Developmental Anatomy of Zucchini Leaves with Squash Silverleaf Disorder caused by the Silverleaf Whitefly

Abstract

Squash silverleaf (SSL) is a physiological disorder of vegetables in the genus Cucurbita L. caused by feeding of nymphs of the silverleaf whitefly (SLW) (Bemisia argentifolii Bellows & Perring). SSL causes a silvering of the leaves and a blanching of fruit. Leaf silvering is caused by developmental separation of the upper epidermis and the palisade mesophyll layer resulting in additional air space and altered light reflection. The anatomical development of SSL was analyzed in young leaves of zucchini squash (Cucurbita pepo L.) using two susceptible genotypes (`Elite' and YSN347-PMR) and two tolerant genotypes (ZUC76-SLR and ZUC33-SLR/PMR). SLW nymphs were allowed to feed only on the mature leaves and the anatomy of the developing leaves was observed by light microscopy and transmission electron microscopy. Silvering began at the apex of young leaves and developed basipetally. The increased percentage of air space that resulted between the upper epidermis and palisade layers was the result of an increase in the duration of air space development in young, developing leaves. Chloroplasts in silvered tissue of mature leaves and in tissue of young leaves that later became silvered, were smaller and contained less starch than chloroplasts in tissue from noninfested plants. In contrast, development of genetic silvering, a condition not related to whitefly feeding, occurred throughout the entire leaf at one time, beginning as yellowed tissue in the axils of leaf veins then turning silver. Chloroplasts were normal in genetically silvered tissue of genotype YSN421-PMR. The SSL-tolerant genotype, ZUC76-SLR, did not show alteration in chloroplast structure or air space development when challenged with SLW; however, it had shorter and paler petioles, as did the susceptible genotypes. SSL symptoms were induced with spraying of gibberellic acid synthesis inhibitor chlormequat chloride in SSL-susceptible but not in SSL-tolerant genotypes. Reciprocal grafting between susceptible and tolerant plants showed that tolerance resides in the developing tissue and not the mature tissue on which the whiteflies feed.