Spot on: managing Panama disease of banana in the Philippines

Banana either cooking or dessert type is regarded as one of the most important crops being staple food or cash crop. Currently the global banana production is threatened by a destructive soil-borne fungus Fusarium odoratissimum colloquially called Tropical race 4 (TR4) causing Panama disease in banana. The on-going dispersal of the pathogen raises the fear of the demise of our beloved banana. As of this writing, there is no concrete solution available to combat the disease, hence manifold of management strategies are explored. With the use of molecular diagnostic tools, this thesis describes the spatial dispersal, epidemiology and management options of Panama disease in the banana belt of the Philippines. This contributes to the development of evidence-based and cost-effective management strategies. In the first chapter the subject is presented by unfolding the biology of banana, brief history of its origin, distribution and its cultivation as a staple and cash crop. Having limited genetic variation, the vulnerability of banana to various diseases is described. Panama disease causing serious devastation in banana is elaborated with emphasis on its disease development, spatial and temporal dispersal, disease diagnostics employed and management strategies explored. Statement of the scope of the thesis and outline of the different chapters concluded this chapter. The second chapter describes the development of DNA-based diagnostic tool for TR4. In here, a rapid and highly specific detection assay based on Loop Mediated Isothermal Amplification (LAMP) was developed that is pertinent under laboratory and field conditions. The TR4-specific DNA sequence was obtained by Diversity Arrays Technology sequencing of representative genetic diversity of Foc. The specificity of the assay was tested extensively on both target and nontarget isolates and successfully used to detect TR4 in artificially inoculated and naturally infected Cavendish banana corm and pseudostem in the Philippines. Hence, the developed assay offers a powerful tool for epidemiological study on TR4 and is indispensable for implementing quarantine measures. The third chapter uncovers the unknown epidemiological phenomena of TR4 dissemination in the banana belt of the Philippines and validate the efficacy of applied management strategy. In here, the spatial distribution of TR4 across soil profiles cropped with either Cavendish or local banana cultivars reveals that the pathogen is distributed across soil layers of up to 1 m below. The dissemination through contaminated soil and water is presented suggesting the importance of deploying biosecurity measures. In the absence of banana host, the long term survival of TR4 on weeds was confirmed emphasizing the importance of weed management in combating Panama disease. Data supporting the efficacy of field sanitation by burning was presented revealing the inefficiency of the procedure to eliminate TR4 propagule in the soil and should be reconsidered. Together, this findings emphasize the need for an integrated and evidence-based Panama disease management strategies. The fourth chapter put emphasis on the efficacy of disinfectants used as front-line defense against TR4 spread in the banana production areas in the Philippines. In this study, disinfectants of different active ingredients, rate and exposure time were tested in vitro against different propagules of TR4 to include mycelia, conidiospores, chlamydospores suspensions and chlamydospores-infested soil. The TR4 mycelia and conidia are sensitive to disinfectants tested at the manufacturer’s recommended concentration. The thick-walled chlamydospores were remarkably sensitive to disinfectants in suspension but the same compounds were largely ineffective against chlamydospores in soil except for diamidine-based disinfectant. Summing up, the rational choice of appropriate disinfectants should be facilitated by reliable data on efficacy under various biological, environmental and temporal conditions, corrosiveness and costs. The utilization of disinfectants should be maximized and properly implemented to stop or slow down TR4 dissemination within and between farms at local, regional or international scale. The fifth chapter investigates the potential of anaerobic soil disinfestation (ASD) as a biological option for Panama disease management. In this study, two commercially available organic amendments were tested both under laboratory and field conditions in the naturally infested soils in the Philippines. Controlled condition in the laboratory resulted to reduction of viable TR4 chlamydospores to 0.01% in plastic containers after four weeks treatment supported by the production of toxic gases and volatile fatty acids. The pilot field experimentation displayed a hundred-fold reduction of TR4 chlamydospores level relative to the control after eight weeks field treatment. A significant reduction of Panama disease recurrence was recorded on replanted Cavendish “Williams” banana monitored for one cropping cycle. These promising results might contribute to short-term management options to continue banana production in Panama disease affected farms. The last chapter summarizes the findings gathered in this PhD project. The continued local and international TR4 dispersal is discussed in relation to containment strategies and management options currently employed. The importance of the molecular diagnostic tool to chart progression of disease spread is discussed. The systematic approach to manage soil-borne pathogens are reviewed linking to the current practices to continue banana production despite the presence of TR4. The pathogen inoculum density and disease incidence curve was correlated to the current knowledge. This gives rise to the statement that partial resistance to Panama disease does not ultimately contributes to sustainable disease control. The intended long-term perspective for banana growing lies in an integrated management strategy that includes the use of multiple resistant banana varieties.