The Past and Current Approaches for Control of Papaya Ringspot Virus in Taiwan

Production of papaya has been limited in all papaya-growing areas in Taiwan due to the disease caused by Papaya ringspot virus (PRSV)since 1975. Agricultural practices such as eradication of diseased plants, cultivation of a high-stem barrier crop, silver mulching, and protection with open plastic bags at the seedling stage were mostly in vain or resulted in only marginal benefits. A PRSV-tolerant variety, 'Tainung No. 5', was released in late 1980s, but not welcome by the farmers and consumers because of its susceptibility to root- and fruit-rot. Between 1984 and 1993, control of PRSV by a protective mild strain became the main strategy for promoting papaya yield amidst prevalence of otherwise uncontrollable PRSV. A mild strain designated as HA 5-1 was induced by nitrous acid treatment and used to inoculate more than 4 million papaya seedlings (>2,000 hectares)during that decade. The control by cross protection assured the success of the first harvest and was welcome by the farmers. However, highly strain-specific nature of this strategy rendered its application unpromising in the areas exhibiting heavy challenge pressure or different PRSV strains. After 1994, despite its odds, protective netting has become the widely followed strategy to control PRSV. As revealed by a rough estimation in 2005, over 2,000 hectares of papaya in Taiwan were under netting, as an effort to ensure high yield, by blocking the access of aphids to papaya plants. Despite its popularity among the farmers, netting has several limitations such as relatively inferior quality of fruits in sweetness and flavour and high costs of the investment, apart from the environmentally hazardous nature of the net material. Hence, currently, the attention of the investigators to improve papaya crop against PRSV infection has been shifted to transgenic approach, with particular emphasis to transform the crop with the coat protein (CP)gene of PRSV. Several transgenic papaya lines that provide broad-spectrum resistance against different geographic strains have been evaluated under field conditions. Potential risks of breakdown by super strains of PRSV and Papaya leaf-distortion mosaic virus are being addressed. The approaches to generate transgenic resistance to different viruses or super strains of PRSV are proposed. Furthermore, the suggestions for improving papaya-transformation mediated by Agrobacterium are described.