The Viruses Causing Mosaic of Cowpeas and Azuki Beans, and Their Transmissibility through Seeds

Abstract

Three kinds of viruses were isolated from mosaic diseased plants of cowpeas (Vigna catjang and V. sesquipedalis). One of them is a seed-borne virus in cowpea plants, which is transmitted by sap and by at least three species of aphids. Its host range is restricted to several genera of Leguminosae, and Chenopodium. Infectivity of the crude sap was lost by heating at 50-60°C for 10 minutes, and by aging at room temperature for 5 to 6 days, and by diluting at 1:104-1:105. The electron microscopy of virus preparations by dip method always revealed a small amount of elongated virus particles of about 750mμ in length. Thus, according to Anderson's classification scheme (1955), the virus in question was considered to be a member of the Group 2 among Vigna and Crotalaria viruses. To avoid the present confusion in virus names of the Group 2, the authors proposed to use a name of cowpea aphid-borne mosaic virus (CAMV) (Lovisolo and Conti, 1966) for all members of the Group 2. The remaining two viruses isolated from cowpea plants were cucumber mosaic virus (CMV) and subclover mottle virus. The latter is considered to be a virus belonging to broad bean wilt virus group. Neither of these two viruses is transmitted through seeds of cowpea. More than fifty collections of cowpea plants showing mosaic symptoms from commercial fields in Kanto district, were tested for identification of the causal virus by sap inoculations to a set of differential hosts. The results showed that CAMV was found predominant. Viruses causing mosaic of azuki bean (Phaseolus angularis) were also investigated. Azuki bean mosaic virus (AzMV), CMV, and CAMV were isolated from azuki bean plants showing mosaic symptoms. AzMV is a seed-borne virus in azuki bean, Whereas both CMV and CAMV are not transmitted through seeds of azuki bean. Host range of AzMV is restricted to leguminous plants. The virus in the crude sap is inactivated by heating at 55-60°C for 10 minutes, and by aging at room temperature for 2 to 7 days. The dilution end point lies between 1:104 and 1:105. AzMV preparations examined under the electron microscope by dip method always showed a small amount of elongated virus particles of about 750mμ in length. AzMV seems to be closely related to CAMV, because the former virus protects azuki bean plants from infection by the latter virus. Fifty collections of mosaic plants of azuki bean from commercial plantings in Kanto district, and identification of the causal virus involved was undertaken by sap inoculations to a set of differential hosts. The results showed that the percentages of infection by CMV, CAMV, and AzMV were 55, 26, and 32%, respectively. It was also noted that some of azuki bean plants were infected simultaneously with CMV and either of CAMV and AzMV.