Consultant’s Corner*
AN UPDATE ON SPOT FORM NET BLOTCH (SFNB) IN
THE NORTHERN REGION
1. The research view - Greg Platz QDPI&F Hermitage
Spot form of net blotch (Pyrenophora teres f. maculata) is the most widespread
and prevalent disease of barley crops in north-eastern Australia. This situation
has materialized through the widespread adoption of conservation tillage and the
fact that most commercial varieties are susceptible to very susceptible to the
disease.
Spot form net blotch (SFNB) is a classic stubble borne disease. It is an
aggressive pathogen during crop growth but once the crop matures it survives in
the straw as dormant mycelium and exists essentially as a saprophyte. Under
wet conditions, it will resume growth and may produce asexual spores (conidia)
or undergo mating to produce the sexual stage. This manifests as small black
fruiting structures (pseudothecia) which when mature contain many long sausage
shaped structures (asci) each enclosing eight sexual spores (ascospores).
The presence of these raised black fruiting bodies that appear like black pimples
on the stubble are a good indication that net blotch is present in the straw;
however often many of these fruiting bodies are immature and do not contain
ascospores. It would appear that most new season inoculum is in the form of
conidia arising directly from the straw when suitable wet conditions prevail.
SFNB requires free moisture to produce spores and free moisture to infect.
Consequently epidemics are a function of the frequency and duration of wet
periods. Severity of epidemics is further determined by varietal susceptibility and
the amount of initial inoculum.
Once a viable spore lands on a susceptible host in the presence of free moisture,
it infects the leaf in a minimum of 3-4 hours at 20°C. Lesions become apparent a
few days later and appear as small dark spots. These expand over time to
produce the typical SFNB lesions, killing leaf tissue as they grow. Under further
wet conditions infected leaves produce conidia which are responsible for
spreading the disease from leaf to leaf, plant to plant and crop to crop. Conidia
are released as humidity decreases and carried by wind to new infection sites.
When barley is sown into infested stubble, wet conditions trigger the formation
and release of conidia and/or ascospores. These infect the new crop, leaf tissue
is killed, and in the presence of free moisture more conidia are produced and the
cycle continues.
While we have a reasonable understanding of the epidemiology and control of
SFNB, there has been limited research into the disease in north- eastern
Australia. Trials conducted by QDPI&F in 2005 indicate that losses in yield of
21% could be expected from a moderate epidemic in a very susceptible variety.
The effects of SFNB are a product of severity and duration of the epidemic,
varietal susceptibility and possibly soil moisture status. High disease levels
coupled with low soil moisture had severe effects on some crops in the North
Star area in 2007. The interaction between disease level and moisture stress
warrants investigation.
The main thrust of research into SFNB by QDPI&F has been disease control
through genetic resistance. It appears that our most resistant varieties carry
three genes for resistance and these have additive effects. One gene is believed
to confer the majority of resistance while the other two genes contribute minor
effects. Even when the three genes are present SFNB can still infect and cause
leaf damage but lesions are much smaller than would occur in a susceptible
variety and the progress of the disease is greatly reduced.
Currently, there is limited diversity in sources of resistance to SFNB; however
recent genetic studies have identified what appears to be a different source of
resistance and crosses will be made in 2008 to determine if in fact the resistance
is different and if it can be combined with the resistances already utilized.
In 2008, QDPI&F is conducting several experiments to better understand the
epidemiology and control of SFNB in this region. A field trial examining the
effects of an epidemic on 16 varieties of differing susceptibilities and maturities
will be conducted at Hermitage Research Station under supplementary irrigation.
This should provide useful information on the levels of resistance required to
prevent yield loss and the magnitude of losses that may occur. This trial will
complement a second experiment where foliar applied fungicide will be used to
control SFNB in large plots of 9 varieties in a semi-commercial scenario to gauge
the impact of the disease under commercial conditions.
Another trial will determine the effects of several fungicides when applied to
fertilizer and drilled with the seed. Fungicide amended fertilizers have been used
in other regions of Australia with some success in disease control and their
efficacy in reducing losses to SFNB in the heavy clay soils of the northern region
will be assessed.
Over recent years there has been mounting evidence that SFNB may overseason on stubbles other than barley. With the cooperation of staff of the
Northern Grower Alliance (NGA), stubble samples from the 2007 harvest have
been collected and will be examined for the presence of SFNB. It is reasonable
to expect that the fungus may survive in residues of other crops, such as wheat,
as P. teres is a good saprophyte as well as a good pathogen. It is anticipated
that this work will clarify the role of wheat stubble as a substrate for oversummering of SFNB and in providing new season inoculum.
It will be several years before a suite of varieties with adequate resistance to
SFNB is available so management of the disease will have to rely on alternative
strategies. Given an average season, work currently being done and planned for
the coming season will assist in formulating improved control strategies to
minimize the effects of the disease in the Northern Region.
The Commercial Experience with Spot Form Net Blotch
By Paul Castor MCA
Spot form of net blotch was commonly present at worrying levels in commercial Border
Region barley crops during 2007.
The disease was most severe in districts where barley is grown more intensively eg.
North Star, and particularly where:
• Crops were planted into previous barley stubble (2 year old in one case) and/or
• Where early planting dates were used (Early to mid May)
Over the last 4-5 seasons SFNB has made regular appearances in crops within the region.
This has already prompted a number of growers to reduce their barley plantings.
Whilst the impact of the disease is yet to be measured definitively, it is highly probable
that significant yield and quality losses are occurring. It is most likely a key cause of the
increasing issues with high screenings in regional barley crops.
Commercial experience with fungicide management of SFNB prior to 2007 had been
very limited. A similar approach to the management of Yellow Leaf Spot in wheat had
been thought appropriate. This involved crop monitoring during the later stages of
jointing and treatment with fungicide if the severity of leaf symptoms on the flag and the
2 lower leaves exceeded a set threshold level.
With YLS, these threshold levels are very rarely exceeded except under wet seasonal
conditions (such as in 1998). With SFNB however, disease incidence has been much
more regular even under the relatively dry conditions of recent years.
Fungicides treatments of these diseases at the flag leaf stage are unlikely to provide a
visually pleasing result, particularly under high innoculum situations. In the absence of
trial strips, it can be difficult to assess the benefit of any fungicide program. I feel that
this has limited the use of fungicides to help manage these diseases. We need to
remember that we will only be able to manage and not control these diseases with
fungicides.
Earlier treatments with fungicide may help reduce the level of innoculum development
within crops and should provide the opportunity to reduce application costs. Earlier
applications are more likely to be applied with groundrigs (rather than by contract aerial
application) and may in some cases be tank mixed with herbicides.
During 2007 a number of crops were treated with propiconazole from GS32. Disease
incidence and severity on the 3 youngest leaves was evaluated to support treatment
decisions. Treatments were generally felt to reduce the severity but perhaps not the
incidence of disease symptoms. Whilst not closely evaluated, it was also felt that
treatment provided improvements in grain yields and quality.
The trial work conducted by NGA during 2007 will help provide greater confidence in
the future management of SFNB with fungicides.
Further trial work testing fungicide programs under different environmental conditions
would be useful. This may also allow further development of threshold levels of
incidence and/or severity of SFNB that would justify fungicide treatment at these earlier
crop stages.
Anecdotal evidence suggests that some cultural practices may play an important role in
the development of the disease. Later planting dates, isolation from previous barley crop
residues and limiting early nitrogen availability may be important in limiting the
development and impact of the disease. Research to test these observations and theories
may be useful in the development of an integrated management approach for SFNB.
Earlier application may yield benefits – The NGA View
By Richard Daniel, Northern Grower Alliance
Northern Grower Alliance project direction is determined by the issues and
priorities established by five local consultative committees. These committees
cover an area from the Liverpool Plains in the south, west to Gilgandra and
Walgett and north to Moree/ Narrabri and Goondiwindi. One new priority project
for 2007 was to generate additional data on the management of SFNB.
Key questions
1. What is the level of yield and grain quality impact from spot form of net
blotch on current barley varieties?
2. Is there any benefit in fungicide application prior to flag leaf emergence?
3. Can we provide additional data to support commercial management
practices?
What we did in 2007
Two detailed small plot trials were conducted on SFNB susceptible barley
varieties. One trial was in the North Star district on the variety Skiff, with a
second site near Bellata on the variety Kaputar. Both crops had been planted into
2006 barley stubble with SFNB present naturally before the first spray was
applied.
In each trial eleven different fungicide approaches were compared, involving a
combination of application timing, product, rate and also the impact of multiple
applications (applications were at ~ jointing (T1), ~ 2nd node (T2) and full flag leaf
emergence (T3). Plot sizes were 4 x 10 m with three replicates in both trials. All
treatments were applied with DG nozzles as Fine/ Medium droplets in 50 L/ha
total volume.
The primary focus of this work was to evaluate the impact of fungicide timing, ~
two (T2) and four weeks (T1) earlier than current recommendations (T3), but also
to compare single applications with a number of multiple spray approaches.
Propiconazole (Tilt®) was the main fungicide evaluated with use rates of 250 and
500 mL/ha and comparisons of single application versus multiple timing.
Amistar® Xtra was also evaluated but only at 400 mL/ha and when applied at T2
and T3.
Results
Disease development
SFNB is generally described as a ‘wet weather’ disease. At both sites conditions
were dry during July and most of August with only dews providing any leaf
surface moisture. This level of moisture was clearly sufficient for the disease to
continue to infect new leaves at both sites. This ability for the disease to ‘tick
along’ - even without rain events – supports agronomists’ field observations in
recent dry years. However the disease levels did increased dramatically following
a three day rain period in late August. Although SFNB will be much more
active under rainy conditions, it can clearly progress up the plant even in
the absence of rain events.
Disease management
Disease management was assessed by incidence (% of leaves with any SFNB
lesions) and severity (% of leaf area affected by SFNB lesions). All fungicide
approaches significantly reduced the severity of SFNB but with much less impact
on disease incidence. There was no consistent difference between products,
rates or timings at either site. High levels of visible symptoms were still evident at
both sites even when a 2 or 3 spray program had been applied. The key
message was that all treatments suppressed this disease but none fully
controlled it.
Level of disease pressure
The North Star trial experienced very conspicuous levels of SFNB from jointing
onwards. At the time of first spray, 8 out of 10 uppermost emerged leaves had
any level of SFNB lesions, with 100% incidence on the next oldest leaf. Even so,
the level of disease in the untreated was still at or below current thresholds at 18
days after the 3rd spray timing.
The Bellata trial had trace levels of SFNB from jointing onwards. At the time of
first spray, 2 out of 10 uppermost emerged leaves had any level of SFNB lesions,
with 93% incidence on the next oldest leaf. The level of disease in the untreated
was still below current thresholds at 20 days after the 3rd spray timing.
Photograph 1 – SFNB levels in untreated Skiff at time of first spray, North Star
2007, NGA0720
Yield
Table 1 – Yield and grain quality summary data
North Star (high disease)
Untreated Fungicide ave
1217
1569 (+29%)
62
65
31
17
54
Bellata (low disease)
Untreated Fungicide ave
2969
3189 (+7%)
60
61
34
28
103
Yield (kg/ha)
Test weight (kg/hL)
Screenings (%)
Net return v
untreated ($/ha)
Net return = Gross return – fungicide product and application cost
Application cost = $15/ha each application
Grain yield at the North Star was severely limited by prolonged moisture stress,
with the untreated only yielding ~1200 kg/ha. Although not statistically significant,
all eleven fungicide approaches recorded higher yields than the untreated with an
average advantage of ~350 kg/ha (+29%).
Grain yield at the Bellata site was affected by post-anthesis moisture stress but
with the untreated still yielding ~3000 kg/ha. Again not statistically significant, ten
of the eleven fungicide approaches recorded higher yields than the untreated
with an average advantage of ~220 kg/ha (7%).
Early application (at T1 and T2) provided at least equivalent levels of yield
benefit to application at full flag leaf emergence (T3) or multiple spray
approaches.
Grain quality
Clear trends were apparent in grain quality assessment but again with no
consistent difference between products, rates or the application timing.
At North Star all eleven fungicide treatments recorded higher test weight and
lower screenings levels than the untreated.
At North Star, grain from all fungicide treatments was eligible for Feed 1 or
Feed 2 whilst untreated grain was downgraded to Feed 3.
At Bellata all eleven fungicide treatments recorded higher test weight than the
untreated with nine of the eleven also recording lower screenings levels.
At Bellata, grain from five fungicide treatments was eligible for Feed 2 with
the untreated and all other treatments downgraded to Feed 3.
Net return
Despite all yields being 1800 kg/ha or less at North Star, ten of the eleven
fungicide approaches resulted in increased net returns compared to the
untreated. The only treatment that did not provide a net benefit was where a
multiple application of two sprays of Tilt at 500 mL/ha were applied at T2 and T3.
Even under the low disease conditions at Bellata, nine of the eleven fungicide
approaches resulted in increased net returns compared to the untreated. The
only treatments that did not provide a net benefit were Tilt 250 mL/ha applied at
T3 and a multiple application of Tilt 250 mL/ha applied at all three timings.
In a year not considered conducive to SFNB, net benefits were obtained in
19 of 22 fungicide treatments across the two trials. The average level of net
benefit was $103/ha at North Star and $54/ha at Bellata.
Conclusions
The work conducted in 2007 has started to provide some answers to the original
questions posed by growers and agronomists.
1. Across the two trials, SFNB caused average yield losses of between 220350 kg/ha together with test weight reductions of 1-4 kg/hL and screenings
increases of 6-14%
2. Fungicide application up to 5 weeks earlier than currently recommended
provided equivalent benefits to a flag leaf spray under these conditions
3. Economically attractive levels of benefit were obtained managing SFNB in
two susceptible varieties even in a drier season
Where to next?
This is the first work done in the north-west looking at early timing of fungicide
application for SFNB management. Although the results are very encouraging, it
is important that we repeat this work to examine the performance under different
conditions. NGA plans in 2008 include conducting four trials that will also include
an even earlier timing in an attempt to identify ‘what is too early’.
For more information, contact Greg Platz on (07) 4660 3633, Richard Daniel on
(0428) 657 782 or Paul Castor on (0428) 712 003.
* Consultant’s Corner is a new initiative by Australian Grain that will highlight
current GRDC-funded research with a particular focus on the commercial
implications of adopting cutting-edge research.
1 Queensland Department of Primary Industries and Fisheries
2 MCA Goondiwindi
3 Northern Grower Alliance