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Dependent Virus Transmission by Rhopalosiphum padi From Mixed Infections of Various Isolates of Barley Yellow Dwarf Virus. W. F. Rochow, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, also Professor of Plant Pathology, Cornell University, Ithaca, NY 14853; C. C. Gill, Plant Pathologist, Agriculture Canada, Research Station, 25 Dafoe Road, Winnipeg, Manitoba R3T 2M9. Phytopathology 68:451-456. Accepted for publication 17 August 1977. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-451.

Rhopalosiphum padi rarely transmits virus from oats infected only by the MAV isolate of barley yellow dwarf virus (BYDV), but it often transmits MAV, together with the RPV isolate, from plants doubly infected by MAV and RPV. Since previous work on this dependent virus transmission phenomenon was restricted to these two virus isolates, we studied interactions of eight additional BYDV isolates as a way to assess potential relevance of dependent transmission in the field. Six isolates were from collections of oats made in 1968 in fields in New York or Illinois. From all nine combinations of double infections made with the three RPV-like isolates, R. padi transmitted each of the three MAV-like ones. Dependent transmission of the paired isolates occurred from 76% of 193 doubly-infected oats. Two BYDV isolates from Canada were used in other experiments. One of the Canadian isolates (isolate 6524) was similar to RPV; the other (isolate 6407) was similar to MAV. In experiments in Ithaca, R. padi transmitted either MAV or 6407, together with RPV or 6524, from 81% of 89 doubly-infected plants. In Winnipeg, dependent transmission of MAV in the presence of RPV occurred in five of six cases studied; dependent transmission of isolate 6407 in the presence of isolate 6524 occurred from three of six mixed infections. In almost all tests, Macrosiphum avenae transmitted only MAV or MAV-like isolates from the mixed infections. These data show that dependent virus transmission occurs with a range of BYDV isolates, and support the possibility that such virus interactions affect BYDV transmission in nature.