THE NATURAL OCCURRENCE, TRANSMISSION, PROPERTIES AND POSSIBLE AFFINITIES OF COWPEA MILD MOTTLE VIRUS

Cowpea mild mottle virus (CMMV), although thought to be of only local importance when first found in 1973 infecting cowpeas (Vigna unguiculata) in Ghana, has since been shown to have a very extensive geographical distribution and a wide natural host range. It occurs in Africa, Asia, South America and Oceania in tomato (Lycopersicon esculentum) and a range of leguminous crops including cowpeas, groundnuts (Arachis hypogaea), soybean (Glycine max) and French beans (Phaseolus vulgaris). The virus has physico-chemical properties resembling those of aphid-borne carlaviruses; it has filamentous particles ca. 650 nm long which contain a single polypeptide of 31-33 KDa and single-stranded RNA of 2.5 X 10(6). Unlike carlaviruses, however, CMMV is transmitted in a non-persistent manner by whiteflies (Bemisia tabaci) and the particles occur in vivo in characteristic brush-like inclusions. It is also seed-borne in some, but not all, cultivars of cowpea and soybean, but seed-transmission is probably dependent upon the interaction of genotype, virus strain, time of infection and environmental factors. CMMV is serologically closely related to, and thus probably synonymous with, viruses previously designated groundnut crinkle, Psophocarpus necrotic mosaic, Voandzeia mosaic and tomato pale chlorosis. It is, however, serologically unrelated to 18 recognized carlaviruses; because it is also transmitted by whiteflies and induces the formation of unusual brush-like inclusions, it is probably best left unclassified or tentatively placed in a sub-group of the carlavirus group until the taxonomic significance of these features has been fully evaluated.