PARTIAL CHARACTERIZATION OF A DISTINCT POTYVIRUS ISOLATED FROM WATERMELON IN FLORIDA

Conspicuous, unusual nuclear inclusions in stained epidermal strips of leaves implicated a virus (designated isolate 2932) as the cause of f oliar mosaic in a watermelon plant (Citrullus lanatus) received for an alysis from South Florida in 1990. In greenhouse tests, mechanically i noculated plants of Cucurbita pepo (Small Sugar pumpkin and Early Prol ific Straightneck squash) and watermelon (Crimson Sweet) developed mos aic or mottle symptoms. Isolate 2932 caused foliar symptoms in 16 cult ivars of Cucurbita pepo, including Freedom II and Prelude II, and in s ix cultivars of watermelon. None of five cultivars of melon (Cucumis m elo) or 11 cultivars of cucumber (Cucumis sativus) developed consisten t, distinctive symptoms, but all of these cultivars were systemically infected based on back-inoculations to squash. No systemic infection o f mechanically inoculated plants of 25 species representing 13 noncucu rbitaceous plant families was detected. Crystalline nuclear inclusions , cytoplasmic amorphous inclusions, and cytoplasmic cylindrical inclus ions were detected by light and electron microscopy in leaf tissues of infected squash and watermelon. Electron microscopy of squash leaf ex tracts revealed filamentous particles, and 86% of 159 particles measur ed ranged from 800 to 890 nm in length. The virus was transmitted in a nonpersistent manner by Myzus persicae from squash to squash in two o f three trials. Immunodiffusion tests with polyclonal antisera prepare d to partially purified 2932 or its capsid protein showed that the iso late was antigenically different from papaya ringspot virus type W, wa termelon mosaic virus 2, and zucchini yellow mosaic virus. In limited testing of field samples of squash and watermelon since 1990, no addit ional isolates of the 2932 type have been found. The characteristics o f isolate 2932 obtained thus far indicate that it is a distinct potyvi rus. It is tentatively named watermelon leaf mottle virus to distingui sh it from other potyviruses commonly isolated from cucurbits in Flori da.