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Effect of protective mild strains of PRSV-W and ZYMV on the yield of four varieties of Cucurbita pepo Zucchini squash (Cucurbita pepo L.) is widely cultivated in Brazil, especially in the State of São Paulo, which is the major producer. Nine viruses capable to infect this species have already been described in Brazil. The most frequent and responsible for significant yield losses are the potyviruses Papaya ringspot virus type W (PRSV-W) and Zucchini yellow mosaic virus (ZYMV), for which zucchini squash is highly susceptible and intolerant. Preimmunization with mild strains of both viruses have been investigated in Brazil for the last 10 years and appears as and interesting option for the control of these viruses. As preimmunization have been evaluated only for zucchini squash cv. Caserta, the purpose of the present work was to investigate the effect of the mild strains PRSV-W-1 and ZYMV-M on the quantitative and qualitative yield of four other commercial zucchini squash varieties: Samira, Novita Plus, AF-2847 e Yasmin. The results of an experiment carried out under plastic-house showed that mild strain ZYMV-M induced mild symptoms on the leaves of the plants of all varieties, but did not affect the yield of marketable fruits, as compared to those from the respective healthy controls. On the other hand, plants from all varieties exhibited accentuated mosaic and leaf malformation when infected with mild strain PRSV-W-1, alone or in mixture with ZYMV-M. The amount of fruits harvested (number and average weight) from these plants was similar to that from the respective healthy controls. However, the quality of the fruits was severely affected, since the mild strain PRSV-W-1 induced alteration on the texture and color of the skin. When plants infected only with mild strain PRSV-W-1 were grown simultaneously under field and plastic-house conditions, it was noticed that leaf and fruit symptoms were more intense on those maintained in the plastic-house. Once more the quantitative fruit yield was not affected by this mild strain. Zucchini squash cv. Samira was the most sensitive to PRSV-W-1 under both conditions. Together theses results suggest that environmental variables, besides the interaction between the variety and the mild strain, might influence the expression of symptoms shown by plants infected with PRSV-W-1. In addition to this, the present work also tried to identify the most appropriate host, among some cucurbit species/varieties, for multiplication of both mild strains, and the efficiency of two species of aphids (Myzus nicotianae and M. persicae) on the transmission of the mild strain ZYMV-M. The mild strain ZYMV-M attained the highest concentration in C. pepo cv. Caserta and Cucumis melo cv. Casca-de-Carvalho. The most appropriate hosts for multiplication of mild strain PRSV-W-1 were C. pepo cv. Caserta followed by Citrullus lanatus cv. Crimson Sweet and C. sativus cv. Safira. M. nicotianae and M. persicae transmitted the mild strain ZYMV-M to 11.7% and 0% of the test-plants of zucchini squash cv. Caserta. The transmission of the severe strain of ZYMV by both species of aphids occurred for 47% and 80% of the test-plants, respectively.

Efeito de estirpes fracas do PRSV-W e do ZYMV sobre a produção de quatro variedades de >i/i<