Malvastrum bright yellow mosaic virus is transmitted by Bemisia tabaci MEAM1 and infects cotton in addition to Malvastrum

In 2016, a novel bipartite begomovirus, malvastrum bright yellow mosaic virus (MaBYMV), was discovered from Malvastrum sp., a weed that is widespread in South Texas. To demonstrate Koch’s postulate for the virus and investigate its host range, virus-free Bemisia tabaci MEAM1 adults were introduced into insect-proof cages with MaBYMV-positive Malvastrum plants for an acquisition access period of 48-h. Subsequently, 50 -100 potentially viruliferous whiteflies were released into separate cages containing 10 plants each of healthy Malvastrum and cotton. The experiment was replicated three times. After 48-h of inoculation access period (IAP), the cages were treated with a mixture of contact and systemic insecticides. The test plants were maintained in insect-proof cages and monitored daily for symptoms development. They were tested individually at 7, 14, and 21-days post-IAP for MaBYMV using newly designed detection primers. Symptoms induced by MaBYMV appeared in 50% of Malvastrum plants in two weeks post-IAP, while 50% of the cotton plants became symptomatic a week later. PCR assays resulted in the amplification of the expected 233-bp MaBYMV-specific fragment in 80% and 70% of the test Malvastrum and cotton plants, respectively. The MaBYMV-specificity of the obtained amplicons were confirmed via cloning and Sanger sequencing. In addition to confirming the whitefly-transmissible nature of MaBYMV, the results demonstrated Koch’s postulate for the virus, and it revealed its pathogenicity on cotton. Consequently, Malvastrum, a widely prevalent weed, could serve as virus inoculum source for cotton, an important cash crop.