Incidence and distribution of viruses infecting cucurbit crops in the Northern Territory and Western Australia

During 2003–04, a survey was done to determine the incidence and distribution of virus diseases infecting cucurbit crops growing in the field at Kununurra, Broome, and Carnarvon in north-western Australia, Perth in south-western Australia, and Darwin and Katherine in the Northern Territory. Overall, 43 cucurbit-growing farms and 172 crops of susceptible cultivars were sampled. From each crop, shoot samples were collected from plants chosen at random and from symptomatic plants. Shoot samples were sometimes also collected from potential alternative virus hosts (cucurbit volunteer plants and weeds). All samples were tested by enzyme-linked immunosorbent assay (ELISA) using antibodies to Cucumber mosaic virus (CMV), Papaya ringspot virus-cucurbit strain (PRSV), Squash mosaic virus (SqMV), Watermelon mosaic virus (WMV), and Zucchini yellow mosaic virus (ZYMV). Samples from one-third of the crops were also tested by tissue blot immunosorbent assay (TBIA) using generic luteovirus antibodies. Overall, 72% of farms and 56% of crops sampled were virus-infected. The growing areas with the highest incidences of virus infection were Darwin and Carnarvon, and those with the lowest incidences were Katherine and Perth. For WA, overall 78% of farms and 56% of crops were virus-infected, and in the NT the corresponding figures were 55% of farms and 54% of crops. Overall virus incidences in individual crops sometimes reached 100% infection. Crops of cucumber, melon, pumpkin, squash, and zucchini were all infected, with squash and zucchini being the most severely affected. The most prevalent viruses were ZYMV and PRSV, each being detected in 5 and 4 of 6 cucurbit-growing areas, respectively, with infected crop incidences of <1–100%. SqMV was detected in 2 cucurbit-growing areas, sometimes reaching high incidences (<1–60%). WMV and CMV were found in 3 and 4 of 6 cucurbit-growing areas, respectively, but generally at low incidences in infected crops (<1–8%). Infection with luteovirus was found in 3 growing areas but only occurred in 16% of crops. Beet western yellows virus was detected once but at least one other luteovirus was also present. Infection of individual crops by more than 1 virus was common, with up to 4 viruses found within the same crop. Virus-resistant pumpkin cultivars (6 crops) had little infection when adjacent virus-susceptible cucurbit crops had high virus incidences. Viruses were detected in cucurbit volunteer plants and weeds, suggesting that they may act as important reservoirs for spread to nearby cucurbit crops. In general, established cucurbit-growing farms in close proximity to others and with poor crop hygiene suffered most from virus epidemics, whereas isolated farms with large-sized crops or that had only recently started growing cucurbits had less infection. The extent of infection revealed in this survey, and the financial losses to growers resulting from virus-induced yield losses and high fruit rejection rates, are cause for concern for the Australian cucurbit industry.