By Noeleen Warman

Panama disease, caused by the fungus Fusarium oxysporum f.sp. cubense, threatens Australia’s banana production. Knowing how the fungus moves through the banana plant and when and where it produces spores, is an important component in containing and managing the disease, as there are severe limitations in the way this fungus can be controlled.

The infection process of the Fusarium fungus from the soil into the roots of the banana plant has been widely studied. However, there is little information available detailing the movement of the fungus into the rest of the plant. Understanding this process was the basis of research recently conducted at the University of Queensland, in conjunction with the Queensland Department of Agriculture and Fisheries.

Under strictly controlled conditions, both in the laboratory and glasshouse, Cavendish ‘Williams’ and Lady Finger banana plants were inoculated with an isolate of the Sub Tropical Race 4 of the Fusarium fungus, which had been transformed with a Green Fluorescent Protein (GFP) gene. This gene, when inserted into Fusarium allows scientists to visualise the movement of plant pathogens, but does not change the virulence of the pathogen.

Once the banana plants were inoculated with the Fusarium containing the GFP gene, they were destructively harvested every 10 days for 80 days. All parts of the plant, including the roots, corm, pseudostem and leaves, were examined after harvest.  A confocal microscope allowed the fluorescing fungus to be visualised, providing a detailed look into the movement of Fusarium, spore formation and disease development in both banana cultivars.

We found that, in both cultivars, the presence of the fungus had extended much further into the plant than was apparent from the corresponding external symptoms.  In as little as 10 days after inoculation, the fluorescing Fusarium could be found within the roots of both cultivars. At this same time, the fluorescing fungus was also found in the corm and lower parts of the pseudostem in the Cavendish plants, but had not yet reached the corm in the Lady Finger. By contrast, external symptoms were not observed in Cavendish until 30 to 40 days after inoculation and 40 to 50 days after inoculation in Lady Finger. By the time external symptoms were observed, the fungus was visualized internally all the way up the plant and into the mid rib of the leaves above the throat in both cultivars.

An important observation from this work was the movement of Fusarium from inside the leaf sheaths, which make up the banana pseudostem, to the outer surface of decaying leaf sheaths via the stomata; the pores responsible for gas exchange. Once present on the outer surface, the fungus produced masses of spores (macroconidia) as well as the long lived resting spores, called chlamydospores. In addition, chlamydospores were also produced profusely in the air chambers on the inside of these decaying leaf sheaths. These observations suggest that decaying leaf sheaths or pseudostems are a potential source of inoculum and emphasize the importance in limiting the movement of plant material both within a farm and externally.

The results from this study provide a valuable insight into the movement of Fusarium wilt fungus through the entire banana plant. It also raises further questions, such as the potential spread of the pathogen from aerials parts of the plant. Furthermore, it stresses the importance of early identification and eradication of infected plants.

·         This study was jointly conducted at the School of Agriculture and Food Sciences, The University of Queensland, with the Queensland Department of Agriculture and Fisheries under the supervision of Prof. Elizabeth Aitken. This project has been funded by Horticulture Innovation Australia Limited with co-investment from the Queensland Department of Agriculture and Fisheries (DAF) and funds from the Australian Government. Needs a Hort Innovation/DAF logo.