Panama disease caused by the soil-borne fungus Fusarium oxysporum f. sp. cubense (Foc) has destroyed thousands of hectares of banana plantations throughout the world in the last 100 years. Currently this disease affects not only dessert bananas (including the cultivar Cavendish), but also cooking bananas (including the cultivar Kepok). In the last 50 years the resistant cultivar Cavendish selected as a replacement for the susceptible cultivar Gros Michel, has succumbed to a new race of the pathogen designated as race 4. In 1995 the cultivar Goldfinger was released as a replacement for Cavendish for the Australian market, providing growers with a cultivar resistant to race 4. However more recently a more virulent strain of the pathogen (tropical race 4) has been recorded in the Northern Territory of Australia, and the resistance of Goldfinger to tropical race 4 is unknown. Moreover increasing evidence indicates that the resistance of cultivars to a specific pathogenic strain can be broken down under adverse environmental conditions.
To date there are no cultural or chemical techniques available to control this disease. Breeding remains the only feasible alternative. However most of the previously available screening techniques have relied on unrealistically high levels of artificial inocula, in simplified soil systems, seldom reproducing the disease resistance of cultivars observed in the field. Panama disease is complicated by the existence of different pathogenic and saprophytic strains of the fungus, and the role of soil type in determining disease severity in the field. In this study soil types known to be naturally infested with only one strain of the pathogen were used to study the role of pathogen activity in the soil and invasion of banana plantlets, in determining disease severity. The Vegetative Compatability Group test and the Volatile Odour technique were used to confirm the genotype of isolates and to distinguish between pathogenic and saprophytic strains of the fungus. The addition of composted sawdust and aged chicken manure, and solarisation were used to manipulate levels of pathogen activity and disease severity in glass-house bioassays, transplanting susceptible banana plantlets at four-weeks post-deflasking into the different soil treatments.
The results indicate that Panama disease does not follow a dose-response model. Solarisation effectively reduced the survival of Foc in soil, but rates of invasion and disease severity in treated soil were not significantly different from untreated soil. The addition of composted sawdust neither reduced pathogen activity nor affected invasion or disease severity. In contrast chicken manure significantly enhanced pathogenic invasion and disease severity, with no significant effect on the activity of the pathogen in the soil. The findings on the effect of chicken manure were used to develop a screening technique to select banana cultivars with improved resistance to Panama disease.
By manipulating the amount of chicken manure added and the age of banana cultivars at transplanting, appropriate disease severity levels in naturally infested soil were reproduced to screen the responses of resistant and susceptible cultivars. A rating system based on the development of foliar disease symptoms over time was developed to compare the relative performance of the cultivars, using ordinal logistic regression techniques. The performance of six cultivars was ranked against cultivar Williams, the Australian industry standard resistant to Foc race 1. The response of six of the cultivars segregated into three groups, reflecting their known field performance as susceptible, moderately resistant and resistant. The exception was cultivar Sugar, unexpectedly exhibiting extremes of resistance and susceptibility. Further testing is required to confirm the phenotype of this material, as evidence suggests either mislabelling or somaclonal variation has occurred.
In a pilot survey of West Sumatra Indonesia, 37 isolates of Fusarium oxysporum f. sp. eubense (Foc) derived from 15 banana cultivars showing symptoms of Panama disease were recovered from six of the 14 districts surveyed. Affected cultivars included the most popular dessert bananas Pisang buai (AAA), Pisang raja (AAB), Pisang raja serai (=raja sereh AAB), Pisang ambo(u)n (AAA), and the most popular cooking type Pisang kepok (BBB). During this survey, no Panama disease symptoms were observed on wild Musa species.
The distribution of the VCGs identified in this pilot study indicate that human activity is the most important agent in the dispersal of pathogenic strains of Foc. There are two key ethnobotanical factors responsible for the current distribution. The first is the traditional cultural practice of transplanting suckers and corms, irrespective of the the disease status of the parent plant. The second is the pattern of migration operative within the different districts. The development of an extension program to educate both traditional and commercial growers about appropriate hygiene and quarantine practices is required to control the spread of different VCG strains of Foc. High priority must also be given to mapping the current distribution of VCGs in large plantations, and to provide a disease risk assessment service for proposed new plantations. Traditionally many different genotypes of bananas have been cultivated in Indonesia, which may provide alternatives to the currently preferred Cavendish varieties grown for export. The application of the chicken manure screening technique to rank these cultivars against Cavendish, and in the detection of novel sources of resistance in wild germplasm, will add considerably to the success of world banana breeding programs.