Hypovirulence of Didymella bryoniae Associated with dsRNA

Didymella bryoniae, isolate 98–18, recovered from watermelon seedlings with symptoms of gummy stem blight, showed abnormal growth, mycelial lysis, sectoring, barrage and limited production of fruiting bodies in culture. A dsRNA (approximately 6.5 kbp) was associated with isolate 98–18 and other isolates showing abnormal mycelial growth. Transfer of dsRNA from 98–18 to virulent isolates Tk659 and TK671 via hyphal anastamosis was achieved only by consecutive exposure to 98–18, but not to virulent isolates RJ1 and RJ2. Transfer of dsRNA via infiltration through growth media to virulent isolates RJ1, RJ2, TK659 and TK671 was unsuccessful. The disease severity index of isolate 98–18 was reduced 53% on watermelon, 32% on cataloupe and 26% on yellow squash compared with isolate RJ2, suggesting that the presence of the dsRNA is associated with reduced virulence of D. bryoniae isolate 98–18. Zucchini and yellow squash were resistant to both isolates when tested. Didymella bryoniae (syn. Mycosphaerella melonis and its anamorph Phoma cucurbitacearum), the causal agent of gummy stem blight disease can affect stems, leaves and fruits of all cucurbits (Zitter et al., 1996). An isolate of D. bryoniae, 98–18, recovered from diseased watermelon plants, showed abnormal growth, including sectoring and lysis on V-8 agar amended with bactericidal compound, 30 lg/ml rose bengal and 100 lg/ml streptomycin sulphate. Lysis in mycelial mats persisted in subcultures in the growth medium with or without antibacterial compounds, suggesting that an infective agent may be involved. Similar phenotypic alterations in fungi are believed to be influenced by genetic and/or cytoplasmic determinants, including dsRNA (Hammar et al., 1989; Chen et al., 1994, and Zhang et al., 1993). The infective nature of dsRNA not only alters the fungi phenotypically but also affects the virulence of pathogenic fungi. Hypovirulent, dsRNA-containing fungal isolates were successfully used to reduce severity of several plant diseases (Zhang et al., 1993). The objectives of this study were to (i) identify and characterize the infective agent; (ii) transmit this agent through hyphal anastamosis and/or by extract absorption; and (iii) examine its effect on virulence of the pathogen.