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Association of Double-Stranded Ribonucleic Acids with Lettuce Big Vein Disease. T. Erik Mirkov, Department of Plant Pathology, University of California, Riverside 92521; J. Allan Dodds, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 75:631-635. Accepted for publication 16 November 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-631.

Healthy roots and leaves of lettuce (Lactuca sativa) plants lacked detectable double-stranded ribonucleic acid (dsRNA). Disease-specific dsRNAs were readily detected in lettuce plants experimentally infected with each of four isolates of Olpidium brassicae known to transmit the lettuce big vein (LBV) agent. No dsRNAs were detected when an isolate of O. brassicae that does not transmit the LBV agent was used. The highest concentration of dsRNA was in the roots of plants maintained at 18 C for 30?40 days after inoculation, but dsRNA could be detected as early as 6 days after inoculation. No dsRNA has been detected before or after symptom development in extracts from up to 100g of leaves of plants infected by the LBV agent. The dsRNAs were detected in roots, but not leaves, of lettuce plants vegetatively propagated from shoots from plants infected by the LBV agent. These roots were no longer infected with O. brassicae, and an isolate of O. brassicae known to be free of the LBV agent acquired the dsRNAs from these roots and transmitted them to healthy plants. These plants developed LBV symptoms 3?4 wk later, and dsRNA was detected in their roots. The dsRNAs of tobacco necrosis virus are distinct from, and do not hybridize to, the dsRNAs associated with LBV disease. The number (between one and six) and the relative amounts of dsRNAs associated with LBV disease varied with the experiment, the isolate, and the time of harvest. Three dsRNAs (MW = 2.3, 2.1, and 0.72 ? 106) have been detected more consistently than the others. The results suggest that an RNA virus causes LBV disease.

Additional keywords: soilborne viruses, viroid.