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Effectiveness of Cross-Protection by Mild Mutants of Papaya Ringspot Virus for Control of Ringspot Disease of Papaya in Taiwan. H. -L. Wang, Research Associate, Department of Plant Protection, Fengshan Tropical Horticultural Experiment Station, Fengshan, Taiwan, Republic of China. S.-D. Yeh, R.-J. Chiu, and D. Gonsalves. Associate Professor, Department of Plant Pathology, National Chung Hsing University, Taichung, Taiwan, Republic of China; Senior Research Specialist, Council of Agriculture, Taipei, Taiwan, Republic of China; and Professor, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Plant Dis. 71:491-497. Accepted for publication 19 November 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0491.

Two mild nitrous acid-induced mutants (PRV HA 5-1 and 6-1) of papaya ringspot virus (PRV) were tested under greenhouse and field conditions to study the potential of cross-protection for control of papaya ringspot disease of papaya in Taiwan. Under greenhouse conditions, both mutants caused only mild or symptomless infection on test plants in Chenopodiaceae and Cucurbitaceae families and on the major papaya cultivar of Taiwan, Tainung No. 2. Also, under greenhouse conditions, HA 5-1 and HA 6-1 provided a high degree of protection in papaya against the severe effects of two prevalent PRV strains of Taiwan. Large numbers of papaya seedlings were infected by applying inocula with a spray gun (equipped with a 1.2-mm-diameter nozzle), using pressures of 48 kg/cm2 at distances of 1020 cm. Field cross-protection trials were started in severely diseased areas in fall 1983. When protected papaya plants were planted randomly with unprotected controls or row by row under high disease pressure, unprotected plants showed severe symptoms 23 mo after planting and protected plants showed severe symptoms 13 mo after the controls. Under these conditions, cross-protection did not provide economic benefit. However, in a test where protected and unprotected plants were established in solid blocks and where the disease pressure inside the test orchard was minimized by roguing once every 10 days up to the flowering stage, protected trees showed 82% higher fruit yield than unprotected plants. This resulted in a 111% increase in income because of a much higher yield of good-quality fruit from protected trees than from the controls. Because of these initial successes, the government of Taiwan initiated large-scale cross-protection field trials in 1984 and 1985 of 244,000 and 400,000 seedlings planted over 122 and 200 ha, respectively.