Fusarium wilt of banana, also known as Panama disease, is one of the most destructive diseases of banana (Musa spp.) and is regarded as one of the most significant threats to banana production not only in Australia, but worldwide. The causative agent is the soilborne fungus, Fusarium oxysporum Schlechtend.: Fr. forma specialis (f. sp.) cubense (E.F. Smith) Snyder and Hansen (Foc) for which options of control are limited, with no commercially viable means of chemical control. Micropropagated bananas are increasingly being used by the Queensland banana industry. This is because they are the best source of disease and pest-free planting material and productivity on the farm can be greatly improved with their use. However, when planting into soils heavily infested with Foc, these plants have been found to be more susceptible than conventional planting material. It is believed that microorganisms such as bacteria and arbuscular mycorrhizal fungi (AMF), which are not available to micropropagated plants raised in a sterile environment, may protect vegetative planting material dug from the field. Adding beneficial microorganisms to micropropagated plants in the nursery before planting in the field may provide some protection from soilborne diseases such as fusarium wilt and lead to improved performance. This research was undertaken to identify endophytic and rhizosphere inhabiting bacteria (including fluorescent Pseudomonas spp., Plant Growth Promoting Rhizobacteria (PGPR), spore-forming and chitinolytic bacteria) and fungal endophytes such as AMF that enhance plant growth and inhibit or reduce the infection of Foc in micropropagated banana plants. It is widely known that suppressive soils exist where fusarium wilt does not reach epidemic proportions. Laboratory results to date support a microbial basis for this suppressiveness. Wilt suppressive sites were located in Queensland and northern New South Wales and roots of banana plants were collected. Rhizosphere root washings and surface sterilised root segments were plated onto solid growth media to isolate both rhizosphere and endophytic bacteria. Bacterial isolates were initially evaluated using various in vitro tests including antagonism on agar, chitinolytic ability, production of fluorescent siderophores when grown on Kings B media, spore production and ability to lyse Foc mycelia using Foc as a sole carbon source. This process assisted with selection of isolates that may have the most biological control potential. Based on these tests 41 isolates were chosen for further investigation. Arbuscular mycorrhizal fungi were isolated from banana roots growing in disease suppressive soils and were also obtained from a culture collection at the Gatton Campus of The University of Queensland. A total of 10 pot cultures were prepared using sunflower as a host plant. Mycorrhizal fungi produced typical intraradical structures (arbuscules, vesicles, hyphae) when banana plants were inoculated with AMF. To assess the biological control potential of bacteria and AMF against Foc, a reliable small plant bioassay system was required. Attempts by various researchers to develop a reliable bioassay have been made in the past however infection of banana plants by the pathogen was inconsistent. The inconsistencies of previous work may microorganisms in the growth of micropropagated banana plants and protection of young plants from disease. However, ability to reduce disease assessed by internal rating of vascular discolouration appears to be influenced by host genotype. While reduction in disease severity and promotion of plant growth was observed in glasshouse trials, this was not observed in field trials at harvest. If plants had been assessed for differences in growth and susceptibility to disease plus nutrient uptake at the early stages of growth following transplanting into the field rather than at harvest, differences between microbial treated and untreated plants may have been determined. In the field, environmental factors such as drought influenced disease development and plant growth of inoculated plants at the race 1 field site at Terranora in NSW, which was not irrigated. Six of the bacterial isolates tested (25, 26, 2bb, 3b-1, 48 and S-5) and two soilborne isolates of Fusarium oxysporum (Calcutta-1 and K17pi) significantly reduced plant height of cv Lady finger at harvest. Six bacterial isolates (4a, 4b, 84, 8-7, 92 and S-24) and AMF isolate LJS1 had no effect on plant growth. A reduction in plant growth following application of micropropagated banana with microbial inoculants was not observed at the ‘subtropical’ race 4 field testing site that was irrigated. Further research is required to understand the influence of microorganisms applied to nursery grown plants on growth under different environmental and soil conditions when transplanted. The results of this investigation illustrate how selected rhizobacterial and arbuscular mycorrhizal amendments applied to micropropagated banana as a pre-plant treatment in the nursery could be used to enhance plant growth and vigour and reduce disease caused by the fusarium wilt pathogen, thereby improving root health in young plants following transplanting. However, further trials are necessary to determine if these microbial inoculants can be practically produced and applied to commercial operations in field soil conditions to improve root health.
| Identifer | oai:union.ndltd.org:ADTP/253395 |
| Creators | Smith, Linda Jane |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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