Biotechnology Centre, AFNS, University of Alberta, Edmonton, AB T6G 2P5, Canada

Background and objectives
Olpidium brassicae has been known to be a vector of at least three viruses: tobacco necrosis (TNV), tobacco stunt (TSV), and lettuce big-vein (LBV7). There is renewed interest in the virus-Olpidium associations due to recent progress in the study of some properties of both TSV and LBVV. These three viruses offer interesting contrasts in their properties; TNV is usually stable and is readily sap-transmissible, whereas TSY and LBVV are relatively unstable and are sap-transmitted only with the aid of chelating or reducing agents. Furthermore, there are differences in the relationship of the viruses with their vector fungus. TSV and LBVV are readily maintained in the resting spores for an extended period of time, whereas INV is not. TNV is readily acquired from suspension by zoospores whereas TSV and LBVV are acquired by the fungus only within cells of infected plants [1]. The objective of this study is to shed light on the mode of transmission by zoospores of tobacco and lettuce strains of O. brassicaeusing TNV, TSV and LBVV in various combinations.

Results and conclusions
Transmission of TNV with TSV or LBVV. The results from experiments to test dual transmission of TNV with TSV or LBVV clearly indicated that both the tobacco and lettuce strains of Olpidiumwere efficient vectors of TNV and were also capable of transmitting TSV and LBVV in combination with TNV. However, when secondary inoculations were made with inocula prepared from previously infected mung bean roots to tobacco or lettuce, the number of stunt- or big-vein-diseased plants was significantly lower in the cases of dual transmission, suggesting some deleterious effects of TNV on transmission of these two viruses. The final TNV assays suggested that INV became dominant in the dual transmission system.
Effect of systemic hosts on dual transmission. There were no differences in transmission of TSV or LBVV between single and dual transmission in their systemic host plants. However, an increase in TNV infectivity was detected in duplicated tests in one case of dual transmission with TSV-tobacco Olpidium in both mung bean root and cowpea primary leaf assays.

This study demonstrated that a tobacco strain of O. brassicae is capable of transmitting TNV as well as TSV using an in vitro mixture of viruliferous zoospores. Since Olpidium can acquire TSV only in vivo, it is presumed that TSV is borne internally in Olpidium zoospores. On the other hand, Olpidium zoospores acquire TNV externally from a virus suspension [2]. The fact that both TNV and LBVV, or TNV and TSV are transmitted by a lettuce strain or a tobacco strain of O. brassicae clearly suggests a reason for the frequent reports of the isolation of TNV from big-vein-affected lettuce or from stunt-affected tobacco plants. However, whether or not there is mutual interference or synergism between them in dual or multiple transmissions was not determined in this study and must await further clarification.

1. Hiruki C, Teakle DS, 1987. In Harris KF, ed, Current Topics on Pathogen-Vector-Host Research 3. Springer-Verlag, New York, pp. 177-215.
2. Temmink JHM, Campbell RN, Smith PR, 1970. Journal of General Virology 9, 201-213.