Morphological Responses of Strawberry Leaves to Infestations of Twospotted Spider Mite1

Abstract

Injury to strawberry leaves caused by various infestation levels of Tetranychus urticae Koch was studied. Reduction in leaflet chlorophyll content by twospotted spider mite feeding was more evident on the lower leaflet surfaces where mites fed. Chlorophyll content of upper leaflet surfaces was not significantly reduced by mite feeding from the lower surface. On the lower surface, infestations of 24.4 and 46.1 mite-days/cm2 significantly reduced chlorophyll content compared to the mite-free control. Percent leaflet area scarred by mite feeding was correlated with increasing mite infestation levels. Histological observations indicated that mite feeding injury at 4.8 mite-days/cm2 was restricted to localized areas of the spongy mesophyll and lower palisade mesophyll layers. Injury was more extensive at the higher densities and resulted in destruction of the spongy mesophyll and substantial injury to the lower palisade layer. Significant differences in stomatal opening occurred between mite-infested and mite-free leaflets with the majority (61%) of the stomata closed in infested tissues. The mechanisms of mite injury and plant responses are discussed.