Selected mild strains of Passion fruit woodiness virus (PWV) fail to protect pre-immunized vines in Brazil

The Passion fruit woodiness virus (PWV) is the most important virus affecting passion fruit (Passiflora edulis f. flavicarpa Deg.) crops in Brazil. The main purpose of this work was to select mild strains of PWV and to evaluate their protective effect against a severe strain of the virus. Three mild strains were selected from outstanding plants found in orchards severely affected by the virus (F-101, F-102 and F-103) and three others were obtained from blisters formed in passion fruit vine leaves showing mosaic (F-99, F-144 and F-145). The protective effect of the mild strains was evaluated for vines under greenhouse and field conditions. Plants pre-immunized with mild strains F-101, F-102 and F-144, in a greenhouse, had partial protection against the severe strain PWV-SP. In a first field experiment, all passion fruit vines pre-immunized with the six selected mild strains showed severe symptoms of the disease, approximately four months after the challenge inoculation with the PWV-SP strain. Results from a second field experiment, with vines pre-immunized with strains F-101 and F-144, followed by a quantitative evaluation of the mild strains in different leaves of the protected plants, indicated that breakdown in protection seems to be related to the low concentration and/or irregular distribution of the mild strains in leaves, which allows the existence of infection sites available for the establishment of the severe strain. Pre-immunization was not an appropriate alternative for the control of the passion fruit woodiness disease. O endurecimento dos frutos do maracujazeiro (Passiflora edulis f. flavicarpa Deg.), causado pelo Passion fruit woodiness virus (PWV) é a virose mais importante da cultura dessa frutífera no Brasil. Este trabalho teve por objetivo selecionar estirpes fracas do PWV e avaliar o seu efeito protetor para o controle do endurecimento dos frutos por premunização. Foram selecionadas três estirpes fracas a partir de plantas de elite em pomares afetados pela doença (F-101, F-102 e F-103) e três a partir de bolhas que aparecem em folhas de maracujazeiro com mosaico (F-99, F-144 e F-145). O efeito protetor das estirpes fracas foi avaliado em maracujazeiros, em casa de vegetação e em campo. Plantas premunizadas com as estirpes F-101, F-102 e F-144, em casa de vegetação, ficaram parcialmente protegidas contra a estirpe severa PWV-SP. Em um primeiro experimento em campo, todos os maracujazeiros premunizados com as seis estirpes fracas seleciona