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.

RESUMO -Neste trabalho avaliou-se o efeito da premunização com duas estirpes fracas do vírus-domosaico-do-mamoeiro -estirpe melancia (papaya ringspot virus -type W, PRSV-W), combinada com a tolerância das plantas, no controle do mosaico e na produtividade da abóbora Menina Brasileira. Testes realizados em casa de vegetação mostraram que as plantas dessa cultivar premunizadas com as estirpes fracas PRSV-W-1 e 2 ficaram totalmente protegidas contra a infecção por uma estirpe severa de Campinas. Em condições de campo avaliou-se comparativamente a produção de plantas premunizadas, infectadas com a estirpe severa de Campinas, e sadias e expostas à infecção natural (controle). A proteção foi medida com base na produção individual das plantas, cujos frutos foram classificados em comerciais e não-comerciais. As plantas premunizadas tiveram uma produção média de frutos comerciais (peso) 33% superior à daquelas naturalmente infectadas em campo. Quanto ao número de frutos comerciais, o aumento foi da ordem de 50%. A premunização combinada com a tolerância da abóbora Menina Brasileira permitiu um melhor controle do mosaico, com ganhos na produção de frutos comerciais.Termos para indexação: Cucurbita moschata, controle de doença. EFFECTS OF CROSS PROTECTION WITH MILD STRAINS OF PRSV-W ON MENINA BRASILEIRA SQUASHABSTRACT -Cross protection with mild strains combined with tolerance was evaluated for control of papaya ringspot virus -type W (PRSV-W) in squash cultivar Menina Brasileira. Tests carried out in the greenhouse showed that plants premunized with PRSV-W-1 and 2 mild strains were protected against infection with a severe strain of the virus. Protection in the field was evaluated comparing plants premunized with the mild strains, plants infected with severe strains and healthy plants exposed to natural infection (control). Protection index was obtained through yield of marketable and non marketable fruits harvested from individual plants. Average yield (weight) of marketable fruits from protected plants was approximately 33% higher than that of naturally infected plants in the field. Combination of mild strain protection and tolerance of Menina Brasileira provided a better disease control with a significant increase of fruit yield.

Pre-immunization with mild strains of Papaya ringspot virus -type W (PRWV-W) has allowed the mosaic disease to be controlled in different cucurbit species, with increases in marketable fruit yield. The objective of this study was to compare virus concentration, biomass and symptomatology of 'Caserta' zucchini squash, 'Menina Brasileira' long-neck squash and 'Crimson Sweet' watermelon plants infected by three mild strains and one severe strain of PRSV-W. Plants were inoculated at the cotyledonary stage, under greenhouse conditions, sampled at 7, 14, 21, 28 and 35 days after inoculation (DAI), and analyzed by PTA-ELISA. The severity of the symptoms was scored according to a scale from 1 to 5, and the fresh and dry biomass of the aerial part of the plants were evaluated at 40 DAI. Concentrations of the mild strains, based on absorbance values of the PTA-ELISA, were lower than the concentration of the severe strain for all species. The mild strains did not cause mosaic in infected plants of all species. Plants of zucchini squash and watermelon infected by the severe strain exhibited severe mosaic symptoms, but the same was not noticed for infected long-neck squash plants. Biomass values from zucchini squash and watermelon plants infected by the mild strains were 1.7 % to 12.4 % lower as compared to healthy plants. Biomass values of zucchini squash and watermelon plants infected by the severe strain presented greater reduction, varying from 29 % to 74 %. However, biomass values of long-neck squash plants infected by the mild and severe strains were similar for all treatments. Key words: PTA-ELISA, Cucurbita pepo, Cucurbita moschata, Citrullus lanatus BIOMASSA, CONCENTRAÇÃO VIRAL E SINTOMATOLOGIA DE CUCURBITÁCEAS INFECTADAS POR ESTIRPES FRACAS E SEVERA DO Papaya ringspot virusRESUMO: A premunização com estirpes fracas do Papaya ringspot virus -type W (PRSV-W) tem proporcionado o controle do mosaico em diferentes cucurbitáceas, com ganhos na produção de frutos comerciais. Neste trabalho foram comparadas as concentrações virais, severidade dos sintomas e biomassa das plantas de abobrinha-de-moita 'Caserta', de abóbora 'Menina Brasileira' e de melancia 'Crimson Sweet' infectadas com três estirpes fracas e uma estirpe severa do PRSV-W. As plantas foram inoculadas em estádio cotiledonar e mantidas em casa de vegetação. Aos 7, 14, 21, 28 e 35 dias após a inoculação (DAI), amostras foliares foram analisadas por PTA-ELISA para avaliar a concentração viral. A severidade dos sintomas foi avaliada por uma escala de notas de 1 a 5 e a biomassa da parte aérea das plantas foi determinada aos 40 DAI. As concentrações das estirpes fracas foram menores do que a da estirpe severa nas três espécies. As estirpes fracas não causaram sintomas de mosaico nas plantas das três espécies estudadas. A estirpe severa causou sintomas extremamente acentuados em abobrinha-de-moita e melancia e sintomas relativamente fracos em abóbora 'Menina Brasileira'. Os valores de biomassa das plantas de abobrinha-de-moita e melancia infectadas pelas e...

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