Association of Double-Stranded Ribonucleic Acids with Lettuce Big Vein Disease

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Abstract
Healthy roots and leaves of lettuce (Lactuca sativa) plants lacked detectable double-stranded ribonucleic acid (dsRNA). Disease-specific dsRNA were readily detected in lettuce plants experimentally infected with each of 4 isolates of Olpidium brassicae known to transmit the lettuce big vein (LBV) agent. No dsRNA were detected when an isolate of O. brassicae that does not transmit the LBV agent was used. The highest concentration of dsRNA was in roots of plants maintained at 18.degree. C for 30-40 days after inoculation, but dsRNA could be detected as early as 6 days after inoculation. No dsRNA had been detected before or after symptom development in extracts from up to 100 g of leaves of plant infected by the LBV agent. The dsRNA were detected in roots but not leaves, of lettuce plants vegetatively propagated from shoots from plants infected by 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 dsRNA 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 dsRNA of tobacco necrosis virus are distinct from, and do not hybridize to the dsRNA associated with LBV disease. The number (between 1 and 6) and the relative amounts of dsRNA associated with LBV disease varied with the experiment, the isolate, and the time for harvest. Three dsRNA (MW = 2.3, 2.1 and 0.72 .times. 106) have been detected more consistently than the others. The results suggest that an RNA virus causes LBV disease.