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Ovine Observer

Have you seen sub clover red leaf syndrome in your pastures?  

Paul Sanford (DPIRD), Kevin Foster (UWA) and Ben Congdon (DPIRD)
Author correspondence: paul.sanford@dpird.wa.gov.au

This research is funded by DPIRD and UWA.

Introduction

Subterranean clover is the most widely used annual pasture legume in WA with around eight million hectares sown and is the backbone of our southern feedbase systeem. Consequently, there has been considerable concern amongst sheep producers regarding the recent outbreaks of subterranean clover red leaf syndrome, which have led to a substantial loss of quality forage in the form of clover biomass. In 2017 for example the subterranean clover red leaf syndrome most likely caused the failure of clover to establish in a number of sheep enterprises in the Brookton area resulting in significantly more supplement being fed.
 
In response to these concerns, the Department of Primary Industry and Regional Development (DPIRD) with the University of Western Australia (UWA) investigated the cause of subterranean clover red leaf syndrome.

Materials and methods

In the late spring of 2017 samples of subterranean clover plants with the symptoms of red leaf syndrome and samples of healthy clover plants in close proximity without symptoms where collected from 12 commercial pastures located in Woodanilling, Tambellup, Wandering, Mt Barker, Narrikup, Kendenup, Manypeaks, South Stirlings and Torbay. In most cases ten or more whole live plants with roots were collected, both with and without symptoms. In three cases samples numbered less than ten. The samples were chilled as soon as possible and remained cold during transport to the DPIRD Diagnostic Laboratory where they were tested for a range of plant viruses using Polymerase chain reaction (PCR) and sequencing.

Results and discussion

The DPIRD laboratory found that 80% of clover plants with obvious red leaves were infected with the soybean dwarf virus (SbDV) compared to only 2% of ‘healthy looking’ plants. This result strongly suggests that SbDV infection was the inciting cause of the subterranean clover red leaf syndrome in the spring of 2017. Other factors such as environmental stress and root disease may have compounded the severity of the syndrome.
 
The type and severity of SbDV leaf symptoms differ depending on plant age at infection. Infection targets the older leaves of subterranean clover, causing leaf margin reddening. Plants infected early in the season exhibit more severe symptoms than those infected later and are often stunted with small leaves and produce few seeds.
 
A comparison between two sub clover plants (cultivar Dalkeith), one infected with soybean dwarf virus, the other uninfected, grown under the same conditions in the DPIRD glasshouse
A comparison between two sub clover plants (cultivar Dalkeith), one infected with soybean dwarf virus, the other uninfected, grown under the same conditions in the DPIRD glasshouse
 
A challenge of identifying SbDV disease in subterranean clover is that environmental stressors can result in the same red leaf symptoms, such as nutrient deficiencies, water-logging and cold stress. It has also been identified that SbDV infection in some cases coincides with secondary infection by fungal root rots pathogens which can lead to pruning of the roots. Hence SbDV symptoms, severity and rates of plant death can be compounded by other environmental factors.
 
Previous research demonstrated that SbDV infects legume species and does not infect grasses. SbDV is hosted by live plants over summer and spread to subterranean clover plants by aphids. Foxglove and pea aphids are the primary vectors of SbDV and once infected the aphids carry the virus for life. There is no seed-borne spread of SbDV. Summer rains favour SbDV outbreaks. An increase in weed growth from summer rains allows the virus to survive and growth of aphid populations. Rising virus and aphid levels coincide with the critical point of new autumn pasture growth thus increase the risk of an SbDV outbreak.
 
Depending on the location, legume crop volunteers, subterranean clover, white clover, red clover, strawberry clover, lucerne and medics are also likely over-summer hosts for SbDV in WA. Due to the reliance on pre-season rainfall, the increased presence of its external hosts and aphid vector species SbDV is more common in medium to high rainfall areas. Dry springs can also favour aphid activity and the development of red leaf symptoms. However, aphid numbers are often low in years with cold wet weather or the presence of high numbers of aphid parasites.

Integrated disease management

Please refer to the following DPIRD factsheet for management information.