RESUMO
Two mild strains of papaya ringspot virus-type W (PRSV-W) were tested under greenhouse and field conditions to study the potential of cross-protection for control of zucchini squash mosaic. Protected plants of zucchini squash cultivars Caserta and Clarinda were challenged with three severe strains from different geographic regions of the country in tests carried out in the greenhouse. Challenge inoculations were done mechanically 10 and 20 days after the protective inoculation. The mild strains did not have a visible negative effect on the development of the plants and offered effective protection against the severe strains. Field tests of protected Caserta plants were carried out in Piracicaba County, São Paulo. Comparative evaluation based on the symptoms and development of protected and unprotected plants of zucchini squash showed that both mild strains effectively protected the plants against the effects of the severe strain present in the field. Yield of marketable fruits harvested from protected plants was only 10% less than that of the healthy plants in one field trial. Compared with the yield from plants infected with the severe strains, protected plants showed an increase of 511 and 633% in the number and weight of marketable fruits, respectively, in a test in 1994. In a second experiment in 1995, an increase of 327% in the number and 344% in the weight of marketable fruits was recorded. These studies demonstrate the effectiveness of cross-protection for the control of the mosaic disease caused by PRSV-W in zucchini squash and offer growers a method for large-scale application of this technology.