Australian Pulse Bulletin

Managing viruses in pulses

Key points

• Virus management aims at prevention through integrated management practice that involves controlling the virus source, aphid populations and virus transmission into pulse crops.
• Rotate legume crops with cereals to reduce virus and vector sources and where possible avoid close proximity to perennial pastures (eg lucerne) or other crops that host viruses and aphid vectors.
• Eliminate summer weeds and self sown pulses 'green bridge' that are a host for viruses and a refuge for aphids.
• Aphid activity is influenced by seasonal conditions and will require early monitoring in nearby crops and pastures and possible use an aphicide or cultural controls to reduce numbers.
• Sow directly into cereal stubbles, (preferably standing) and encourage rapid canopy cover through early planting, high planting density as bare soil is more attractive to some aphid species.
• Purchase virus tested seed or have farmer seed virus tested as PSbMV, CMV, BYMV and AMV depend largely on seed transmissions for survival.
• Gaucho® 350SD is now registered and when applied as seed treatment will help protect faba bean, field pea and lentil seedlings from early season aphid attack and reduce virus spread.

Viruses differ from most fungal diseases in that they infect plants systematically and no curative treatment is available. Virus infections are spasmodic and levels depend heavily on seasonal conditions and differ greatly between years and locations. Early infection can lead to stunting, reduced tillering and plant death and losses can be high. Late infections have less impact, but can still affect seed quality.

High levels of virus infections have occurred in recent years resulting from infected plants in the previous spring as a virus source and a 'green bridge' of summer plant material to carry over these viruses and as a refuge for aphids. Warm dry conditions during autumn have favoured increased aphid activity and virus transmission.

Some aphid species prefer to land on plants surrounded by bare ground and favour thin crop stands or areas within the crop which have low plant densities (figure 2).

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  Faba bean crop infected with bean leaf roll virus (BLRV). Deniliquin.

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  Kabuli chickpea (centre) with low plant stand and high virus infection compared to kabuli (right) and desi (left) with good canopy.

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  Field pea infected with seed borne mosaic virus (PSbMV).

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  Symptoms of early viral infection appearing at shoot tips rather than lower leaves, as is the case with root diseases.

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  BLRV in faba beans, showing stunted plants with leathery upturned leaves. These plants also failed to nodulate.

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  Cowpea aphids colonising vetch, Vic Mallee.

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  CMV along a crop edge showing stunted plants and small leaves, but also bare ground.

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  Lupins in standing stubble as early ground cover. This is particularly important in wide row pulses.

Stressed plants are also more attractive to aphids, possibly due to a higher level of plant sugars, and are vulnerable to colonisation and can become a source of virus spread. Environmental factors that impacted on chickpeas in 2009 were extremely dry conditions early in the season that favoured aphid build and this was particularly evident in vetch crops. Then followed cold and wet conditions that included some transient water logging that stressed plants making them more venerable to root diseases and aphid attack.

Figure 2: Kabuli chickpea (centre) with low plant stand and high virus infection compared to kabuli (right) and desi (left) with good canopy. DPI Vic, Minyip 2009

Virus damage was more severe in chickpea as aphids do not colonise this crop, resulting in more frequent probing and movement to spread the virus, often without detection. Chickpea that border lentil, canola or lucerne crops can be subjected to larger numbers of aphids, as they can readily colonise these crops and multiply quickly. Controlling aphids in these nearby host crops can potentially decrease aphid numbers moving through chickpea crops.

Types of transmission

Pulse viruses are transmitted either in a persistent or non-persistent manner by insects (mostly aphids). The mode of transmission has implications for the way a virus develops in the field and its management.

Persistently transmitted viruses

• Bean leafroll virus (BLRV)
• Beet western yellows virus (BWYV)
• Subterranean clover red leaf virus (SCRLV)
• Subterranean clover stunt virus (SCSV)

Persistent transmission means that once the insect becomes infectious, it remains so for the rest of its life. After an insect vector feeds on an infected plant, the virus has to pass through its body and lodge in the salivary glands before it can be transmitted to healthy plants. Not all aphid species are vectors of this kind of virus in pulses so the identification of aphid species is very important.

BWYV is the main virus and most common occurring in chickpea and lentil crops in Victoria in 2009. It has a diverse natural host range including canola, pasture plants, lucerne and many weeds such as paddy melons, wild radish and some native legumes. BLRV is another but is limited to fabacae (faba bean, field pea, chickpea, and lentil), lucerne, clovers and summer legumes. It has also caused considerable damage and yield loss to faba bean crops in some southern regions of NSW and SA in recent years, as has infections of subterranean clover stunt virus SCSV in southern Vic.

Persistently transmitted viruses typically start with a random distribution of infected plants in autumn and increases during the season as vectors colonise the crop. Transmission rates can dramatically increase with large aphid flights that will often coincide with aphid activity and build up prior to sowing.

Non-persistently transmitted viruses

• Alfalfa mosaic virus (AMV)
• Bean yellow mosaic virus (BYMV)
• Cucumber mosaic virus (CMV)
• Pea seedborne mosaic virus (PSbMV)

Non-persistently transmitted viruses can be seed-borne (depending on the virus – crop combination), but require aphid vectors to spread during the season.

Figure 3: Field pea infected with seed borne mosaic virus (PSbMV), Murray Valley 2006

Non-persistent transmission means that the insect vector can acquire the virus through a brief probe on a virus infected plant and can transmit the virus directly afterwards, again through brief probes of healthy plants. It will lose the virus after probing only one or two times. Insecticides do not act fast enough to prevent transmission, and can make things worse by making the aphids hyperactive, flitting from plant to plant. Many aphid species are vectors of this type of virus including ones that do not colonise legumes but just land and probe while searching for their preferred hosts.

These non-persistently seed borne viruses, AMV, CMV and BYMV can sometimes cause heavy yield losses. PSbMV is particularly dangerous because of high seed to plant transmission rates and effect on grain quality, especially in faba beans and field peas destined for human consumption markets, while CMV has been reported at up to 75 percent transmission in lupins.

Virus symptoms

Viruses exhibit a varied range of symptoms and severity from relatively unapparent to plant death. The intensity and symptoms depend on virus and pulse species and to a lesser extent on virus strain, pulse variety, climatic conditions and plant stage at infection.

Plants infected at an early stage or through seed (figure 3) will usually show more uniform discoloration and stunting, but when infected at the later stage will usually occur at the leaf tip (figure 4) before the whole plant starts to deteriorate.

Figure 4: Symptoms of early viral infection appearing at shoot tips rather than lower leaves as is the case with root diseases.

Foliage symptoms are often more visible on young leaves and can include yellowing (sometimes reddening), vein clearing, leaf mottle, leaf distortion, curling of leaves, reduced size, chlorotic or necrotic spotting, or more widespread necrosis. Shoot symptoms may be seen as bunching of young leaves, growth of auxiliary shoots, bending over of the growing point, tip or apical necrosis, streaking of stems, stunting and wilting or plant death.

Symptoms such as leaf yellowing, veining, mottling, and wilting can often be confused with nutrient deficiencies, herbicide damage or water stress unless sufficiently distinct. It is also difficult to tell which virus is present without resorting to laboratory tests on plant samples.

It is best to collect living tissue samples and collection and packaging of fresh samples is simple. Instructions from local agronomists or Pulse Australia need to be heeded. Immediately place the sample with paper towelling into a plastic bag, seal it and refrigerate it until dispatched. Send the sample by priority post and do not leave it sitting around.

Figure 5: BLRV in faba beans showing stunted plants with leathery upturned leaves. These plants also failed to nodulate.

Aphids and their lifecycle

Aphids are small soft-bodied insects that do not like temperate extremes, and become more active in autumn and spring when temperatures are 18–270 C. They can survive over summer in low numbers as winged adults or eggs, usually on alternate host plants such as weeds, perennial pastures or grasses around fences or roadsides. Nearly all aphids are female and reproduce asexually and are capable of giving birth to up to 12 live young per day that can develop into reproducing adults in just 7–8 days. Given ideal conditions an aphid could produce over half a million offspring within 3 weeks. Immature aphids are wingless but can grow wings as adults when populations are too high or the quality of their food source deteriorates. Aphids will often fly low and relatively short distances to the nearest food source and are first detected entering a crop around the perimeter or thin areas within.

In the absence of a nearby suitable crop and given favourable winds, they are capable of much greater distances and landing further into crops.

There is known to be at least 18 aphid species that inhabit broadacre crops but not all are of economic importance. Pea aphids (Acyrthosiphon pisum), are the most efficient BLRV and PSbMV vector colonising peas, along with faba beans and other pulses. Cowpea aphid (Aphis craccivora) is a major BLRV vector as well as the most efficient SCSV vector and a vector of CMV. The foxglove aphid (Aulacorthum solani) is reported to be the most efficient SCRLV vector and green peach aphid (Mysus persicae) is well known for spreading BWYV.

Aphids have a number of natural predators such as parasitic wasps, lady beetles, lacewings and syrphid flies, and these beneficial’s are usually very effective when aphid numbers are relatively low. In an integrated pest management approach (IPM) aphids are allowed to initially build up to allow the beneficial’s to increase so that they can later control aphid numbers.

However this approach can be deleterious for virus incidence as only a few virulent aphids can spread viruses quickly.

Figure 6: Cowpea aphids colonising vetch. Vic Mallee.

Integrated Disease Management Strategy

A virus management strategy to reduce the risk of infection may require a number of control measures relevant to the various virus and pulse types.

Non-Persistently transmitted viruses

(e.g. CMV, BYMV, AMV, and PSbMV) the initial and main source of infection is contaminated seed, with further transmission in-crop by aphids.

Management steps include:

* Source healthy seed that has been tested free of CMV, BYMV, AMV and PSbMV virus. Tested seed should have less than 0.1% virus infection and field peas should have less than 0.5% for PSbMV.

* Farmer retained seed should only come from crops with no visible virus symptoms and seed testing should be a priority.

* Some cultivars have virus resistance such as CMV in many new lupin varieties and in Jenabillup (available in 2011). Yarrum field pea has resistance to BLRV and PSbMV. Increased emphasis on virus resistance is a priority of Pulse Breeding Australia.

* Controlling aphid’s in-crop is not an effective means of controlling non-persistently transmitted types of viruses.

* Sow direct into retained cereal stubble and preferably standing as some aphid species are attracted to bare earth. This has been effective in minimising CMV spread in lupins (figure 7).

Figure 7: CMV along a crop edge showing stunted plants and small leaves, but also bare ground.

Persistently transmitted viruses

(e.g. BLRV, BWYV, SCSV) are not seed–borne, and the virus is transmitted from live infected plants to healthy plants primarily by aphids or other insect vectors.

An integrated management strategy involves the use of both cultural and chemical measures that aim to eliminate any virus sources, minimise aphids and deter aphids from entering the crop. Often by the time aphids are detected, the virus spread has already occurred.

Figure 8: Lupins in standing stubble as early ground cover. This is particularly important in wide row pulses.

Management steps include:

* Minimise the “green bridge” for virus and aphid survival over summer. Control volunteer pulses, legumes and weeds well before sowing and early crop weeds that may carry viruses and aphids.

* Minimise bare earth through sowing into previous cereal stubbles and early sowing with adequate plant population (germ & vigour test seed). Use narrow rows in the absence of stubble to minimise exposed bare soil to deter aphids entering the crop.

* Avoid crop stress through good paddock selection (soil type, no hard pan, low weed burden) adequate nutrition, no herbicide stresses and good inoculation.

* Consider a seed treatment with Gaucho® 350SD which is now registered to protect faba bean, field pea and lentils from early season aphid attack. It is compatible to tank mix with inoculants and does not affect the viability of the rhizobia.

* Avoid sowing pulses close to each other and broadleaf crops such as canola, and be aware of proximity to perennials (e.g. lucerne).

* Monitor crops and neighbouring areas using a sweep net or beat sheet. Yellow sticky traps on crop perimeters can also be a handy check for aphid presence. Identify the species present and be prepared to use a ‘soft’ insecticide such as pirimicarb if there is a chance of localised flights.

* Use of ‘soft’ insecticides soon after emergence has been shown to help control persistently transmitted viruses only. Use of an SP is controversial as while it prevents early colonisation due to ‘anti feed’ properties, it can also agitate aphids not controlled and increase virus spread. It should not be used when green peach aphid is present as this major vector for BWYV has resistant populations. Impact on natural beneficials could also lead to higher aphid build up.

REFERENCES & FURTHER READING:

* VicGov Agnotes http://agriculture.vic.gov.au/agriculture/grains-a... Agriculture and Food > Fact Sheets > Agnote name

o Temperate Pulse Viruses: Alfalfa Mosaic Virus (AMV) - Angela Freeman & Mohammad Aftab Vic DPI

o Temperate Pulse Viruses: Cucumber Mosaic Virus (CMV) – Angela Freeman & Mohammad Aftab Vic DPI

o Temperate Pulse Viruses: Bean Lafroll Virus (BLRV) – Angela Freeman & Mohammad Aftab Vic DPI

o Seed Health Testing in Pulse Crops - . Angela Freeman & Mohammad Aftab Vic DPI

o Cucumber mosaic virus of lupins – Ivan Mock Vic DPI

* Winter Crop Variety Sowing Guide – NSW DPI www.agric.nsw.gov.au

* Grain Legume Handbook 2008 (sections 6 & 7) – Pulse Australia etc.

* Viral Diseases of Pulses in Western Australia - Dept of Agriculture and Food WA www.agric.wa.gov.au

* Viruses in pulse crops in 2009 and planning for 2010. - Angela Freeman, Mohammad Aftab and Helen Richardson, VicGov. GRDC Update paper Vic 2010

* Viruses in pulse crops – Jenny Davidson, SARDI, Mohammad Aftab & Angela Freeman, Vic Gov. GRDC Update paper SA 2010

* Aphids, natural enemies and smarter management. – Kym Perry, Ken Henry, & Judy Bellati, SARDI, Stuart McColl, CESAR Consultants, and Paul Umina, CESAR. GRDC Update paper SA 2010

* Aphids, natural enemies and smarter management. – Stuart McColl, CESAR Consultants Paul Umina, CESAR, Kym Perry, Ken Henry, & Judy Bellati, SARDI. GRDC Update paper Vic 2010.

AKNOWLEDGEMENTS:

Wayne Hawthorne and Trevor Bray formally of Pulse Australia.

Joop van Leur (NSW DI & I), Angela Freeman and Mohammad Aftab (VicGov), Jenny Davidson (SARDI),

Key contacts

Last updated: 20 November 2015