BARLEY YELLOW DWARF VIRUS WIDESPREAD IN NORTHERN NSW IN 2009

Background

Barley yellow dwarf virus (BYDV) is the most widespread and damaging virus disease of winter cereals worldwide. BYDV has a wide host range which includes all the winter cereals (wheat, barley, oats, durum and triticale) and over 150 grass species. BYDV is NOT seed-borne and survives from one season to the next in over-summering grasses. BYDV is generally spread to cereal crops in autumn and early winter by aphid flights from infected grasses which transmit the virus when they feed. BYDV affects the vascular system of infected plants restricting water and nutrient movement up the stem. In sensitive wheat, barley and oat varieties WA research has shown that plants infected before the end of tillering are stunted with up to 50% yield loss. Early infections can also result in increased screenings. In contrast, later infections post-tillering in the same varieties had little impact on yield or quality.

BYDV in 2009
Agronomists reported that leaf symptoms characteristic of BYDV were widespread in northern NSW in 2009. Incidence of infected crops was particularly high on the Liverpool Plains, New England Tablelands and around Tamworth. BYDV was first reported in grazing oat crops in late July around Coolah in the very susceptible variety Eurabbie. Symptomatic plants were submitted to the diagnostic service at Tamworth throughout the season and BYDV confirmed by ELISA at I&I NSW Diagnostics Laboratory at Camden. BYDV was positively confirmed in six oat crops from around Coolah, Boggabri, Gunnedah, Tamworth, Glen Innes and Moonie (Qld). BYDV was also confirmed in samples from a cv. Crusader  and Ventura bread wheat crop and Jandaroi  durum crop on the Liverpool Plains and a cv. Ventura crop at Narromine. BYDV was also confirmed in a range of bread wheat and durum varieties displaying leaf symptoms in variety trials at Tamworth (I&I NSW) and Blackville (Liverpool Plains, Pursehouse Rural).

Symptoms
BYDV symptoms take at least three weeks to appear after infection and can easily be confused with symptoms of nutrient deficiency, water logging or other stresses that cause yellowing, reddening and striping of leaves. However, BYDV can usually be easily distinguished from other problems based on distribution within the paddock. Nutrient problems tend to be fairly uniform across paddocks or related to soil types while water logging is generally more severe in lower lying areas. BYDV is first introduced into a crop by winged aphids flying in from outside. These flying aphids may infect individual or groups of plants randomly throughout a paddock which creates primary infection points. These initial winged aphids however, only produce wingless offspring which can only crawl to adjacent plants and the infection spreads outwards in an approximate circle, radiating from the original infected plant. Hence, BYDV first appears as patches of yellow or red stunted plants which increase in size over time and when infection is high, patches merge which can result in complete crop infection. BYDV is often also first found along the edge of crops adjoining pastures, other crops or grass weed patches along fence lines where wingless aphids have crawled into the crop and spread infection.
When infection occurs early in crops development (before end of tillering) stunting can occur and is sometimes quite severe. Leaf symptoms vary between wheat, barley and oats with severity dependent on time of infection and variety susceptibility.

Wheat
In young leaves slight to severe yellowing or pale striping between veins (interveinal chlorosis) occurs. Leaf tips may also die. In some wheat varieties there is yellowing and in others there is reddening of older leaves which is particularly evident in the flag leaf.

BarleyInfection causes a characteristic bright yellowing of leaves (particularly older leaves) and interveinal chlorosis starting from the leaf tips and moving towards the base. In some varieties, reddening of leaf tips may also develop. Late infections do not result in severe stunting but young leaves may turn yellow.

OatsBYDV symptoms are most striking in susceptible oat crops. Infected oats develop reddening (crimson-pink) in leaves from the tips down, which sometimes begins as blotches especially in older leaves. Young leaves often have yellow interveinal chlorosis. Stunting of plants and white sterile florets are also quite distinctive symptoms in oats. Late infections usually result in characteristic reddening of new leaves.

Generally a fairly characteristic leaf symptom in all cereals is that the yellowing/reddening extends from the leaf tip down and usually runs down the edges of the leaves faster and further. Hence, a green wedge remains in the middle of the leaf. Further, be aware that BYDV symptoms are much less obvious through winter.

Disease cycleBYDV requires living plant tissue to survive and cannot survive in stubble, soil or seed. BYDV has a very wide host range in the grass family. It survives between cropping seasons (‘green bridge’) in volunteer cereals, annual and perennial pasture grasses and grass weeds. Grasses and/or cereal volunteers also harbour the aphid vectors over summer. Autumn and early winter aphid flights from infected grasses establish infections in cereal crops.

BYDV is usually worse in seasons with a wet summer followed by mild winters. Wet summers encourage summer growth of grasses and cereal volunteers which increases the build-up of both BYDV and aphid vectors. This is more likely to result in early migration of higher aphid populations into crops. Because the aphids have built-up on infected grasses the proportion of aphids carrying BYDV into crops will be much higher and results in more extensive autumn spread. Mild winters favour the further development and spread of aphid populations within crops. However, considerable BYDV can still be caused by relatively few infective aphids in a crop if they arrive early in the growing season and move around a lot as appeared to have happened in 2009.

Although several aphid species can transmit BYDV, three aphids common in northern NSW, the oat aphid (Rhopalosiphum padi), corn aphid (R. maidis) and rose grain aphid (Metopolophium dirhodum) are all documented to transmit BYDV. In other states the oat and corn aphid are generally considered to be the main vectors. Infection in a plant is restricted to the vascular system (phloem) and once an aphid acquires the BYDV after feeding on an infected plant it will continue to transmit the virus to any plant it feeds on for its entire life. Aphids need to feed on an infected plant for at least 15 minutes before the virus is either acquired or later transmitted to a healthy plant as they need to probe deep into the phloem. Hence, insecticides with anti-feeding activity (foliar sprays of synthetic pyrethroids or imidacloprid seed treatment) are quite effect at reducing the spread of BYDV.

ManagementThere appears to be limited information on the management of BYDV in northern NSW so the following information is based on recommendations from trial work conducted in WA and Victoria.

The best way to control BYDV is to sow tolerant varieties but if they are not an option then management of aphid activity especially early in the season to limit spread of BYDV is critical.

Tolerant varieties
Tolerant or resistant varieties when available are the best option for reducing losses to BYDV. There is reasonable information on the relative tolerance of oat varieties in NSW with Bass  and Nile having good tolerance while varieties like Cooba, Coolabah and Yiddah  are rated as moderately tolerant (Refer to NSW Winter Crop Sowing Guide). NSW information on tolerance of barley varieties to BYDV does not appear to exist but Gairdner  is considered to have moderate resistance (MR) in other states while Commander  and Hindmarsh  are susceptible (S). Rating of other barley varieties is unknown. In terms of bread wheats and durum there simply appears to be no information available on the relative tolerance of varieties commonly grown in northern NSW to BYDV.

BYDV was evident and confirmed in a seed increase trial at Tamworth in 2009. Symptomatic and non-symptomatic plants in three durum and nineteen bread wheat varieties were tagged during flowering. These samples were harvested separately and are being used to look at the relative impact of BYDV on these varieties commonly grown in northern NSW.

Insecticides
It is vital to prevent spread of BYDV by aphids during the first 8-10 weeks after crop emergence, as this is when plants are most vulnerable. When aphids start migrating into crops, insecticides can be used to kill these and deter further arrivals. In WA this usually means applying insecticides based on risk before aphids and/or BYDV symptoms are evident, at 3 and 7 weeks after crop emergence. This is because BYDV symptoms are usually not obvious until 3 weeks after the aphids feed on plants, meaning the aphids may have moved on prior to their presence and the problem being noticed. Considerable BYDV spread can also occur even when aphid numbers are low and symptoms can be hard to see in winter.

Five years of trial data from WA has shown that foliar sprays of synthetic pyrethroids such as alpha-cypermethrin at 3 and 7 weeks after full crop emergence decreased incidence of BYDV by 87% and increased yield by up to 41%. Pyrethroids have been found to control BYDV better than pirimicarb or dimethoate in WA because they not only kill aphids present at application but also having an anti-feeding effect that deters new aphids from feeding for a further 3-4 weeks.

Imidacloprid seed dressing also gave good early season control of aphids and hence BYDV in WA. Trial work by I&I NSW and Northern Grower Alliance over the last two seasons in northern NSW has found that imidacloprid seed treatments provide around 70-90 days of aphid control. A second seed treatment with a higher rate of imidacloprid appeared to provide even longer aphid control. BYDV was not evident in these trials so impact of aphid control on virus transmission could not be determined.

In years conducive to aphid build-up a follow-up insecticide application in spring, with both the early foliar or seed treatment strategies, may be required to limit feeding damage. The effect of late BYDV infection by itself is generally not sufficient to warrant spraying in spring so the decision should be purely based on aphid pressure.

Delayed sowing
Delayed sowing avoids the main autumn peak of aphid flights can significantly reduce the incidence of BYDV. However, other yield penalties associated with late sowing make this option generally considered a poor choice over using insecticides in other states.

Green-bridge
Spraying out of volunteer cereals and grass weeds within paddocks is important from a moisture/nutrient conservation perspective in northern NSW and may also reduce the source of infection. Spraying out perennial grasses near and around cereal paddocks at least three weeks prior to sowing may also reduce aphid numbers but would usually be impractical.

Acknowledgements
Funding for virus testing of suspect plant samples was provided through I&I NSW diagnostic surveillance. The authors kindly thank I&I NSW district agronomists and commercial agronomists for assisting with surveillance and sample submission in 2009. Further information can be found through the Department of Agriculture and Food –WA website at:

www.agric.wa.gov.au.
Contact details
Steven Simpfendorfer
I&I NSW Tamworth
Ph: 02 6763 1261
Email: steven.simpfendorfer@industry.nsw.gov.au