A disease of French beans and soybean caused by Cowpea mild mottle virus in Queensland

In March 2016, a disease causing pod distortion, discolouration and leaf mottling in fresh market beans (Phaseolus vulgaris) occurred in the Fassifern area of south-east Queensland. Disease incidence in crops which are harvested nine to ten weeks after planting was frequently 60% to 100%, resulting in losses approaching $400,000 over a period of only two months. In April 2016, samples of soybean (Glycine max) cv. Zam-1 displaying virus-like symptoms of distorted leaves and mottling were collected from a crop near Gatton, approximately 50 Km from the infected bean crops. Disease incidence in this crop was 5-10%. Symptomatic plants from bean and soybean crops had slightly flexuous filamentous particles. Samples were negative for potyvirus when tested using a potyvirus group ImmunoStrip test and in RT–PCR using degenerate primers Poty 1 and U341. These samples were positive for Carlavirus using Carlavirus group specific primers and for Cowpea mild mottle virus using CPMMV –specific primers. Phylogenetic analysis of sequences obtained from PCR products confirmed the identity of CPMMV from bean and soybean with the closest matches over the region obtained being 85% nt and 76% nt identity to CPMMV isolates from Brazil and India respectively. The isolates from bean and soybean were transmitted by manual inoculation to several cultivars of bean and soybean which developed symptoms very similar to those seen in the original samples. Both virus isolates were transmitted by Bemisia tabaci (MEAM1) following exposure of young healthy bean and soybean plants to whitefly previously allowed access to bean plants infected with these isolates. CPMMV has been detected in bean crops in the Fassifern area in 2017, but at much lower levels than in 2016. The detection of CPMMV in 2016 was the first detection of this virus in Australia and of Carlavirus infection of Fabaceae species in Australia.