Diagnosing virus damage in field peas

It can be difficult to distinguish plant disease symptoms caused by viruses in pea plants, as viral foliage symptoms are often similar to those caused by nutritional deficiencies, herbicide damage or waterlogging.

What to look for

Paddock

Seed-borne virus infection causes stunted plants that are scattered thoughout the crop and act as within crop infection foci for virus spread by aphids.
When the virus source is external to the crop, plants often become virus-infected first and in greatest numbers close the the paddock edges or around thin or bare areas, particularly on the paddock,s windward side.
Patches of virus infection are common with infection spreading out from initial infection foci in their centres.

Plant

The intensity and types of virus symptoms depend on the strain of the virus, climatic conditions (especially temperature), and stage of plant growth at infection.
Depending on the virus, field pea plants can develop various degrees of stunting and mature later than healthy plants
In plants infected early by aphids and in plants infected via seed, all the foliage may show symptoms, which vary from pale and mild mosaic to necrosis and plant death depending on the virus present.
Plants infected later often have normal older leaves. When a virus causes mild symptoms in foliage, typical symptoms are often most clearly visible in young leaves.
Peas are rarely colonized by aphids under WA conditions

Where did it come from?

Insect vector

Green bridge

Viruses infecting pulses in Western Australia are either transmitted persistently or nonpersistently by aphids
Persistently aphid-borne viruses: The common virus in in this category in WA is BWYV. Other viruses in this category have not been reported in peas in WA but occur in the eastern states in peas. Persistent transmission means that when an insect vector feeds on an infected plant, the virus has to pass through the its body and lodge in its salivary glands before it can be transmitted to a healthy plant, a process that takes more than a day. Once the insect is infectious it remains so for the rest of its life. Important aphid vectors such as the green peach aphid tends to colonize the hosts they transmit the virus to. Because acquisition of the virus is slow insecticides that kill aphids work well in suppressing virus spread. Luteoviruses are not seed-borne and require a continuous "green bridge" of host plant material, such as an infected lucerne pasture or broad-leafed weeds surviving in isolated wet spots over summer, in order to pass from one crop to the next.
Non-persistently aphid borne viruses are seed transmitted to varying extents. PSbMV is the most common and economically damaging virus that reduces yield and seed quality in peas. The other ther viruses in this category infecting pea in WA are AMV, BYMV, and CMV. Non- persistent transmission means that the insect vector can land on a virus infected plant, make a brief probe, acquire the virus on its mouth parts within seconds and then transmit it immediately when probing on a healthy plant. The aphid loses the virus after it probes a healthy plant one or two times. After this, the insect does not infect further plants. The whole process is so quick that 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 colonize legumes but just land and probe pulse crops while searching for their preferred hosts, such as oat and turnip aphids.
Close proximity to a substantial external virus reservoir, seed infection and high summer and autumn rainfall before the growing season are the most important factors that predispose pea crops to severe virus infection. Summer and autumn rainfall stimulate early growth of pastures, weeds and crop volunteers upon which aphids build up before the growing season starts. This results in early aphid flights to newly emerged crops and early virus infection. The infected plants then act as reservoirs for further spread of infection within the crop so the final virus incidence is high. In contrast, dry starts to the season and minimal virus sources result in little virus spread and absence of any economic losses.

Management strategies

Clean seed

Rotation

Weed and pasture control

Resistant varieties

Practices that minimise virus damage are;
Where the virus source is internal, especially with seed-borne PSbMV, sow healthy seed stocks to minimise initial infection sources within the crop.
Where the virus source is external, sow non-host barrier crop strip and spray pastures adjacent to the crop. These measures decrease virus spread into crop from an external source.
Sow at high seeding rates to generate high plant densities and promote early crop canopy development. These measures minimise virus infection sources (seed-infected and/or early infected plants), diminish aphid landing rates, and dilute numbers of infected plants.
Sow at narrow row spacing and maximise retained stubble. These measures diminish aphid landing rates before the crop canopy develops.
Do not plant peas after another pulse to avoid volunteer seed-borne pulse infection sources within crop.
Maximise weed control to minimise potential weed virus infection sources within crop.
Sow early maturing varieties to decrease final infection incidence, especially in prolonged growing seasons.

Further information

Pulse Australia [1]