COMPARISON OF SILVERLEAF WHITEFLY-INDUCED AND CHLORMEQUAT CHLORIDE-INDUCED LEAF SILVERING IN CUCURBITA-PEPO

Whitefly-mediated squash silverleaf is induced by the feeding of nymph s of the silverleaf whitefly, Bemisia argentifolii, but the mechanism involved in symptom expression is unknown. Several plant biochemical r egulators were tested to determine whether they could mitigate express ion of squash silverleaf in Cucurbita pepo. Application of chlormequat chloride, a gibberellic acid biosynthesis inhibitor, induced leaf sil vering symptoms similar to those induced by the silverleaf whitefly in squash plants, cvs, Senator and Dixie, but not in cv. Small Sugar Pum pkin. Chlormequat chloride-induced silvering never extended over more than 80% of the upper leaf surface, compared with 100% for whitefly-in duced silverleaf. Paclobutrazol, but not Alar, also produced leaf silv ering symptoms. Chlormequat chloride treatment also resulted in intern odal stem shortening, increased chlorophyll levels, and increased root and stem weight in Small Sugar Pumpkin and Senator but not in Dixie. Chlormequat chloride did not produce detectable double-stranded RNAs ( dsRNAs) or changes in intercellular fluid proteins in any of the C. pe po cultivars tested. Silvered leaves from whitefly-infested plants, in contrast, had 15 to 40% lower chlorophyll levels, two induced interce llular leaf proteins, and some reductions in foliar and root biomass. No dsRNAs were detected in whitefly-silvered tissues or plants. Gibber ellic acid applied after chlormequat chloride of paclobutrazol treatme nt resulted in less internodal shortening and decreased both chemical- and whitefly-induced leaf silvering. Chlormequat chloride-treated pla nts attracted greater whitefly oviposition and had more nymphs than un treated controls in greenhouse tests. These data suggest that leaf sil vering induced by gibberellic acid biosynthesis inhibitors and by the silverleaf whitefly results from hormonally mediated alterations in th e plant's physiology. Because the silverleaf whitefly also induces dis coloration disorders in plants other than squash, hormonal inhibition may be a general component of whitefly phytotoxicity.