Resistance to phloem transport of potato leafroll virus in potato plants

A 'double-graft sandwich' technique in which sections of potato stem from different potato cultivars were grafted between a susceptible healthy stock plant and a potato leafroll virus (PLRV)-infected scion was used to study the rate of phloem transport of PLRV in cultivars differing in resistance to PLRV infection (I(R)) and accumulation (A(R)). Resistance to phloem transport (i.e. delayed PLRV systemic movement) was found in Bismark cultivar (I(R) A(S)). This was independent of I(R) and A(R) as the rate of movement in Bismark cultivar was markedly slower than that in Omega and Spunta (I(R) A(R)), Delaware (I(S) A(R)), and Desiree and Renova (I(S) A(S)) cultivars. It operated in Bismark cultivar stems of two different ages, but did not operate against potato virus X (PVX) and was not influenced by previous infection with this virus. Aphid vector (Myzus persicae) feeding preferences and colonization rates differed between cultivars, but the cultivar characteristics responsible were unrelated to I(R), A(R) or resistance to phloem transport. Delayed systemic movement of PLRV out of leaves inoculated with viruliferous aphids was independent of A(R) and resistance to phloem transport, and remained unaffected by previous infection with PVX. It was also independent of cultivar factors causing different aphid feeding preferences and colonization rates, but may be linked to I(R).