Isolates of Fusarium oxysporum f. sp. Race 2 (VCG 0128) was found attacking Bluggoe in cubense from wilted banana plants in Queensland and North Queensland. Race 4 (VCGs 0120, 0129, 01211) New South Wales were characterised for vegetative was detected in Cavendish plantations in southern compatibility. Six vegetative compatibility groups Queensland and in Lady finger plantations in New (VCGs) were identified. Race 1 (VCGs 0124, 01241.5, South Wales and southern Queensland. Isolates of the 0125) was widespread, being detected in northern and race 4 VCG 0129 from Lady finger plantations were southern Queensland as well as northern New South Wales. pathogenic to Cavendish cultivars in glasshouse tests.
The basic infection rate of bunchy top disease in established bananas averaged 0.0342 new infections per infectious plant per day, but varied seasonally with a maximum in summer. The mean distance of spread for the aphid vector was 15.2 m. The latent period was 59.8 days and correlated with the time required for the growth of 3.7 new banana leaves. A microcomputer program was written to simulate spread of banana bunchy top disease in space and time. In the absence of disease control, disease spread from an initial primary infection in July or January to 124 or 153 infected plants, respectively, in one year. When disease control was maintained by removing diseased plants whenever the number of infected plants exceeded a given threshold, the numbers of diseased plants detected each inspection were positively correlated with the infection threshold, but the numbers of inspections required to maintain control increased markedly as the infection threshold was decreased. A practice of removing apparently healthy plants within 5 m of plants detected with bunchy top disease symptoms in five or more leaves was found to locate about 30% of the remaining undetected infected plants when disease was first detected in a plantation. However, its use as a routine control measure was ineffective in reducing the number of inspections required to maintain control or in reducing the risk of disease spreading to adjoining plantations. Removal of apparently healthy plants within 5 m had some bearing on disease control when applied around plants with disease symptoms in two leaves or less, but also caused a significant loss of healthy plants.
An infection chain was reconstructed from historical data of a typical outbreak of bunchy top disease of bananas to determine factors influencing the seasonal occurrence and eradication of disease under roguing. The results were described by a mathematical model involving four random variables (incubation period, relative infection rate, detection efficiency and eradication efficiency) and one fixed variable (inspection interval). The model successfully predicted the course of roguing in an independent disease outbreak. The incubation period of the disease depended on the time taken for two banana leaves to emerge after inoculation. At Alstonville, N.S.W., the incubation period varied from 19 days in summer to 125 days in winter. The relative infection rate varied on average from 0.027 to 0.001 new infections per old infection per day in summer and winter respectively. Efficiency of disease detection by experienced inspectors depended on the number of leaves displaying disease symptoms and, equally, on time of year. Under the normal 3-weekly inspection routine, detection efficiency varied from 0.94 in summer to 0.19 in winter, plants being found on average at the two-leaf stage of disease. The efficiency with which banana growers eradicated diseased plants following their detection was found to be a characteristic of the grower concerned but was, in general, poorest in late summer.
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