Characterization and identification of two distinct strains of Watermelon Mosaic Virus-2 affecting cucurbits in Texas

Abstract

Two unrelated viruses, Watermelon Mosaic Virus-1 (WMV-1) and Watermelon Mosaic Virus-2 (WMV-2), cause the major virus diseases of cucurbits in Texas. WMV-1, with a host range restricted to cucurbits, was reported in Texas in 1979 (20). WMV-2 was reported in the Brazos Valley of Texas by Chala (8). Two virus isolates from Cucurbita pepo L. (squash) and Melothria pendula L. (wild cucumber), designated "S" and "M", respectively, each resembled WMV-2 (8,12,45). Both strains cause disease symptoms when mechanically inoculated onto commercially popular cultivars of watermelon (Citrullus lanatus (Thunb.) Mansf.), cantaloupe (Cucumis melo L.), cucumber (Cucumis sativus L.), squash (Cucurbita pepo L.), and pumpkin (Cucurbita pepo L.). Among currently used commercial cucurbit varieties there is no known resistance. Recently, a virus very similar to WMV-2 was isolated and identified as Zucchini Yellow Mosaic Virus (ZYMV) (25,40,42). Isolates "S" and "M" were purified, characterized, and differentiated from ZYMV based on host range and serological differences. Isolates "S" and "M" have modal lengths of 755 nm and 775 nm, buoyant densities of 1.34 g/cm^3 and 1.33 g/cm^3, A260/A280 ratios of 1.15 and 1.19, respectively, and protein capsid subunits of molecular weights 3.2 x 10^4 daltons. These characteristics are consistent with other members of the potyvirus group. Neither isolate caused symptomatic infections of the differential hosts garden pea, Pisum sativum L. (cultivars Alaska and Little Marvel), and tobacco Nicotiana benthamiana, thus distinguishing them both from type WMV-2 and ZYMV. Isolate "S" caused a systemic infection on bean, Phaseolus vulgaris L. (cultivar Black Turtle 2), whereas isolate "M" did not, thus distinguishing them from each other. When cross absorbed by ZYMV antigens, anti-S-serum and anti-M-serum reacted homologously to WMV-2, S, and M antigens. When cross absorbed against WMV-2 antigens the antisera failed to react with ZYMV antigens and only weakly to S and M antigens. Both isolates closely resemble WMV-2 and ZYMV. Current evidence indicates isolates "S" and "M" are isolates of WMV-2 as opposed to separate and distinct viruses or strains of ZYMV.