Identification of a severe strain of bean common mosaic virus isolated
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A severe mosaic inducing virus, associated with a disease of Phaseolus vulqaris L. 'Canadian Wonder', in experimental plots, at the Field Station of the Faculty of Agriculture, University of Nairobi, was identified on the basis of host range and symptomatology, reaction in differential bean cultivars, seed and aphid-transmission, physical properties, particle morphology and serology as a severe strain of bean common mosaic virus. Field' symptoms were mosaic, downward curling, distortion and stunting. Plants of 'Long Tom' bean were readily infected following mechanical inoculation with crude sap from naturally infected field beans, and produced severe symptoms. When several bean cultivars, generally used as bean differentials were mechanically inoculated with crude sap extracts from infected 'Long Tom' beans, symptoms ranged from chlorotic spots, vein-banding, mottling to severe mosaic on the non-inoculated leaves. Plants of the cultivars 'Pinto UI 111', 'Great Northern UI 59', 'Red Mexican UI 35' and 'Monroe' reacted with the production of necrotic local lesions followed by veinal necrosis on the -xvinoculated leaves. Plants of 'Top crop' developed lethal top necrosis with eventual death, 7 days after inoculation. Of the cultivars tested, 'Amanda', 'Great Northern UI 123', 'Great Northern UI 1140', 'Immuna', 'Kentucky Wonder', 'Mexican 142', 'Monel', 'Porillo', and 'Redland green leaf B' were immune. A comparison of reaction of bean differentials reported in the literature to 7 strains of bean common mosaic virus, including the East African strain and the Tanzanian strain (T-BCMV), indicated that the virus isolated from experimental field beans is a distinct strain. The vir~s was limited in host range to the families Leguminosae and Chenopodiaceae. Plants of Canavalia e~siformis L. D.C. exhibited severe stunting, mottling, downward curling and reduction in leaf size. Cyamopsis tetragonoloba L. Taub, reacted with vein-clearing and a mild mosaic. In the family Chenopodiaceae, the virus induced chlorotic local lesions on ChenopOdium amaranticolor, Coste & Reyn and C~ guinoa Willd. The virus particles were flexous fila~entous rods approximately 755 nm long. It was seedborne in plants of 'Long Tom' (24%), 'Mwezi \ Moja' (19%) and 'Canadian Wonder' (25%), and was successively transmitted non-persistently by -xviaphids (Myzus persicae L. Sulz.). The virus was In~ctivated at 60 C but not at 58 C heat exposure for 10 min.; had a dilution end-point of 10-3 but not 10-4; and the crude sap was still infectious after 36 hr. and not 48 hr. A partia}ly purified virus preparation, obtained by using 0.5 M sodium citrate buffer pH 8.0 in 0.5% sodium sulphite, and clarified with n-butanol-chloroform mixture (1:1), gave a light scattering zone containing infectious particles in a rate zonal gradient centrifugation. Ultraviolet absorption of the virus preparation indicated that the virus contained approximately 1% nucleic acid. The virus was serologically related to the East African strain of BCMV (B18) and the Egyptian isolate of BYMV. The antiserum made against it had an homologous titre of 1/1024. On the basis of these criteria., the virus ~solated from the bean cultivar 'Canadian Wonder' is considered a new strain of common bean mosaic virus. It is designated as Kenya Strain of bean common mosaic virus (K-BCMV).