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Use of Resistant Cucumis metuliferus for Selection of Nitrous-Acid Induced Attenuated Strains of Papaya Ringspot Virus. Shyi- Dong Yeh, Department of Plant Pathology, National Chung Hsing University, Taichung Taiwan, Republic of China; Ying-Huey Cheng, Department of Plant Pathology, National Chung Hsing University, Taichung Taiwan, Republic of China. Phytopathology 79:1257-1261. Accepted for publication 21 June 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1257.

Papaya ringspot virus (PRV) HA 5-1 and HA 6-1 are two nitrous-acid induced mild strains that cause infection without conspicuous symptoms in papaya and Cucumis metuliferus line Acc. 2459. These strains were able to overcome the resistance governed by a single dominant gene Wmv in Cucumis metuliferus line PI 292190, in which resistance was defined by immunity to the parent severe strain PRV HA and susceptibility to the mild strains as manifested by systemic mosaic and necrosis symptoms. Moreover, the systemic and necrosis symptoms caused by HA 6-1 were found in all F1 test plants of Acc. 2459 PI 292190, and perfectly followed the segregation of the Wmv gene in the F2 population, indicating that the symptoms are direct interactions between the mild virus strain and the Wmv gene. The possibility of using the resistant line of C. metuliferus to screen useful attenuated strains of PRV for cross protection was investigated. Virus in crude sap from susceptible C. metuliferus Acc. 2459 infected with PRV TM, a local strain prevalent in Taiwan, was treated with nitrous acid; enrich-propagated in plants of line Acc. 2459; and then transferred to plants of the resistant line PI 292190. Virus isolates that induced systemic symptoms on line PI 292190 were selected and followed by repeated serial dilutions on the same line. The isolates were then transferred to papaya to determine their pathogenicity. Among 20 isolates selected, six induced mild symptoms on papaya; and one, designated as PRV TM-1, caused infection without conspicuous symptoms. Under greenhouse conditions, papaya inoculated with PRV TM-1 were completely protected against PRV TM when challenge inoculations were at 23, 45, or 90 days after preimmunization. The results indicated that the resistant C. metuliferus can be used as a selective host for screening attenuated virus strains of PRV from artificial induction.

Additional keywords: artificial induction, Carica papaya, cross protection.