Manage the Risk of Beet Western Yellow Virus
(BWYV) and Other Diseases in 2015
Frank Henry1, Jenny Davidson2 and Paul Umina3
1Department of
Economic Development, Jobs Transport and Resources (DEDJTR), 2South
Australian Research & Development Institute (SARDI), 3cesar
Summary
Significant summer rainfall over much of the southern cropping zone during January 2015 has
stimulated plant growth providing a "green bridge" for last year’s insect pests and diseases to move
into this season. Important pests and diseases to watch out for in 2015 include Beet western yellows
virus (BWYV), the green peach aphid (GPA) (Myzus persicae) and the cereal rusts. Significant rainfall
events over the summer will also alter inoculum levels of soil borne diseases like crown rot, rhizoctonia
root rot and take-all.
BWYV is of particular concern as widespread infestations of GPA contributed to an outbreak of BWYV
in southern Australia during autumn and winter of 2014. The virus severely affected canola crops
across South Australia, Victoria and some parts of NSW, leading to significant yield losses in 2014.
BWYV is not seed borne and is transferred into canola crops by aphids carrying the virus. GPA is the
principal vector of BWYV, although other aphid species like the cabbage aphid can transmit the virus
at a low level. While further rain and mild temperatures during summer and autumn this season are
necessary for an increase in the GPA populations and the associated virus risk, early weed control will
minimise the risk.
Growers should continue to pay attention to weather conditions (rainfall and mild temperatures)
leading into the growing season as this will provide a good indication of the likely risk of GPA and
BWYV, as well as the cereal rusts and soil borne diseases.
Key Messages
• Substantial summer rains have increased the risk of GPA, BWYV and cereal rusts for 2015.
• Remove the "green bridge" over summer and autumn prior to sowing to reduce the risk of
BWYV and its vector the green peach aphid (GPA), as well as cereal rusts.
• Use a current disease guide to check the varietal rust ratings before sowing, and take
appropriate action if sowing cereal varieties susceptible to rust.
• To reduce the risk of BWYV, do not sow canola into desiccating weeds/canola volunteers as
aphids will move directly from the weeds to the emerging seedlings.
• Monitor GPA populations on weeds, volunteers and seedling crops.
• For early control of GPA use seed treated with a neonicotinoid insecticide, and if farmer
applied, ensure proper application and coverage of seed dressing for efficacy against GPA.
• Ensure correct identification of GPA before applying insecticides to protect seedling canola.
• Use a PreDicta B soil test before sowing to identify paddocks at risk of soil borne diseases.
Page | 1
Beet western yellows virus in canola
In 2014 canola crops across the lower and mid north regions of South Australia, the Eyre Peninsula,
Victoria and some parts of NSW were severely affected by the BWYV, leading to significant yield losses.
The severity of the BWYV outbreak was most likely due to a combination of the following factors:
• Summer rainfall which resulted in a "green bridge" of weed hosts of aphids and BWYV, and
aphid flights to areas without a "green bridge".
• The early start to the season and early sowing.
• Very mild autumn conditions which contributed to early (and extended) levels of aphid activity
through until late June.
• Crop management practices including short intervals between weed control and sowing of
the crop.
• The prevalence of insecticide resistance in GPA (particularly to pyrethroids, organophosphates
and carbamates).
• The low proportion of canola seed treated with neonicotinoids i.e. Gaucho® (imidacloprid)
and Cruiser Opti® (thiamethoxam + lamda-cyhalothrin), in some areas.
• Poor coverage of insecticide on canola seed which reduced effectiveness.
Paddock management when sowing into high virus risk
• Control the "green bridge", particularly volunteer canola, wild radish, wild turnip,
marshmallow and other broadleaf weeds, which host BWYV and GPA. Apply appropriate
herbicides soon after weeds begin germinating. Re-apply herbicides following any further
rainfall events that encourage additional weed germination. Ensure paddock is weed free for
10-14 days before sowing.
• Use insecticide seed dressing with a neonicotinoid. Ensure proper application and coverage
of the seed dressing for efficacy. This will protect seedling crops from aphid infestation for
approximately 6 – 8 weeks after sowing.
• Do not sow crops early in the season, sow at normal dates, to avoid early aphid flights.
• Sow into standing stubble, where feasible, to reduce aphid landings.
• Sow at high seeding rates; high plant density reduces aphid landing rates.
• Monitor seedling crops before the rosette stage for aphids and symptoms of virus. Aphids can
be difficult to find when their presence is low, but GPA is often found on the underside of the
oldest leaves. Control aphids if required.
• Ensure correct identification of GPA before considering chemical sprays. Cesar and GRDC have
recently developed a Backpocket guide to aid in the identification of crop aphids. GPA is most
likely the only aphid species on juvenile canola plants, while other aphid species are present
in spring.
Page | 2
Controlling GPA using chemicals
High levels of resistance to carbamates (e.g. pirimicarb) and pyrethroids (e.g. bifenthrin and alphacypermethrin) are now confirmed to be widespread across Australia. Moderate levels of resistance to
organophosphates (e.g. dimethoate and chlorpyrifos) have been observed in many populations.
Growers should consider using Transform® when aphid numbers have reached threshold levels, but
this product must be used as part of a broader resistance management program. While Transform®
will control GPA populations that are tolerant to carbamates, pyrethroids and organophosphates, it
has – as most insecticides – only a limited systemic action. In order to control GPA populations
colonising the older canola leaves, applications should be made before crop closure and using
appropriate spray technologies. Transform® is not registered for use in pulse crops.
A Resistance Management Strategy for GPA in Australian grain crops was developed in 2014. This can
be downloaded directly from the National Invertebrate Pest Initiative (NIPI).
It is of utmost importance to only use insecticides registered for the crop and situation and comply
with all label directions for the application method. Do not exceed application or frequency rates. In
addition, the decision to spray should also consider the proximity and impact on local beehives and
that beekeepers are provided with sufficient advanced notice so that bees can be withdrawn if
necessary.
Soil borne diseases
Soil borne diseases can have serious impacts on grain yield in the absence of adequate control. The
key to preventing these diseases is to identify paddocks at risk by inspecting the roots of previous
cereal crops or by using a PreDicta B soil test prior to sowing. Knowledge of the potential diseases in
a paddock then enables the most appropriate control strategies to be implemented prior to and/or at
sowing. Management must be implemented prior to sowing as there are no in-crop management
options available for the control of soil borne diseases.
Crown rot will be an important disease during 2015 if the season finishes with a dry spring as inoculum
levels are high from the 2014 season. Reduce risk by rotating to non-cereal crops.
The crown rot fungus can survive for up to two years on infected cereal stubble from previous cereal
plants, volunteer plants or grass weeds. This is why stubble management is an important aspect in the
control of crown rot. Crown rot survival is reduced by summer rains that increase stubble breakdown,
while infection early in the season is favoured by moist, humid conditions. Dry conditions in the spring
will also favour crown rot as the fungus grows faster in stressed plants.
Rhizoctonia root rot will likely be a low risk in 2015 if there is a wet summer with multiple rainfall
events, provided summer weeds are controlled.
Soil moisture plays an important role in rhizoctonia survival. The fungus is capable of growing below
permanent wilting point and can survive dry summers. This gives rhizoctonia a competitive advantage,
and disease damage is often severe following dry summers. However, summer rainfall (more than 30
mm in a week) will reduce inoculum as the fungus does not compete well with other soil
microorganisms in a warm and moist environment. Multiple rainfall events over summer and autumn
can reduce inoculum levels to low risk.
Though, where there are long periods between rainfall events (about four weeks) inoculum levels can
recover. This can make it difficult to predict the amount of rhizoctonia inoculum in the soil prior to
Page | 3
sowing. Fortunately, a PreDicta B soil test can be used to determine the amount of rhizoctonia
inoculum in the soil and identify high risk paddocks before sowing.
Take-all will be a low risk in 2015 as the dry spring in 2014 would have limited inoculum build up, while
rainfall during January 2015 will have reduced inoculum further.
Take-all can only survive between susceptible crops in the root and tiller bases of previously infected
plants. Following autumn rains, the take-all fungus grows out of this material and attacks the roots of
susceptible plants. In the absence of hosts, the fungus is unlikely to survive for more than one year.
The build-up of take-all is greater during wet springs, but its carry over is reduced following significant
summer rain events (more than 25 mm in a single event). Host plants that germinate during summer
will generally not increase take-all levels if soil temperatures remain above 25°C.
Cereal cyst nematode (CCN) will be a very damaging if nematode populations are allowed to increase
by growing susceptible cereal varieties.
A PreDicta B soil test before sowing can be used to identify paddocks that are most at risk of soil borne
diseases like crown rot, rhizoctonia root rot, take-all, root lesion nematode and cereal cyst nematode.
Cereal rusts
Self-sown volunteer wheat plants that grow during the summer and autumn can carry over inoculum
of cereal rusts. In seasons when wheat volunteers are widespread during the summer and autumn,
remove volunteer wheat plants "the green bridge" that will support rust inoculum in the 6 weeks prior
to sowing. In high risk years, increased emphasis should be placed on the management of rusts using
resistant varieties, seed or fertiliser treatments, as well as on monitoring crops for disease to ensure
the timely application of foliar fungicides.
A new strain of wheat leaf rust was identified in 2014. It has changed the rust resistance ratings of a
number of wheat varieties. Provisional resistance ratings to the new strain of wheat leaf rust include
Axe S-SVS, Corack SVS-VS, Emu Rock SVS, Grenade CL Plus MSS-S, Mace MS-S, Revenue MSS-SVS and
Wallup S-SVS.
Acknowledgements
Grains Research and Development Corporation (GRDC) and South Australian Grain Industry Trust
(SAGIT) for financial support. Bill Kimba, Ken Henry and Greg Baker (PIRSA-SARDI), Joop van Leur and
Don McCaffery (NSWDPI), Kym Perry (SARDI), Brenda Coutts (DAFWA), Grant Hollaway, Mohammad
Aftab and Piotr Trebicki (DEDJTR), project contributors.
Page | 4
Further information
CropPro diagnostic support for the southern region, DEDJTR
Diagnosing beet western yellow virus in canola (updated 2014), DAFWA
Foliar and soil borne diseases of cereals, eXtension aus
Green bridge control is integral to pest and disease management (2009), GRDC
Green peach aphid and beet western yellows virus – Update (Oct 2014), eXtension aus
Honeybees, PestFacts South East (August, 2014), cesar Australia
Insecticide resistance in GPA across southern Australia (2014), cesar
NVT description of rust resistance ratings
NVT rust resistance ratings and cereal disease guide eXension aus
Pesticides and bees, (2014) DEDJTR (formerly DEPI)
Resistance management for green peach aphids, GRDC
Resistance management strategy for GPA in Australian grain crops (2014) NIPI
Spread of Beet western yellows virus (Sept, 2014), eXtension aus
PreDicta B: a soil analysis service delivered by accredited agronomists. PreDicta B can detect range of
soil-borne diseases. Contact your local agronomist, or to locate your nearest supplier, email your
contact details and location to: [email protected]
Page | 5