Bean common mosaic virus infecting manteiguinha cowpea (Vigna unguiculata): seed transmission, evaluation of yield loss, and genetic resistance
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The manteiguinha type cowpea has extra-small grains with a creamy coloration and constitutes an important component for the culinary in the state of Pará, Brazil, with potential for acceptance in the whole country. A high incidence of plants presenting symptoms of viral infection, such as mosaic, leaf rolling, leaf epinasty, and vein necrosis, was observed in a genetic improvement assay of this cowpea cultivar in the experimental field of Embrapa/Amazônia Oriental in Belém, PA. After negative results for detection of Cucumber mosaic virus, Cowpea aphid-borne mosaic virus, and Cowpea severe mosaic virus by ELISA, the potential agent of the disease was identified as bean common mosaic virus (BCMV). For that, an RT-PCR assay was conducted using primers MJ1 and MJ2, universal for potyviruses, followed by sequencing of the amplified fragment, part of the coat protein gene. The sequences of two isolates (Br:Para:1 – access MK431152 and Br:Para:2 – access MK431153) showed 99% nucleotide and 100% aa identity with BCMV isolates from Taiwan, Thailand, the USA, and China. Cytopathology revealed the presence of Edwardson’s type I cylindrical inclusions, typical of potyvirus infection, in the cytoplasm of symptomatic leaf cells. Assays performed to estimate seed transmission of BCMV-Br:Para:1 isolate by infected manteiguinha cowpea indicated a transmission rate ca. 35%. Yield loss evaluation assays of genotype TUC 170 of manteiguinha cowpea indicated a 53% reduction in grain production and 47% fewer pods/plant. Genotype TUC 178, registered as cultivar BRS Natalina, revealed to be immune and may serve in BCMV resistance breeding programs.