Comparison of Silverleaf Whitefly-Induced and Chlormequat Chloride-Induced Leaf Silvering inCucurbita pepo

Abstract

Whitefly-mediated squash silverleaf is induced by the feeding of nymphs of the silverleaf whitefly, Bemisia argentifolii, but the mechanism involved in symptom expression is unknown. Several plant biochemical regulators were tested to determine whether they could mitigate expression of squash silverleaf in Cucurbita pepo. Application of chlormequat chloride, a gibberellic acid biosynthesis inhibitor, induced leaf silvering symptoms similar to those induced by the silverleaf whitefly in squash plants, cvs. Senator and Dixie, but not in cv. Small Sugar Pumpkin. Chlormequat chloride-induced silvering never extended over more than 80% of the upper leaf surface, compared with 100% for whitefly-induced silverleaf. Paclobutrazol, but not Alar, also produced leaf silvering symptoms. Chlormequat chloride treatment also resulted in internodal 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. pepo cultivars tested. Silvered leaves from whitefly-infested plants, in contrast, had 15 to 40% lower chlorophyll levels, two induced intercellular leaf proteins, and some reductions in foliar and root biomass. No dsRNAs were detected in whitefly-silvered tissues or plants. Gibberellic acid applied after chlormequat chloride or paclobutrazol treatment resulted in less internodal shortening and decreased both chemical- and whitefly-induced leaf silvering. Chlormequat chloride-treated plants attracted greater whitefly oviposition and had more nymphs than untreated controls in greenhouse tests. These data suggest that leaf silvering induced by gibberellic acid biosynthesis inhibitors and by the silverleaf whitefly results from hormonally mediated alterations in the plant's physiology. Because the silverleaf whitefly also induces discoloration disorders in plants other than squash, hormonal inhibition may be a general component of whitefly phytotoxicity.