Aust. J. Agric. Res., 1993, 44, 1891-1904 Resistance to Potato Leafroll Virus Infection and Accumulation in Potato Cultivars, and the Effects of Previous Infection with other Viruses on Expression of Resistance C. R. ~ilson~>">~ and R. A. C. ones^>^ A Department of Agriculture, Baron-Hay Court, South Perth, W.A. 6151. Department of Soil Science and Plant Nutrition, The University of Western Australia, Nedlands, W.A. 6009. Present address: New Town Research Laboratories, Department of Primary Industry and Fisheries, New Town, Tas. 7008. Corresponding author. Abstract A selection of potato cultivars and breeding lines was evaluated for presence of resistance to infection with potato leafroll virus (PLRV) via viruliferous aphid vectors ( I ~ ) and/or resistance to accumulation of PLRV antigen ( A ~ ) in infected leaf tissue. Cultivars Aracy, Delcora, Omega and Spunta, and breeding lines BR63.15 and B71.240.2 carried both and AR , Bismark, Serrana INTA and L/T1 had alone and Delaware had AR alone. The other cultivars tested had neither. Within both the resistant and susceptible classes for AR, the level of PLRV antigen accumulation achieved varied with cultivar. Previous infection with potato virus X (PVX) or potato virus S (PVS) either alone or together did not diminish the expression of or AR. However, the presence of PVX sometimes significantly increased the accumulation of PLRV in susceptible cv. Desiree and this effect was most pronounced in mature leaves of older plants. In contrast, presence of PVX in susceptible cv. Desiree did not increase the numbers of plants becoming infected with PLRV. Identifying potato genotypes with and AR will help the Australian potato industry to select cultivars which become infected with PLRV more slowly under conditions of high infection pressure and/or are suitable for use as parental lines in breeding virus-resistant cultivars. Keywords: viruses, potato, resistance, PLRV, leaf roll, aphid vectors, virus accumulation, co-infection, virus interactions. Introduction Potato leaf roll virus (PLRV) seriously diminishes potato tuber yields and is the mast important virus disease of the potato crop worldwide. Control measures normally involve certification schemes to produce healthy seed potato stocks and use of appropriate crop management strategies, e.g. application of insecticides to kill its aphid vectors (Jones et al. 1982; de Bokx and van der Want 1987). However, growing PLRV-resistant potato cultivars is also possible. Two main components of virus specific resistance to PLRV have been studied, resistance to infection (IR) by viruliferous aphid vectors (Cockerham 1945; Ross 1958) and resistance to virus accumulation (AR) in infected plants (Joces 1979; Barker and Harrison 1985,1986). Both operate independently (Barker l989a). IR is polygenically inherited (Ross 1958; Davidson 1973, 1980) and overcome if inoculum dosage is sufficient, while AR, which decreases but does not prevent