Mechanisms of resistance to whitefly-induced squash silverleaf disorder in zucchini

Feeding of immature Bemisia argentifolii Bellows & Perring induces squash silverleaf disorder, an economically important physiological disorder of commercially grown Cucurbita species. Two zucchini squash breeding lines, ZUC76-SLR and ZUC33-SLR/PMR, which did not show silverleaf symptoms in field tests, were evaluated under greenhouse conditions. Tests designed to evaluate antixenosis, antibiosis, or tolerance to whiteflies as mechanisms of resistance to the disorder were conducted using 'Elite', a commercial zucchini variety susceptible to the disorder, as a control. No significant differences were found in oviposition preference by the whitefly for any of the 3 genotypes. Immature survival on ZUC33-SLR/PMR was significantly higher than on Elite and percentage female emergence from ZUC33-SLR/PMR was significantly higher than from ZUC76-SLR. Development of immature whiteflies was significantly different among the 3 genotypes tested only in the first trial conducted during the spring season. In this trial, time to 50% emergence was highest for ZUC33-SLR/PMR and lowest for Elite. In tolerance tests, ZUC76-SLR did not show silverleaf symptoms at any of the whitefly infestation levels tested (0, 40, 80, or 160 pairs). ZUC33-SLR/PMR showed mild silvering (maximal rating of 3 or less) only at infestation levels of 80 whitefly pairs or higher. Elite showed severe silvering (maximal silverleaf rating of 5) even at the lowest infestation level of 40 whitefly pairs. Number of silvered leaves and average silverleaf rating were significantly higher for Elite than for ZUC33-SLR/PMR at all infestation levels. Results from this study strongly suggest that tolerance to whitefly feeding is the major mechanism of resistance to the disorder in ZUC33-SLR/PMR and ZUC76-SLR.