Studies on the lettuce big‐vein virus and its vector Olpidium brassicae (Wor.) Dang

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Abstract
SUMMARY: It was demonstrated by serial graft transmission experiments that the big‐vein disease of lettuce was caused by lettuce big‐vein virus (LBVV).An Olpidium sp., always found in the roots of naturally affected plants, induced the disease when its zoospores were transferred to healthy plants. The fungus was identified as O. brassicae (Wor.) Dang.The same fungus was found in the roots of healthy lettuce, but zoospores from this source would not induce big‐vein when transferred to healthy plants.When a non‐viruliferous isolate of O. brassicae was grown in the roots of lettuce affected by big‐vein the fungus acquired LBVV and was able to transmit the disease. Conversely, the viruliferous Olpidium was converted to a non‐viruliferous form by culture in the roots of Plantago major or Veronica persica. These results were not caused by the selection of non‐vector strains of the fungus during its culture in these species, as the fungus reacquired and transmitted LBVV when again cultured in big‐vein affected lettuce roots. The results suggested that LBVV did not multiply in P. major or V. persica, or in the fungus during its growth in them.Attempts to transmit LBVV with Cuscuta campestris, C. gronovii, C. reflexa and C. subinclusa and by sap inoculation were unsuccessful.No causal relationship between tobacco necrosis virus (TNV) and big‐vein was found: the reasons for the frequent association of TNV with the disease are discussed.