RESISTANCE OF TRANSGENIC HYBRID SQUASH ZW-20 EXPRESSING THE COAT PROTEIN GENES OF ZUCCHINI YELLOW MOSAIC-VIRUS AND WATERMELON MOSAIC-VIRUS-2 TO MIXED INFECTIONS BY BOTH POTYVIRUSES

The resistance of three transgenic yellow crookneck squash lines expressing the zucchini yellow mosaic virus (ZYMV) and/or watermelon mosaic virus 2 (WMV 2) coat protein (CP) genes was investigated under field conditions. Resistance was evaluated under high disease pressure achieved by mechanical inoculations of ZYMV and WMV 2 and natural challenge inoculations by aphid vectors. Assessment of infection rates was based on visual monitoring of symptom development, enzyme-linked immunosorbent assays (ELISA), infectivity assays, and analysis of virus occurrence in fruits. Horticultural performance was evaluated by counting the number of mature fruits per plant and estimating their fresh fruit weight. The transgenic line ZW-20B expressing both the ZYMV and WMV 2 CP genes showed excellent resistance in that none of the plants developed severe foliar symptoms, although localized chlorotic dots or blotches appeared on some leaves. In contrast, the two transgenic lines expressing only single CP genes, either the ZYMV or WMV 2 CP gene, developed severe symptoms by the end of the trial period, as did the nontransformed control lines. Differences between transgenic squash lines were even more dramatic in fruit quality: All transgenic ZW-20B fruits were symptomless, while all fruits from the two transgenic lines with single CP genes showed symptoms, and thus were unmarketable. ZW-20B is a hybrid of commercial quality, and its high level of resistance to ZYMV and WMV 2 should enable the production of yellow summer squash in areas where both potyviruses are prevalent. This transgenic yellow crookneck squash line is especially valuable in light of the difficulties of obtaining similar resistance by conventional breeding strategies.