Patterns of mixed virus infections: a 3-year study of symptomatic cereal and grass hosts in Australia

Context: Yellow dwarf viruses (YDVs) form a complex of economically important pathogens that can significantly reduce grain yield in cereals. Mixed infections, or infection with two or more YDV species, can be particularly damaging. Aims: We aimed to examine the proportion of single and multiple virus infections present in symptomatic cereal and grass plants in Victoria, south-eastern Australia. Methods: Over 3 years (2020-2022), symptomatic cereal and grass plants from within and around cereal fields in Victoria, Australia were individually tested using tissue-blot immunoassay (TBIA) for barley yellow dwarf virus PAV, barley yellow dwarf virus MAV, cereal yellow dwarf virus RPV, wheat streak mosaic virus, and with a generic TBIA test that can detect multiple luteovirus and/or polerovirus species. Key results: Across 2020-2021, 34% of virus-positive plants were infected with multiple YDV species. The proportion of mixed infections was similar in each individual year. However, higher proportions of wheat (Triticum aestivum, 47%) and wild oat (Avena fatua, 36%) plants were infected with multiple YDV species compared to barley (Hordeum vulgare, 8%) and brome grass (Bromus spp.,17%). Conclusions: The proportion of virus-positive plants infected with multiple YDV species found was almost four times higher than previously reported in a similar study in Victoria, Australia in 1985. The proportion of plants infected with multiple YDV species varied more with host type than between individual years. Implications: These findings demonstrate the complex epidemiology of these damaging viruses, and the challenges associated with developing virus-resistant cereal cultivars, while also highlighting the importance of regular surveillance over multiple years.