Avaliação dos efeitos e transmissão de isolados de Bipolaris em sementes de Brachiaria brizantha cv. Marandu

Abstract

Brazil is the largest producer, consumer and exporter country of forage seeds in the world, but faces difficulties in the international market due to phytosanitary barriers. According to the literature there is little information on the sanitary quality of forage seeds produced and offered to farmers in Brazil. Therefore, the objective of this work was to evaluate the effects and transmission of two isolate of Bipolaris, which represent the most frequent population of this genus in seed lots which have been commercialized in Brasil. Effects of these isolates on germination and vigor of B. brizantha cv. Marandu and its seed transmission to plants were evaluated under controlled conditions. The two selected isolates of Bipolaris, LAPS808 and LAPS809, collected in different regions in Brazil were also characterized in parallel by morphological and molecular analysis with the purpose of determining their identities and pathogenicity in B. brizantha seeds at different levels of inoculum potential. The evaluations showed significant differences in behavior between the two isolates, being the temperature, electrical conductivity and fungal inoculum potential factors that influenced the performance of seeds infected by the isolates in focus. The germination rate, vigor (emergence speed index and electrical conductivity), plant weight were reduced at variable levels by the pathogens. The effects were proportional to the amount of initial inoculum of both isolates in seeds. Both isolates are seed transmitted at variable rates according to the inoculum potential in both cases. By this study it was possible to detect differences in agrressiveness between the two isolates and to verify that they are members of the species Bipolaris gossypina. Both isolates are transmitted from seed to plants at variable rates and cause increasing damage and proportional to increases in seed initial seed potential levels.