Anatomy of a cereal killer
Growers report that barley yellow dwarf virus (BYDV) went on a rampage in 1992 with losses in wheat in the south-west of Western Australia alone in excess of $5 million.
The areas most severely affected are in the high-rainfall zones with 450 mm or more annually. Findings in WA should find applications in cereals and grasses right across southern Australia, particularly in the high-rainfall states of Victoria and Tasmania.
The severity of the disease has been underestimated by farmers and scientists until relatively recently. BYDV infects most cereals grown in Australia — from wheat, oats and barley through to wild grasses and pasture species. Annual losses, even in non-epidemic years, are heavy, especially in the higher-rainfall areas.
Infected crops often show no symptoms, but yield well below their potential. When symptoms do appear the blame has sometimes fallen on water stress, nutrient deficiencies or other problems.However recognition of this killer has been growing since 1989. Western Australian Department of Agriculture scientists, with financial assistance from growers through the Grains Research and Development Corporation (GRDC), have tackled the problem from a number of directions at once.
Team scientists under Dr Roger Jones have been quantifying the losses in wheat production caused by the virus, particularly in the West. They have published new information on BYDV epidemiology (viral strains, aphid vectors, reservoirs of infection, etc.).
More importantly, they have been screening locally recommended wheat cultivars and advanced breeding lines for useful sources of tolerance to BYDV. They have also been studying how important fungal pathogens such as take-all, rhizoctonia, etc. interact with the virus; assessing the effectiveness of control by cultivation and/or insecticide; and investigating how the time of sowing affects the level of subsequent BYDV infection in wheat crops.
Four types of virus In WA
Using a specialised immunological test (ELISA), plant virologist Simon McKirdy established that cereals and grasses in Western Australia can contain four serotypes of the virus, of which PAV appears the most prevalent, but with RPV, MAV also quite common. The different serotypes are transmitted by different aphid species.
ELISA also proved useful in confirming infection levels during varietal tests with 15 different cultivars and breeding lines.
Prime suspect: the oat aphid
The investigators discovered the principal agent involved in spreading infection was the oat aphid. Infection patterns in trials closely resembled field conditions. Symptoms of infection until stem elongation consisted of yellowing leaf tips with some veinal chlorosis. Later, the most distinctive and unmistakable symptom appeared — reddening of the flag leaf.
However, symptom development varied greatly between varieties. Spear, Dagger and Corrigin all had strong symptoms, with herbage dry weight losses up to 62 per cent, but Eradu, Cranbrook and Tincurrin each lost less than 22 per cent, indicating more tolerance of BYDV.
Field trials have consistently supported these results, and so have trials conducted in farmers' commercial crops. The first three cultivars listed above always gave the worst results, said Mr McKirdy.
Direct drilling not conclusive
Despite promising results from trials in the United Kingdom, direct drilling has not yet produced any conclusive benefits in Western Australia's Mediterranean climate. A contributing factor could have been the unusual summer rainfall in 1992, which allowed the aphids to survive and flourish, thanks to the rain-induced long life of their principal oversummering hosts — kikuyu, paspalum, couch and African lovegrass. That meant more aphid flights plagued cereal-growers the following autumn.
The 1992 experience lent additional weight to WA Department of Agriculture advice that later sowing can reduce the impact of BYDV. Even in years of minor infestations, the grower who holds off sowing until June avoids the autumn flights of infective aphids.
The difference in yield is impressive. In time-of-sowing trials in 1992, wheat sown in early June outyielded wheat sown in early May by 60 per cent. BYDV infection levels in the plot sown in May reached 100 per cent while levels in the June-sown plots remained at just over 40 per cent.
Insecticides help, some years more than others
Scientists found that spraying with Pirimor, the recommended aphicide in Western Australia, at the rate of 300g in 200 L water per hectare had a substantial impact on the level of infection in wheat growing at Manjimup (a BYDV prone area) in 1991.
But in 1992 the continual aphid flights reinfected the crops almost as soon as they were sprayed, producing variable results. In the coming season, the team plans to try synthetic pyrethroid sprays, to take advantage of their residual effect. Meanwhile, Pirimor does offer good aphid control in average years, said Mr McKirdy.
Results to date have made some giant strides towards identifying and providing defensive strategies against a stealthy killer most cereal-growers hardly knew was in the neighbourhood until recently.
Region National, North, South, West