Beet western yellows virus (synonym: Turnip
yellows virus) and green peach aphid in canola
Jenny Davidson4, Brenda Coutts1, Roger Jones2, Paul Umina3, Greg Baker4, Mohammad
Aftab5
1Department of Agriculture and Food Western Australia, 2The University of Western
Australia, 3cesar, 4South Australian Research and Development Institute and 5Victorian
Department of Environment and Primary Industries
Why was the project undertaken?
Widespread infestations of green peach aphids (GPA) (Myzus persicae) during autumn and winter of
2014 contributed to an outbreak of Beet western yellows virus (BWYV, syn. Turnip yellows virus) in
southern Australia. Canola crops across the lower and mid north regions of South Australia, the Eyre
Peninsula, western Victoria and some parts of NSW have been severely affected by the virus. GRDC
and SAGIT provided emergency funding to respond to this outbreak.
How was the project done?
Symptomatic plants from 618 canola crops (290 from SA) were submitted by agronomists to VIC DEPI
Horsham for virus testing. Interactions with crop management were observed. Approximately 50 GPA
samples were collected by researchers, advisors and growers in autumn-spring 2014, and
subsequently screened at cesar for insecticide resistance viz. pyrethroids, carbamates and
organophosphates. These populations spanned NSW, Victoria and SA. Results of crop surveys for
BWYV and yield loss trials conducted in WA from 1998 are also presented.
Key messages
• Beet western yellows virus (BWYV, syn. Turnip yellows virus) is widespread throughout grain
growing areas of Australia.
• Early BWYV infection in canola can cause seed yield losses of up to 46%, decrease seed oil content
and increase seed erucic acid and glucosinolate contents.
• Wild radish weeds and volunteer canola are the most important reservoirs but perennials such as
lucerne and many weeds species can also be infected.
• Epidemics are likely to occur when aphids are present early in the season (green bridge prior to
seeding, warm temperatures). GREEN BRIDGE MUST BE CONTROLLED.
• Integrated management strategies have been devised for BWYV in canola.
• Green peach aphid (GPA), the main vector for BWYV, has a high prevalence of resistance to
insecticides. Growers should implement resistance management strategies for insecticides
including neonicotinoids.
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BWYV Observations in South Australia 2014
By mid-September agronomists and researchers across southern Australia had submitted samples
from 618 canola crops for BWYV of which 57% were positive. 290 of these samples were from South
Australia where 86.55% were positive (Table 1). Over 90 weed samples were also submitted for testing
from South Australia and results demonstrated the wide host range for BWYV (Table 2).
Crop management observations made in 2014 are summarised here, but still need to be confirmed
via the management survey being undertaken within the GRDC project DAS00151.
Severity of virus infection was reduced by standing cereal stubble, higher crop density, later sowing
date, seed treatments containing neonicotinoids (e.g. imidacloprid Gaucho®) and where a 10-14 days
break was provided between weed control and sowing.
Yield loss from BWYV infection ranged from 75% loss to negligible effects across South Australia.
Approximately half the crops in lower and mid north region of South Australia yielded 1 kg/ha i.e. half
the normal yield. This was due to BWYV, diamondback moth, frost and waterlogging.
BWYV studies in Western Australia
From surveys of canola crops in WA in 1998, BWYV was found to be the predominant virus infecting
these crops with wild radish (Raphanus raphanistrum) and volunteer canola being identified as major
virus reservoirs. Interestingly, over-summer surveys of weeds found BWYV infecting four other weed
species including wild melon (Citrullus lanatus), fleabane (Conzya spp.), stinkweed (Navarrentia
squarrosa) and blackberry nightshade (Solanum nigrum). In addition seven aphid species were also
found over-summering on weeds. In other surveys, BWYV was also found infecting other crop or
pasture species including mustard, vegetable brassicas, chickpea, faba bean, field pea, lucerne, medic
and subterranean clover. Some native plant species were also infected. BWYV is also found
widespread elsewhere in southern and eastern Australia.
From field trials when canola plants became infected by GPA very early and the BWYV incidence in
crops reached 96-100%, overall seed yield losses were up to 46%. For every 1% increase in the level
of BWYV in a canola crop, there was a 6-12kg/ha decline in yield resulting from formation of fewer
seeds. BWYV infection diminished seed quality by decreasing oil content and increasing erucic acid
and glucosinolate levels. If the crop becomes infected late, spread was much less and yield and quality
losses were minimal.
From field and glasshouse experiments, several canola genotypes (including varieties Tranby and
Trigold) were found with useful infection resistance to BWYV. However, given the often rapid turnover of canola varieties, continued evaluation of new varieties and identification of additional parental
lines of BWYV resistant varieties is required.
In field experiments, use of recommended rates of imidacloprid applied commercially often gave
relatively poor BWYV control. In glasshouse experiments, use of imidacloprid seed dressing applied
commercially was also only partially effective due to not covering all seeds. Care must be used to
avoid development of imidacloprid resistance in GPA from unnecessary or inappropriate use of this
neonicotinoid chemical group.
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2014 GPA Resistance Studies
Methodology
Approximately 50 GPA samples were collected by researchers, advisors and growers in autumn-spring
2014, and subsequently screened for insecticide resistance. These populations spanned NSW, Victoria
and SA. DNA testing by cesar was carried out using TaqMan assays to screen: (i) the kdr and/or superkdr mutations that confer resistance to pyrethroids, (ii) the MACE mutation that confers resistance to
carbamates, and (iii) the overproduction of one of two closely related carboxylesterases (E4 and FE4)
that confers resistance to organophosphates. In each case, 20 individual aphids from each population
were screened.
Results and discussion
Testing revealed widespread resistance to the three chemical groups tested. Following DNA tests,
aphids from all populations were found to be resistant to synthetic pyrethroids, including bifenthrin
and alpha-cypermethrin, and to carbamates, including pirimicarb. The use of these insecticides is not
expected to provide control against these GPA populations in most field situations. The mechanism of
resistance to pyrethroids is also likely to render these products ineffective as an anti-feed.
The story for organophosphates, such as dimethoate and chlorpyrifos, is more complex. The amplified
carboxyl-esterase mechanism leads to organophosphate resistance in GPA. This mechanism is unusual
because it is regulated by DNA methylation, and can be ‘switched on’ in response to pesticide
exposure. As a result, aphid populations can quickly adapt to survive organophosphates, even though
they may have recently been effective. Following DNA tests, all aphid populations tested were found
to contain resistance at the esterase genes. Aphids from these populations are expected to have a
moderate level of resistance (5-20-fold) to organophosphates.
The field efficacy of organophosphates remains uncertain. Some control may be achieved against
populations found to be ‘resistant’, although the use of organophosphate insecticides is risky and may
not be effective (particularly if the population has been exposed to insecticides already this season).
When faced with GPA populations with known resistance to organophosphates, growers are advised
to spray test strips within paddocks to determine field efficacy.
In many cases in 2014, growers were unable to achieve adequate control of GPA populations due to
the prevalence of insecticide resistance. High levels of resistance to carbamates and pyrethroids are
now confirmed to be widespread across Australia. Moderate levels of resistance to organophosphates
have been observed in many populations, and there is evidence that resistance to neonicotinoids (e.g.
imidacloprid) is evolving. Growers and advisors should implement resistance management strategies
for GPA. Further information can be found at the following sites:
http://ipmguidelinesforgrains.com.au/ipm-information/resistance-management-strategies/
http://www.grdc.com.au/Resources/Factsheets/2014/03/Resistance-management-for-green-peachaphids
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Management of BWYV and GPA
An integrated disease management approach using control measures that operate in different ways
is needed to control BWYV and GPA in canola crops:
Control wild radish weeds and canola volunteers. This helps to minimise potential aphid and virus
infection sources near the crop.
Allow a 7-14 day period between weed control and sowing the crop. This reduces the transfer of aphids
from weeds directly onto the seedling crop.
Manipulate sowing dates. Avoid exposure of young canola seedlings to peak aphid flights by delayed
sowing.
Sow at high seeding rates. High plant density helps diminish the rate of virus spread and speeds up
canopy closure resulting in lower aphid landing rates.
Sow varieties with infection resistance. Some varieties will only become infected at low levels, while
others have moderate resistance to BWYV infection. Growers need to be aware that such varietal
differences do exist.
Sow into standing stubble to reduce aphid landings. This will reduce the likelihood of virus infection as
aphids will prefer to land elsewhere.
Use insecticide seed dressings. Insecticide seed dressing applied effectively can provide early control
of GPA at the vulnerable seedling growth stage. However, to achieve effective control application it
must be ensured that sufficient insecticide sticks to each seed.
Observe appropriate use of foliar insecticides for control of aphids to avoid exacerbating insecticide
resistance issues and to protect beneficial insects, including predators and foraging honey bees.
Other resources:
https://www.youtube.com/results?q=groundcover+TV+15
www.eXtensionAUS.com.au
Acknowledgements
Bill Kimber, Helen de Graaf, Kym Perry and Ken Henry (SARDI): Industry meetings and survey of
management practices affecting BWYV in canola.
Michelle Russ and Marzena Krysinska-Kaczmarek (SARDI): Preparation of virus samples.
Siobhan de Little and Anthony van Rooyen (cesar): GPA resistance testing.
Katherine Hollaway (Vic DEPI): Map of BWYV incidence from virus testing
(www.eXtensionAUS.com.au).
Murray Sharman (QDAFF): PCR tests to confirm identity of Turnip
Yellows Virus.
Sam Holmes (Holmes Farm Consulting): Assistance with GRDC and
SAGIT Project applications.
All the growers, advisors and other industry people who submitted
plant and aphid samples for testing.
Page | 4
Table 1 Incidence of BWYV in canola for South Australian regions in 2014 determined by virus testing
of plant material at Horsham VicDEPI.
SA District
Lower North
Mid North
Upper North
Lower Eyre Peninsula
Mid & Upper Eyre Peninsula
Mallee SA
Yorke Peninsula
South East
Total (290)
Number of canola BWYV tests in 2014
Positive
Negative
58
4
10
0
17
1
36
18
36
8
22
1
56
2
16
5
251
39
86.55%
13.45%
Table 2 Incidence of BWYV virus in weed species sampled by agronomists in and around South
Australian canola crops in 2014 determined by virus testing of plant material at Horsham VicDEPI.
*A negative result in the table below does not infer that these species are not hosts of BWYV.
Number of positive
samples
Average infection
rate
Long fruited Turnip,
Wild Turnip, Turnip
17/17
83.60%
Marshmallow
5/22
4.20%
Indianhedge Mustard,
Mustard
3/13
7.20%
Sowthistle
1/3
37%
Yellow Burr Weed
2/2
100%
Milk Thistle
1/2
50%
Salvation Jane
1/1
100%
Oxtongue
1/1
100%
*Other weeds tested Barley grass, bedstraw, charlock, dandelion,
as uninfected
geranium, London rocket, Medic, ryegrass,
soursob, sowthistle, wild oats, short fruited
turnip, capeweed, vetch
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Table 3 Results of visual assessment of virus symptoms (% leaf area diseased of lower canopy) and
BWYV virus testing (% infection rate) at Roseworthy NVT trial, rated and sampled on 6th August 2014.
Virus tests were conducted at Horsham Vic DEPI. Different letters indicate significant differences
(P<0.001)
TT varieties
Mean Virus test
lsd
Page | 6
30 a
66.67 b
80 bc
80 bc
83.33 bcd
91.67 bcde
91.67 bcde
91.67 bcde
93.3 de
93.33 de
93.33 de
93.33 de
93.33 de
93.33 de
95 de
95 de
96.67 de
98.33 de
100 e
16.25
Visual %LAD
3.3 a
51.7 cde
35 abcd
48.3 cde
55 cde
45 bcde
33.3 abcd
55 cde
63.3 de
58.3 de
36.7 abcd
18.3 abc
8.3 ab
31.7 abcd
45 bcde
26.7 abcd
75 ef
55 cde
100 f
lsd
38.05
IMI varieties
Mean Virus test
13.33 a
26.67 a
50 b
71.67 c
76.67 cd
78.33 cd
78.33 cd
85 cd
90 cd
91.67 cd
93.33 d
95 d
96.67 d
lsd
20.22
Visual %LAD
53.3 ab
28.3 a
100 c
31.7 a
86.7 bc
100 c
100 c
76.7 bc
91 bc
98.3 c
93.3 bc
80 bc
100 c
lsd 43.48