Managing sub clover red leaf syndrome in season 2018
Sub clover is the most widely used annual pasture legume in Western Australia with around eight million hectares sown.
As a consequence there has been considerable concern among livestock producers regarding recent outbreaks of sub clover red leaf syndrome, the occurrence of which seems to becoming more frequent and widespread.
Symptoms include reddening of leaves; stunted plants, losses in productivity and even premature plant death.
The Department of Primary Industries and Regional Development investigated the 2017 outbreak finding that 80% of clover plants tested, with obvious red-leaves, were infected with Soybean dwarf virus (SbDV) compared to just 2% of ‘healthy looking’ plants.
A workshop was held to consult with livestock producers, seed industry members and experts in pasture and plant disease last December.
The participants agreed the primary cause of this syndrome is likely to be SbDV infection.
The symptom expression is related to overall plant health so additional stresses such as drought, poor nutrition or root pathogens can weaken the plant further.
In addition, a partnership was formed between DPIRD, the Univerisity of WA (UWA), Meat and Livestock Australia (MLA) and Australian Wool Innovation (AWI) to investigate methods of controlling and managing future outbreaks.
Soybean dwarf virus is not seed borne but is hosted by live plants in summer and spread to sub clover by aphids.
Substantial summer rain can result in a green bridge being present prior to the emergence of sub clover seedlings.
This allows both the virus to survive and aphid populations to increase at a time when new pastures are just developing.
Given that sub clover plants are likely to develop severe symptoms of red leaf syndrome, if infected as seedlings our best advice currently is to control aphids during the period of pasture establishment.
This involves spraying an anti-feeding insecticide at two and six weeks after sub clover seedling emergence using a synthetic pyrethroid at the highest registered rate to deter aphids from feeding and spreading the virus.
Refer to the insecticide label for withholding periods or Safemeat for export intervals.
In addition, oats can be sown as a barrier around pasture paddocks to disperse aphids and slow early spread into pasture from outside sources.
Growing alternative pasture species is one method to lessen the impact of a devastating loss of sub clover as the result of an outbreak.
Serradella is one option as it does not appear to be affected by the syndrome even when growing alongside symptomatic sub clover plants.
Serradella is suited to sandy soils and can be sown into weed free paddocks in summer as pod or seed in autumn/winter.
However a note of caution; it is unknown whether some of the alternative pasture species are hosts of SbDV.
DPIRD and UWA are investigating this possibility.
Grasses are known not to host SbDV so using annual ryegrass or forage oats could be a useful tactic and in the absence of an outbreak would most likely improve feed availability.
If sowing alternative pasture species, seek further advice to be certain they are suited to your soils, rainfall and management.
As a result of the recent summer rain in the south west of WA, DPIRD and UWA have commenced a summer sampling program in an effort to discover which plant species host the SbDV virus.
This is the beginning of a larger research project (funded by MLA, AWI, DPIRD and UWA) designed to understand the life cycle of this virus and its impact on sub clover which is the mainstay of our livestock and grain industries.
The goal over the next three to five years is to deliver proven methods to prevent and manage this syndrome.
For more information contact Paul Sanford, DPIRD Senior Research Officer, Albany on 9892 8475 or Kevin Foster, UWA Pasture Researcher, Nedlands on 6488 2220.