Department of Plant Pathology, Cornell University, NYSAES, Geneva, NY 14456, USA

Background and objectives
A decade has passed since the first report on the use of pathogen-derived resistance to develop virus-resistant transgenic plants. Since then, numerous reports have shown that pathogen-derived resistance is a potentially effective way to control many viral diseases. Recently, transgenic cucurbits and papaya that are resistant to potyviruses have been deregulated in the USA and licences have been obtained for their commercial use. This talk focuses on our evaluation of these transgenic crops in the field and an assessment of their practical impact towards controlling the effects of the target viruses.

Results and discussion
The Freedom II squash, which contains the coat protein genes of watermelon mosaic virus 2 (WMV 2) and zucchini yellow mosaic virus (ZYMV), was developed by the Asgrow Seed Company. Our field trials over several years showed that it was highly resistant to both viruses under conditions where high disease pressure resulted in all non-transgenic plants becoming infected 35 days after planting. Transgenic plants did not show systemic infection of WMV 2 and ZYMV. Under these conditions, fruits from control plants were discoloured and malformed, while fruits from Freedom II were symptomless. Another squash cultivar with both potyvirus genes and the cucumber mosaic virus coat protein gene was also tested under field conditions. This line was highly resistant to all three viruses. Field tests with melons containing these three viral genes also showed that transgenic plants had high resistance to these viruses. Besides resistance, additional benefits of the pathogen-derived resistance approach became evident by the relatively short time it took to develop a horticulturally acceptable virus resistant crop and the relative ease by which the 'resistant' coat protein genes could be transferred to other crops.

The most important disease of the papaya fruit crop is caused by papaya ringspot virus (PRSV). Transgenic papaya that express the coat protein gene of a PRSV strain from Hawaii were developed and a line, designated 55-1, was found to be resistant to Hawaiian PRSV isolates under greenhouse conditions. A field trial was established in Hawaii in 1992. Results showed that line 55-1 was highly resistant even after being exposed to virus for 2 years. Coincidentally, in 1992, PRSV invaded the papaya-growing area of Hawaii island where 95% of the State's papaya are grown. By late 1994, the virus was out of control and nearly half of the papaya acreage was severely infected. A large-scale field trial was started in the middle of the devastated area in October 1995 with the Rainbow hybrid that had been derived from line 55-1. The trial is being carried out using normal farmer operations and fruits are handled and graded by a commercial packing house. Rainbow has shown excellent resistance, plant growth, and fruit quality and production, while non-transgenic plants are totally infected and have almost no fruit production. The transgenic papaya is under the control of a grower organization. It has obtained the necessary licences to grow the papaya commercially in Hawaii. Seeds are projected to be distributed in mid-1998. This transgenic papaya may be the only hope for farmers to grow papaya economically on farms that have been devastated by PRSV.