Bean common mosaic necrosis virus (BCMNV) was isolated from Centrosema pubescens, Crotalaria incana, Lablab purpureus, Phaseolus lunatus, Senna bicapsularis, S. sophera, Vigna vexillata and an unidentified Crotalaria species growing in Uganda. Thirteen distinct isolates were characterized using symptoms, pathogenicity in differential bean cultivars, serology, immunosorbent electron microscopy, and seed and aphid transmission. Some isolates conformed with the characteristics of previously described strains of BCMNV but others showed novel properties. All isolates reinfected the natural host from which they were obtained. The origin and ecological significance of these isolates is discussed.

SUMMARY In host‐range studies, bean common mosaic virus strains (BCMV‐NL1, ‐NL3 and ‐NY 15) usually induced distinct systemic symptoms in susceptible bean cultivars and latent infection in several Vigna genotypes (except NY15 which gave mosaic symptoms in the latter), while blackeye cowpea mosaic virus (B1CMV‐W) caused distinct systemic symptoms in several Vigna genotypes and only weak systemic symptoms in a few bean genotypes only. Biologically, B1CMV‐W was closest to BCMV‐NY15 and less close to ‐NL1. When using antisera to the three BCMV strains and five strains of B1CMV (including a strain originally considered cowpea aphid‐borne mosaic virus CAMV‐Mor) in SDS‐immunodiffusion and ELISA, BCMV‐NL1 and ‐NY15 were found to be closely related to each other and to BICMV‐Fla, ‐NR and ‐W, and less closely to BICMV‐Ind and ‐Mor. Serological relationships of BCMV‐NL1 and ‐NY15 to BCMV‐ NL3 were more distant, which is in line with the biological distinction of NL3 in causing temperature‐independent necrosis in bean cultivars with the necrosis gene I. PAGE analysis of coat proteins revealed that the three strains of BCMV and B1 CMV‐W have similar but non‐identical molecular masses. Although molecular hybridisation may further elucidate quantitative relationships between potyvir‐uses, variation within and among the potyviruses may continue to pose problems in their classification and identification.

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A virus‐like disease of apple was observed for the first time in Canada in 1974 in a three year old planting. The disease was characterised by stem pitting, necrosis and breaking or separation of scion/rootstock at the graft union. Foliage was sparse and leaves were chlorotic and diffusely mottled. Using both frozen and freshly harvested leaves of infected apple trees from rootstock suckers as inoculum, a virus was transmitted to herbaceous indicator hosts. Based on host range symptoms, serology and electron microscopy, this virus was identified as tobacco ringspot virus. Initial counts of nematodes in the rhizophere of apple trees affected with union necrosis showed high densities of Xiphinema americanum Cobb., a known vector of nepoviruses. Reports of similar symptoms have indicated that the disease, apple union necrosis and decline was associated with the recovery of tobacco ringspot virus from infected rootstocks. This is the first report associating a union incompatibility condition of apple and tobacco ringspot virus.