Relationships Between the Sweetpotato Whitefly and the Squash Silverleaf Disorder

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Abstract
Squash silverleaf (SSL), a newly recognized disorder of many squash varieties in Florida (USA), is of unknown etiology. Extracts from SSL-affected tissue did not react with antisera to several cucurbit and whitefly-transmitted viruses. Silverleaf was not graft or mechanically transmitted. Early SSL symptoms were induced in new foliar growth of squash (Cucurbita pepo ''Senator'') after 3 days of feeding by nymphs of the sweetpotato whitefly (SPWF), Bermisia tabaci (average number = 25.4 nymphs per plant). The SPWF nymphs produced chlorotic spots on leaves at their feeding sites, which were clearly different from SSL symptoms. Silverleaf symptoms were always produced on foliage that developed after exposure to nymphs and increased in severity as long as the nymphs fed, indicating that the causal factor was translocated. New growth from SSL-affected plants was symptomless when nymphs were eliminated either by removing infested leaves or by insecticide treatment. Colony nymphs reared on five other host plant species all induced SSL symptoms on test squash plants. Adult SPWFs (up to 200 per plant), however, did not induce SSL symptoms even after 15 days of continuous feeding. Double-stranded RNA (dsRNA) bands of approximately 4.0 and 4.4 .times. 106 daltons were detected by polyacrylamide gel electrophoresis from nymph-infested leaves of SSL-affected plants; dsRNA was, however, not detected from symptomatic tissue free from SPWF contamination, or from cotyledon tissue or healthy controls. Identical size dsRNA bands were detected from both nymph and adult B. tabaci. The relationship between the SPWF and SSL suggested that a toxicogenic factor associated with nymphal feeding may be causing SSL. The role of the dsRNA in SSL etiology, if any, is not yet understood.