Turnip yellows virus detected in migrating cabbage aphids
This season the Department of Primary Industries and Regional Development (DPIRD) staff are monitoring aphid activity using yellow sticky traps on fencelines of canola paddocks in the Geraldton, Kwinana West, Albany and Esperance regions (see the map below). The traps and nearby canola crops are being monitored for green peach aphid (GPA) and all aphids caught on the traps are being tested for the presence of the disease-causing turnip yellows virus (TuYV). TuYV is transmitted primarily by GPA.
Trapping began on the 3 April 2024 and is planned to continue until the start of September.
This trapping surveillance is funded by the Grains Research and Development Corporation (GRDC) project DAW2305-003RTX “Effective virus management in grains crops”.
As of the 29 May 2024, no GPA have been identified on the traps or in the emerging crops. However, aphids of various species, including cabbage and turnip aphids, have been caught in the traps at most sites in both the Esperance and Kwinana West regions. At one site in Kendenup, cabbage aphid have been identified in a dual-purpose canola crop and cabbage aphids were caught on the traps next to that paddock between 14 and 28 May. Small numbers of aphids have been caught on traps at all sites in the Albany region. TuYV was detected in aphids caught at Kendenup, Cranbrook, Tenterden, South Stirlings and Munglinup.
The risk of TuYV is a lot higher when GPA is also present. Cabbage aphid may inefficiently transmit some strains of TuYV but most likely they are feeding on infected plants and the detection of TuYV is from the gut contents of these aphids.
Despite the dry conditions experienced during autumn for much of the grainbelt, aphid flights are likely to be a result of the warmer weather also experienced during that period coupled with moisture stress of aphid refuges such as isolated patches of volunteer canola and wild radish, and dual-purpose canola in some areas.
Due to minimal autumn rainfall, late emergence of many crops in many areas this season will mean later maturation. Therefore, there is some risk that crops will still be in vulnerable growth stages at the time of late-winter/early-spring, at a time when GPA can become increasingly active and virus may spread.
Impact of dry start on aphid populations
Aphids need non-crop hosts to survive over our long hot summer. If we don’t get a lot of summer rainfall, like this season, then aphids are often restricted to isolated damp areas like roadside ditches and home gardens.
When there is little summer and autumn rainfall, aphid reservoirs continue to be limited to isolated pockets of live plants meaning the background aphid population is limited. In such situations, the risk of early aphid infestation and aphid-transmitted viruses is considered low.
However, monitoring of aphid populations via aphid trapping or plant inspections is the best way of estimating local risk, as even small reservoirs of aphids can trigger large-scale crop infestations and virus infection if they are in close proximity to the crop and aphids migrate from the reservoir during the critical window of crop development.
For more information on diagnosing and managing aphids refer to DPIRD’s Aphid management in canola crops and Diagnosing cereal aphids pages.
Early season virus risk
Most broadacre grains viruses that infect our grains crops, particularly Barley yellow dwarf virus (BYDV) in cereals and Turnip yellows virus (TuYV) in canola, only cause significant losses when infection occurs before crop flowering. Furthermore, the risk of feeding damage by the aphid species responsible for spreading these viruses, oat or corn aphids (BYDV) and green peach aphid (TuYV), is greatest when they colonize the crop before stem elongation.
Other viruses, such as cucumber mosaic virus and pea seedborne mosaic virus, are seedborne and do not need to survive in the green bridge. However, these viruses require aphids for plant-to-plant spread, therefore the green bridge is the primary source of these aphid vectors. Some of these viruses have a much broader range of aphid species that can transmit them, and thus total aphid numbers caught on a trap can be a good indicator of risk.
Due to the nature of our WA climate, early season spread of these viruses is relatively sporadic, and occurs under conditions that favor aphid migration from the green bridge into young vulnerable crops during autumn and early winter.
Management
There are a variety of management strategies that growers can employ. The most accessible of these are the use of insecticides (seed treatment or foliar applied) with systemic and/or residual activity which can protect plants for several weeks after application. For some viruses, growers can use resistant varieties, manipulate sowing date and plant density, sow into stubble and applying foliar sprays to reduce aphid numbers.
For more virus management information refer to DPIRD’s Managing barley yellow dwarf virus and cereal yellow dwarf virus in cereals and Turnip yellows virus in canola: diagnosis and management pages.
Effective chemicals currently available in Australia for control of GPA are alarmingly limited as GPA has evolved resistance to many insecticide chemicals. For more information see GRDC’s Aphid and insecticide resistance management in grain crops.
For registered insecticide recommendations, refer to DPIRD’s 2024 autumn winter insecticide guide.
Further information
For further information contact Research Scientist Benjamin Congdon, South Perth on +61 (0)8 9368 3499.
Article author: Benjamin Congdon (DPIRD South Perth).