Occurrence of Beet western yellows virus and its aphid vectors in over-summering broad-leafed weeds and volunteer crop plants in the grainbelt region of south-western Australia

During the summer periods of 2000, 2001, and 2002, presence of Beet western yellows virus (BWYV) was assessed in tests on samples from at least 12 broad-leafed weed species and 5 types of volunteer crop plants growing in the grainbelt region of south-western Australia. In 2000, BWYV was detected in 2 of 35 sites in 2% of 1437 samples, whereas in 2001 and 2002 the corresponding figures were 3 of 108 sites in 0.04% of 8782 samples, and 1 of 30 sites in 0.08% of 2524 samples, respectively. The sites with infection were in northern, central, and southern grainbelt districts, and in high and medium rainfall zones. The hosts in which BWYV was detected were the weeds Citrullus lanatus (Afghan or wild melon), Conzya spp. (fleabane), Navarretia squarrosa (stinkweed), and Solanum nigrum (blackberry nightshade), and the volunteer crop plant Brassica napus (canola). Small populations of aphids were found over-summering at 28% (2000), 4% (2001), and 17% (2002) of sites, mostly infesting volunteer canola and Raphanus raphanistrum (wild radish). They occurred in high, medium, and low rainfall zones, but were only found in central and southern grainbelt districts. The predominant aphid species found was Brevicoryne brassicae, with Acyrthosiphon pisum, Brachycaudus helichrysi, Hyperomyzus lactucae, Lipaphis erysimi, Myzus persicae, and Uroleucon sonchi present occasionally. The importance of these findings in relation to the epidemiology and control of BWYV in the grainbelt is discussed.