Biology and genetic diversity of phasey bean mild yellows virus, a common virus in legumes in Australia

This study examined the natural and experimental host range and aphid and graft transmission of the tentative polerovirus phasey bean mild yellows virus (PBMYV). Eleven complete coding sequences from PBMYV isolates were determined from a range of hosts and locations. We found two genetically distinct variants of PBMYV. PBMYV-1 was the originally described variant, and PBMYV-2 had a large putative recombination in open reading frame 5 such that PBMYV-1 and PBMYV-2 shared only 65-66% amino acid sequence identity in the P5 protein. The virus was transmitted by a clonal colony of cowpea aphids (Aphis craccivora) and by grafting with infected scions but was not transmitted by a clonal colony of green peach aphids (Myzus persicae). PBMYV was found in natural infections in 11 host species with a range of symptoms and severity, including seven important grain legume crops from across a wide geographic area in Australia. PBMYV was common and widespread in the tropical weed phasey bean (Macroptilium lathyroides), but it is likely that there are other major alternative hosts for the virus in temperate regions of Australia. The experimental host range of PBMYV included the Fabaceae hosts chickpea (Cicer arietinum), faba bean (Vicia faba), pea (Pisum sativum), and phasey bean, but transmissions failed to infect several other members of the families Asteraceae, Cucurbitaceae, Fabaceae and Solanaceae. PBMYV was commonly found in grain legume crops in eastern and western Australia, sometimes at greater than 90% incidence. This new knowledge about PBMYV warrants further assessments of its economic impact on important grain legume crops.