Protecting WA crops

Viruses in legume crops and pastures

This issue of Protecting WA Crops has a focus on viruses in legume crops and pastures.

Lupin crop

Take home messages;

  • Legume crops in Western Australia are susceptible to contracting diseases as a result of a number of different viruses.
  • Aphids transmit all legume viruses.
  • Some viruses are seed borne only in their legume hosts, so sowing seed that has a low incidence of virus is an important way of managing these diseases.
  • Unlike fungal infections, the whole plant becomes systemically invaded. The whole plant therefore is either virus-infected or healthy.
  • Yield losses from virus infections are sporadic and vary from year to year and location but can be considerable in some circumstances. Seed quality can also be impaired.
  • Viral symptoms in plant foliage are often confused with nutritional deficiencies, herbicide damage or water-logging, leading to underestimation of the extent of damage caused.
  • The greatest losses to yield due to virus diseases occur when crops are infected early in the growing season.
  • A persistent green bridge can have a large impact on the level of early crop infection.

Viruses survive in the green bridge

Aphids, and many of the plant viruses they spread, rely on live plant hosts (also known as a green bridge) for survival over summer in the Western Australian grainbelt.

In situations when there is substantial summer rainfall across the grainbelt the subsequent green bridge is likely to provide the ideal environment for aphid colonies to build up and move into new crops. This is how turnip yellow virus (TuYV) and soybean dwarf virus (SbDV) enter crops as they are not seed-borne and need to survive between growing seasons in plant hosts to build up.

The greatest crop yield losses occur when crops are infected by viruses early in the growing season when crops are most susceptible.

Destroying the green bridge several weeks prior to sowing can help to minimise the threat from virus diseases. Little or no summer rain delays the arrival of aphids and can reduce crop infections. Some common legume viruses depend entirely on co-survival with these aphid vectors.

The dry conditions experienced through much of the grain belt this season has meant that there has been little or no green bridge developed. Therefore there may be less incidence of early virus infection this season.

Aphid transmission

Blue-green aphid (©2013)
Figure 1 Blue-green aphid (©2013)

There are two main ways aphids can spread viruses and this influences how virus diseases are managed:

  1. Persistently transmitted viruses

Once an aphid feeds on a virus-infected plant the aphid can spread the virus for its entire life. These viruses are found in the phloem of the plant and require the aphid to feed for long periods of acquire the virus. For this reason insecticides can be useful in managing virus spread as they kill the aphid prior to them spreading the virus further. Persistent viruses include; turnip yellow virus (TuYV) and soybean dwarf virus (SbDV).

TuYV is the most widespread of the two viruses. It has a very wide natural host range including non-leguminous crops like canola, and pasture plants such as clovers, medics and lucerne, plus many weeds such as paddy melons and wild radish, and certain native legumes. All of these plants act as reservoirs for TuYV spread to pulse crops.

SbDV hosts are mainly restricted to legumes especially lucerne pastures but also clovers, medics and native legumes.

  1. Non-persistently transmitted viruses

These viruses are found throughout the whole infected plant. The aphid only needs to feed for a few seconds to acquire the virus and then spreads it when it feeds on the next plant. The aphid then loses the virus after feeding. Insecticides are not effective in controlling these viruses as they do not act quick enough to kill aphids prior them moving to new plants. Non-persistent viruses include; cucumber mosaic virus (CMV), bean yellow mosaic virus (BYMV) and pea seed-borne mosaic virus (PSbMV).

For non-persistently aphid-borne viruses, contaminated seed is often the principal source of infection for crops which can then be spread further by aphids. Alfalfa mosaic virus (AMV) and bean yellow mosaic virus (BYMV) are seed-borne in sub-clover and lucerne, respectively. These plants then act as a reservoir for spread into pulse crops such as lupin. Some seed-transmitted viruses such as pea seed-borne mosaic virus (PSbMV) and cucumber mosaic virus (CMV) are predominantly internally sourced from contaminated pulse seed-lots. With all pulse viruses, other pulse crops can act as reservoirs. For example, PSbMV will spread readily from an infected chickpea into nearby pea crops.

Protecting WA Crops Issue 3 focussed on Aphids.

Pea seed-borne mosaic virus (PSbMV)

Figure 2. Pea seed-borne mosaic virus induced seed coat defects in variety Kaspa. Note the necrotic rings and line patterns (sometimes referred to as ‘tennis ball’ symptoms). (©2016)
Figure 2 Pea seed-borne mosaic virus induced seed coat defects in variety Kaspa. Note the necrotic rings and line patterns (sometimes referred to as ‘tennis ball’ symptoms) (©2016)

Sowing seed infected with PSbMV can result in infected plants being distributed at random within the field pea crop which aphids then further spread to healthy plants.

Research officers Brenda Coutts, Rohan Prince and Roger Jones (DPIRD) have demonstrated that the number of plants showing symptoms of PSbMV increases with the amount of primary inoculum (or seed) present.

Field experiments, conducted in 2005 and 2006, investigated the effects of sowing field pea seed with different amounts of infection of PSbMV on virus spread, seed yield, and infection levels in harvested seed. Plots were sown with seed transmission rates of 0.3-6.5% (2005) or 0.1-8% (2006). Disease spread was left to naturally occurring migrant aphids.

The experiments showed that plants with PSbMV symptoms and incidence increased with the amount of primary inoculum present. When final incidence reached 98% (2005) and 36% (2006) in plots sown with approximately 8% infected seed, yield losses of 18-25% (2005) and 13% (2006) resulted.

In the trials described above, seed transmission rates in harvested seed were mostly greater than those in the seed sown when climatic conditions favoured early virus spread but smaller when they did not (See Table 1). For example, when greater than 5% of the seed was infected and was sown early in the season, 100% of the crop became infected resulting in a 25% yield loss. The same seed sown later in the season resulted in only 40% crop infection and less than 3% crop yield loss.

Table 1 Potential effect on yield and subsequent seed transmission rates as a result of sowing PSbMV-infected seed in different conditions (based on the variety Kaspa)
Scenarios 1 2 3 4
Seed infection >5% >5% <1% <1%
Aphid arrival early late early late
Crop infection at full pod set 100% 40% 40% 15%
Yield loss 25% 10-15% 10-15% No effect
Seed transmission 10-20% 1-3% 1-3% 0%

The seed transmission rate of virus to seedlings differs with field pea variety. For example; the seed transmission in Kaspa can be up to 25%, while less than 5% for the variety Twilight.

The transmission efficiencies of aphid vectors was investigated by Ben Congdon (DPIRD) on the susceptible pea variety Kaspa and the partially resistant cultivar Twilight. Ben found that cowpeas aphids, green peach aphids (GPA) and bird cherry-oat aphids were 27%, 26% and 3% effective in transmitting PSbMV to Kaspa, but transmission rates were reduced for Twilight to 12%, 16% and 1% respectively reflecting the partial resistance to PSbMV.

Glasshouse experiments, performed by Ben Congdon in 2017, where wind was simulated through the use of fans, indicate that PSbMV can also be transmitted from infected plants to healthy plants by leaves rubbing on each other. The virus was consistently transmitted when temperatures were cooler (14-20°C) and when the leaves of the healthy plants were wounded (from leaves rubbing on one another in strong winds). Transmission rarely occurred in warmer conditions (20-30°C). This may indicate a decrease in virus concentration, reflecting the co-evolution of the virus with its winter growing (cold adapted) host.

Although infected field pea seed is the main source of PSbMV, infected faba bean, chickpea, lentil, grass pea and vetch crops are occasionally potential sources of virus also. However, lupins and pasture legumes such as clovers and medics do not become infected.

For more information refer to;

Bean yellow mosaic virus (BYMV)

Bean yellow mosaic virus - late season infection causing black pod syndrome (©2018)
Figure 3 Bean yellow mosaic virus - late season infection causing black pod syndrome (©2018)

BYMV is seed-borne in annual clovers, and survives the dry summer period in dormant seeds in the soil. BYMV is not seed borne in lupins but it is spread from infected pastures to lupin crops via aphids.

Yield loss can be 80% or more if all plants become infected by the virus.

Some narrow leafed lupin varieties undergo necrosis (plant death) when infected before pod set. When plants are infected after pod set, black pods develop. This is also known as black pod syndrome (BPS).

Time of infection of BYMV and its influence on severity of plant damage was investigated by Monica Kehoe (DPIRD), while researching her PhD at UWA in 2014. Monica found that where narrow-leafed lupins were inoculated with BYMV at different growth stages, BPS only occurred when plants were inoculated after pods had formed. If inoculated before this growth stage, BYMV resulted in plant death.

These results align with some previous work on narrow-leafed lupins done by Roger Jones and Brenda Coutts from 1988-2001 to quantify yield loss. They introduced clover plants infected with BYMV into plots to provide infection sources and aphids to spread infection to the lupin plants. They found that early infection of BYMV killed lupin plants and resulted in no seed production (Figure 4). In plants that did not die, shoot dry weight, seed yield and seed number all decreased the longer the plant was infected.

Figure 4. The impact time of infection has on lupin seed yield.
Figure 4 The impact time of infection has on lupin seed yield

Increasing the inoculum level of BYMV in lupin crops decreases the seed yield. For example, in previous work, increasing the number of introduced infected clover plants, in a Tanjil lupin crop, from 4 to 16 decreased the lupin yield from over 2 t/ha to less than 1 t/ha.

For more information refer to;

Cucumber mosaic virus (CMV)

Figure 6. Seed-borne infection with CMV in narrow-leafed lupin plants. Plants are stunted with bunched, down curled and mottled leaves (©2018).
Figure 6 Seed-borne infection with CMV in narrow-leafed lupin plants. Plants are stunted with bunched, down curled and mottled leaves (©2018)

CMV infection causes leaves to become pale, bunched and down-curled with faint mosaic patterning. When plants grow from infected seed all leaves develop these symptoms and are severely stunted. Healthy plants that become infected during the growing season have pale, bunched young leaves with faint mosaic while older leaves formed prior to infection appear normal. As growth continues, all new leaves show symptoms and the infected plants become stunted.

The effect of sowing lupin seed with 5% and 0.5% CMV infection on subsequent virus spread, lupin yield and percentage of infection in harvested seed was examined by Roger Jones (DPIRD) in 1988 and 1989. Sowing 5% infected seed resulted in yield losses of 34-53% and seed transmission of 6-13%. The yield in plots sown with 0.5% infection was not significantly decreased. However, late CMV spread in these plots caused >1% seed infection.

For more information refer to the Cucumber mosaic virus page.

Sub clover red leaf syndrome

Figure 7. Sub clover exhibiting red leaf syndrome symptoms at Manypeaks in spring 2017
Figure 7 Sub clover exhibiting red leaf syndrome symptoms at Manypeaks in spring 2017

There has been considerable concern among livestock producers regarding recent outbreaks of sub clover red leaf syndrome (SCRLS), 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 symptom expression is related to overall plant health so additional stresses such as drought stress, poor nutrition or root rot pathogens can weaken the sub clover plant further.

DPIRD officers investigated the 2017 outbreak and found 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.

As a result of concern from growers and industry about the disease a partnership has been formed between DPIRD, the University of WA (UWA), Meat and Livestock Australia (MLA) and Australian Wool Innovation (AWI) to investigate methods of controlling and managing future disease outbreaks.

Given our current understanding of the disease, sub clover plants are likely to develop severe symptoms of red leaf syndrome if infected as seedlings. Therefore, our best advice at present is to control aphids during the period of pasture establishment. SbDV is not seed borne but is hosted by live plants over summer and spread to sub clover by aphids.

Grasses do not host SbDV so using annual ryegrass or forage oats could be a useful tactic and improve feed availability into the bargain.

Growing alternative pasture species is another method to lessen the impact of a devastating loss of sub clover as the result of an outbreak.

Serradella is one alternative option as it does not appear to be affected by the syndrome even when growing alongside symptomatic sub clover plants. 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.

As a result of the recent summer rain in the south west of WA, DPIRD and UWA have commenced a sampling program to discover which plant species host the SbDV virus over the summer.

Producers are invited to participate in a survey about SbDV. As a result of the joint initiative between AWI and MLA producers are able to report incidents of the virus and allow for sample testing to continue. Producers are invited to submit their reports via the survey.

The current take home message is you can still have your sub clover based pastures or sow new pastures as long as you maintain good insect hygiene to reduce or prevent the spread of SbDV. Pending results of the 2018 sampling project, more research may be required to understand the life cycle of this virus and its impact on sub clover which is the mainstay of our livestock and grain industries.

For more detailed information refer to the Red leaf syndrome page.

Impact of insecticide use on severity of SCRLS – some grower observations

Alice Butler and Bec Swift from DPIRD conducted case studies on growers with sub clover red leaf syndrome (SCRLS) in 2017.

Brad Bassett from Brookton sprayed his pastures on the 26 June to control cutworm and the mix contained 15mL Trojan ® (gamma-cyhalothrin). One paddock was sown around 20 years ago with a Dalkeith-Nungarrin sub clover mix and, in this paddock, Brad only managed to get one lap sprayed before the tank ran out. The whole paddock was sprayed again in August for redlegged earth mite (RLEM) and as an anti-feed. The strip that missed out on the spray in June shows clear symptoms of SCRLS while the sprayed section recovered from the syndrome (Figure 8). It is not known whether the SCRLS was caused by a virus or other stress factors in this scenario.

Figure 8. Brad Bassett’s 2017 sub clover paddock sprayed with insecticide on the left, and unsprayed on the right (Photo courtesy of Brad Bassett).
Figure 8 Brad Bassett’s 2017 sub clover paddock sprayed with insecticide on the left, and unsprayed on the right (photo courtesy of Brad Bassett)

Dave Pearce also observed less SCRLS where he had sprayed insecticide on his farm in Woogenellup. Dave had divided a paddock into two less than 18 months ago. These two paddocks contain a mixture of old and new sub clover varieties. In the first week of June 2017 he used an insecticide on one of these paddocks for RLEM; however, he decided against spraying the other paddock as there was no damage from RLEM early in the season. The unsprayed paddock started showing reddening of the sub clover in patches in September 2017 and about 5% of the paddock was affected. In contrast, the paddock that had been sprayed had very little reddening of the sub clover, except for one or two random leaves. Dave said the reddening of sub clover has been less severe in 2017 compared to 2016.

Figure 9. The unsprayed paddock with patches of red-dying sub clover throughout.
Figure 9 The unsprayed paddock with patches of red-dying sub clover throughout
Figure 10. Paddock sprayed with insecticide in the first week of June.
Figure 10 Paddock sprayed with insecticide in the first week of June

These experiences appear to support a correlation between insect control and the impact of the red leaf syndrome. However, the presence of aphid vectors and virus infection were not confirmed.

Virus control

Viruses are controlled through an integrated disease management approach (IDM) where a number of different strategies are used. See the table below for a variety of IDM options;

Table 2 Components of a 'generic' integrated disease management strategy against AMV, BYMV, CMV and PSbMV in pulse crops
Control measure Mode of action
Sow healthy seed stocks Minimises initial virus infection source within crop. For seed-borne viruses such as PSbMV, where seed-transmission rates are high, using healthy seed is an effective control measure. When sowing lupin crops in lower risk areas, a ‘threshold level’ of <0.5% CMV seed infection is sufficiently conservative to avoid serious yield loses but for high risk areas a more stringent threshold of <0.1% seed infection should apply. Similar recommendations apply to PSbMV infection of pea seed stocks.
Sow cultivars with low intrinsic seed transmission rates in high virus risk regions Minimises initial virus infection source within crop
Sow perimeter non-host barrier crop in between adjacent pasture and crop Decreases virus spread into crop from external pasture source
Promote early crop canopy development Shades over infection sources within crop (seed-infected and/or early infected plants) and diminishes aphid landing rates
Sow at high seeding rates to generate high plant densities Minimises infection sources (seed-infected and/or early infected plants) and diminishes aphid landing rates. Dilutes numbers of infected plants
Sow at narrow row spacing Narrow spacing diminishes aphid landing rates
Minimises stubble ground cover using minimum tillage procedures that minimise soil cultivation Diminishes aphid landing rates until crop canopy develops
Spray high value seed crops with pyrethroid insecticide Suppresses virus spread by killing or repelling aphids
Spray adjacent pasture with pyrethroid insecticide in high virus risk regions Suppresses virus spread within external pasture infection source by killing colonising aphids
Avoid fields with large perimeter: area ratios adjacent to pastures in high virus risk regions Decreases ingress of virus into crop from external pasture source
Sow early maturing cultivars Decreases final infection incidence reached, especially in prolonged growing seasons
Isolation from neighbouring legume crops Decreases ingress of virus from any external infected crop source
Maximise weed control Minimises potential weed virus infection sources within crop (especially clovers for BYMV)
Crop rotation Avoids volunteer seed-borne pulse plant infection sources within crop

Cultural control

The cultural control measures of planting upwind and planting non-host barrier crops can diminish vector (aphid) arrivals in plantings, thereby, slowing early infections. The effect of barriers is greatest if they are positioned at the windward edge of the crop so when incoming aphids probe the non-host crop, this reduces the virus load.

CMV and BYMV disease pressure can be reduced through use of higher seeding rates or narrow row spacing.

Retaining crop stubble on the soil surface at seeding will assist in management of BYMV infection in lupin crops. However, wide row spacing in the absence of retained stubble is undesirable.

Trials at Avondale and Badgingarra, run by DPIRD’s Brenda Coutts, Roger Jones and David Ferris in 1991, looked at the effect that mulching, seeding rates and row spacing had on BYMV in narrow leafed lupins. Straw was spread over plots of lupin sown at narrow (17.5cm) and wide (35cm) row spacings and low (25-30kg/ha) and medium (50-60kg/ha) seeding rates. Under high disease pressure, straw was found to decrease the virus by more than 70% and increased the yield by 20%. It’s believed the straw deterred aphids from landing.

In the plots without straw, BYMV infection was decreased in the narrow row spacings treatments while the wider row spacing delayed canopy closure, increasing aphid landings. The greater plant densities resulting from the medium seeding rate also increased grain yield but row spacing did not affect it significantly.