<p style="text-align: justify;">The modified yellow crookneck squash (<i>Curcurbita pepo</i> L.), line ZW-20 was developed using recombinant DNA techniques to resist infection by two plant viruses that frequently infect commercial squash varieties, namely zucchini yellow mosaic potyvirus (ZYMV) and watermelon mosaic potyvirus (WMV2). This novel variety was developed by insertion of the coat protein (CP) encoding sequences from ZYMV and WMV2 into the plant genome. The expression of these introduced gene sequences does not result in the formation of infectious virus particles, nor does it result in any disease pathology, but rather enables ZW-20 squash plants to resist infection by these two viruses.</p>

<p style="text-align: justify;">ZYMV and WMV2 are members of the potyvirus group, which is one of the largest groups of plant viruses. ZYMV produces a severe disease consisting of mosaic (patchwork of yellow chlorotic tissues and green uninfected tissues), yellowing, shoestringing, stunting, and fruit and seed deformations on cucurbits, such as zucchini squash, muskmelon, cucumber, and watermelon. WMV2 causes mosaic and mottle diseases of cantaloupe, pumpkin, squash, and watermelon. On a given cucurbit host, ZYMV usually causes more severe symptoms than WMV2.</p>

<p style="text-align: justify;">The squash line ZW-20 exhibits resistance to infection and subsequent disease caused by ZYMV and WMV2 through a process that is related to viral cross-protection. Although the exact mechanism by which the viral protection occurs is unknown, most evidence suggests that expression of viral CP by a plant interferes with one of the first steps in viral replication, uncoating (removal of CP) from the incoming virus (Register &amp; Nelson, 1992). For example, when ZYMV or WMV2 infect a ZW-20 plant or another cross-protected squash plant, replication is blocked. Other modes of action of cross protection have also been suggested (Matthews, 1991).</p>
