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Black Pod Syndrome of Lupinus angustifolius Is Caused by Late Infection with Bean yellow mosaic virus

Black pod syndrome (BPS) causes devastating losses in Lupinus angustifolius (narrow-leafed lupin) crops in Australia, and infection with Bean yellow mosaic virus (BYMV) was suggested as a possible cause. In 2011, an end-of-growing-season survey in which L. angustifolius plants with BPS were collected from six locations in southwestern Australia was done. Tissue samples from different positions on each of these symptomatic plants were tested for BYMV and generic potyvirus by enzyme-linked immunosorbent assay and reverse-transcription polymerase chain reaction (RT-PCR). Detection was most reliable when RT-PCR with generic potyvirus primers was used on tissue taken from the main stem of the plant just below the black pods. Partial coat protein nucleotide sequences from eight isolates from BPS-symptomatic L. angustifolius plants all belonged to the BYMV general phylogenetic group. An initial glasshouse experiment revealed that mechanical inoculation of L. angustifolius plants with BYMV after pods had formed caused pods to turn black. This did not occur when the plants were inoculated before this growth stage (at first flowering) because BYMV infection caused plant death. A subsequent experiment in which plants were inoculated at eight different growth stages confirmed that BPS was only induced when L. angustifolius plants were inoculated after first flowering, when pods had formed. Thus, BYMV was isolated from symptomatic L. angustifolius survey samples, inoculated to and maintained in culture hosts, inoculated to healthy L. angustifolius test plants inducing BPS, and then successfully reisolated from them. As such, Koch's postulates were fulfilled for the hypothesis that late infection with BYMV causes BPS in L. angustifolius plants.