CHARACTERISTICS OF SWEET-POTATO WHITEFLY-MEDIATED SILVERLEAF SYNDROME AND ASSOCIATED DOUBLE-STRANDED-RNA IN SQUASH

A 48-h feeding by 20 adult sweetpotato whiteflies (SPWF), Bemisia tabacl, resulted in 100% (63/63) of the squash (Cucurbita pepo 'Dixie') plants showing vein clearing and/or leaf silvering. No vein clearing and/or leaf silvering was apparent in squash plants exposed to SPWF for 8 h or less. The ability to mediate silverleaf symptoms was correlated with the density of SPWF. The percentage of plants that exhibited leaf silvering after 48-h feeding by 2, 5, 10, and 20 SPWF were 0, 13, 84, and 100%, respectively. Crude extracts from symptomatic squash leaves increased one hundredfold in RNA-dependent RNA polymerase activity when compared with extracts from asymptomatic, healthy tissue. Enzyme activity was dependent on Mg21 and was insensitive to actinomycin D. Fractions of extracts with RNA polymerase activity also had distinctive 4.2- and 4.6-kb double-stranded RNA (dsRNA). Following glycerol density gradient separation of leaf extracts, dsRNA was found in fractions rich in membranes and nuclei but not in those containing chloroplasts. There were positive correlations between SPWF density, silverleaf rating, and RNA accumulation in symptomatic tissue. The dsRNA associated with silverleaf appears to be translocatable in the plant with limited synthesis in the host plant in the absence of SPWF. The presence of SPWF is necessary for continued silverleaf development. These results are consistent with the suggestion that the causal agent of silverleaf syndrome may be a virus or viruslike agent of SPWF.