more suppressive than soil which had been gently mixed by hand. Results of bioassays with Fusarium oxysporum f. sp. zingiberi were too inconsistent to draw firm conclusions, but the severity of fusarium yellows was generally higher in fumigated fallow soil than in other treatments, with soil management practices having little impact on disease severity. With regard to Pythium myriotylum, biological factors capable of reducing rhizome rot were present, but were not effective enough to suppress the disease under environmental conditions that were ideal for disease development.

Fusarium oxysporum f. sp. cubense ( Foc ) has severely curtailed banana production in the tropical regions of the world. The tropical race 4 (TR4) of Foc was detected in Australia in the 1990s and it is virulent to all Cavendish type banana cultivars, which represents the majority of banana production in Australia. Genetic resistance to Foc race 4 is urgently needed. To characterize sources of resistance, we have assessed the Foc resistance response of 34 Musa cultivars with plants grown under controlled settings. Amongst diploid banana cultivars carrying the AA genome, resistance is found in Musa acuminata sub-species including malaccensis ‘Pahang’ and burmannica ‘Calcutta4.’ In the polyploid group, the hybrids such as ‘FHIA-18’ and ‘FHIA-25’ are highly resistant against both Foc -TR4 and subtropical race 4 ( Foc -STR4). Interestingly, ‘FHIA-2’ and ‘CAM020’ appear to be resistant to Foc -TR4 but susceptible to Foc -STR4, suggesting potential differences in the resistance mechanisms against the different race 4 strains. Using a GFP tagged Foc -STR4 strain challenged onto both resistant and susceptible M. a. malaccensis lines, a high inoculum dosage rapidly induced vascular wilt in the susceptible M. a. malaccensis lines at 2.5 weeks. This was associated with an accumulation of micro-conidia in the rhizome and the movement of the fungus through the xylem vessels. In contrast, the fungal movement was restrained in the rhizome of the resistant M. a. malaccensis lines and no sporulation was observed. Overall, this research suggests that the resistance response is dependent to an extent on inoculum dosage and that the plant host’s response, in the rhizome, plays an important role in inhibiting the fungus from spreading to the rest of the plant. Identifying race 4 resistant accessions can help to understand mechanisms of resistance and provide banana breeders with the genetic resources to integrate resistance genes into commercial varieties.

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The growth and performance of micropropagated ginger (Zingiber officinale Roscoe) was compared with 'seed'-derived plants in field trials conducted in south-eastern Queensland. In the first generation ex vitro, micropropagated plants had significantly (P<0.01) reduced rhizome yield with smaller knobs and more roots. Micropropagated plants had a greater (P<0.01) shoot: root (rhizome) ratio compared with seed-derived plants. Shoots from micropropagated plants were also significantly (P<0.01) smaller with a greater number of shoots per plant. The unusual shoot morphology of the micropropagated plants did not appear to be related to the presence of benzylaminopurine, a plant growth hormone added to the multiplication medium, as plants subcultured for 3 cycles on a hormone-free medium also exhibited similar characteristics. Seed collected from the micropropagated plants and seed-derived plants was harvested and, despite the micropropagated seed being significantly (P<0.01) smaller, by the second generation ex vitro there were no significant differences between the treatments. Factors that can improve rhizome size, while reducing production costs, need to be identified before micropropagated plants can be recommended for routine use in the ginger industry as a source of disease and pest-free planting material.

‘Dwarf Parfitt’, an extra-dwarf Cavendish cultivar with resistance to subtropical race 4 Fusarium oxysporum f. sp. cubense (Foc), was gamma irradiated at a dose of 20 Gy and putative mutants were recovered with improved agronomic characteristics. Further screening of putative mutants for improved yield and fruit size, as well as a degree of resistance to fusarium wilt, led to the selection of a line (DPM25) with improved productivity when grown on soils infested with subtropical race 4 Foc. DPM25 was equal to the industry standard, ‘Williams’, in every agronomic trait measured and it consistently showed a lower incidence of fusarium wilt. Further improvement of field resistance to race 4 Foc is needed in DPM25 and further cycles of mutation induction and selection is an option discussed.

IntroductionThe diploid (2n = 22) cultivar, 'Queensland', is favoured for the production of confectionery ginger in Australia and it is estimated that 40% of the world's confectionery ginger products are based on this single cultivar (G. O'Brien pers. comm.). Unfortunately much of the rhizome that is harvested has small rhizome sections or knobs that are unsuitable for processing into the premium grade, export product.The use of colchicine to produce polyploid plants with larger structural organs is well known. In ginger this strategy is particularly attractive as the crop is vegetatively propagated and any genetic improvements that are made to yield or quality can be preserved over successive generations. Ramachandran (1982) treated sprouting buds on ginger rhizomes in vivo with 0.25% (w/v) colchicine solution and of the 90 buds treated, 4 produced solid, nonchimeral tetraploids. Ramachandran and Chandrasekharan Nair (1992) evaluated these tetraploids and found that there was an overall increase in size of plant parts including the leaves, floral parts and the rhizome. Unfortunately, oil content was lower in the tetraploid than in the original diploid cultivar. In vitro induction of autotetraploids is a more attractive strategy because it is possible to effectively treat larger numbers of explants and to rapidly produce disease-free and pest-free planting material for field evaluation (Hamill et al. 1992). Smith and Hamill (1997) immersed ginger shoot tips in a 0.5% (w/v) colchicine solution with 2% (v/v) dimethyl sulfoxide for 2 h to produce stable autotetraploid lines. Adaniya and Shirai (2001), on the other hand, favoured an 8-day treatment on solid agar-based culture medium containing 0.2% (w/v) colchicine. Their preliminary results also indicate a larger rhizome was produced and the gingerol content of the tetraploid lines were higher than those of their diploid counterparts.The present study reports on a technique to increase rhizome size by the in vitro induction of ginger autotetraploids from the 'Queensland' cultivar and the methods used in the early detection of these lines. Field performance of the tetraploid lines was investigated over several seasons and an assessment made of their suitability for processing into a premium-grade confectionery product. Materials and methods Establishment of culturesA rhizome of ginger (Zingiber officinale Rosc.) cv. 'Queensland' was supplied by Buderim Ginger Limited from a plant exhibiting superior characteristics. To initiate cultures, the rhizome was surface sterilised with 1% sodium hypochlorite for 2 min and then stored in the dark at ambient temperature until it began to sprout. Emerging buds Abstract. Ginger autotetraploids were produced by immersing shoot tips in a 0.5% w/v colchicine, 2% v/v dimethyl sulfoxide solution for 2 h. Stomatal measurements were used as an early indicator of ploidy differences in culture with mean stomata length of tetraploids (49.2 µm) being significantly larger than the diploid (38.8 µm). Of the 500 shoot tips treated, 2% were c...

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