Cereal disease management in Victoria 2016

Background

Growers need to implement disease management strategies for the 2016 season, even though disease pressure was low during 2015. This includes: selecting the most disease resistant varieties possible, avoiding growing cereals in paddocks with high soil or stubble borne disease loads, and planning for the timely application of fungicides if required.

This paper discusses disease management after drought, and provides some research updates on wheat leaf rust, septoria tritici blotch (STB), crown rot, spot form of net blotch (SFNB), scald and nematodes, along with an outlook for the 2016 season.

Disease risk following drought

History tells us that disease pressure can be high in seasons following drought if conditions are conducive. It is therefore important to implement appropriate disease management strategies going into the 2016 season.

Stripe rust was damaging to susceptible wheat crops in Victoria during both 2003 and 1983, with both years following severe drought. This was due to the occurrence of conditions favourable for disease development. If varieties susceptible to rust are grown during 2016 then a suitable rust management plan must be implemented. The presence of volunteer cereals during autumn (the ‘green bridge’) will increase the risk of cereal rust diseases.

Stubble-borne diseases will need to be managed during 2016, due to carryover of inoculum on stubble from both 2014 and 2015 crops as a result of reduced stubble breakdown during 2015. If diseased stubble is present then resistant crops/varieties should be selected where possible.

Our experience following the 2002 drought was that minimal reduction in root disease levels occurred during break crops. Therefore, an additional year of break crop may be required to reduce root diseases to a low risk. If there is a potential for root disease issues, it is advisable to conduct a PreDicta B soil test prior to planting to avoid sowing cereals in high risk paddocks.

Wheat leaf rust

A new strain of leaf rust, first detected during 2014, has become dominant in eastern Australia and will be important if suitable conditions for leaf rust occur. Ratings of many important wheat cultivars (Table 1) are now one rating more susceptible than before. Several cultivars (including Axe, Beaufort, Corack, Derrimut, Mace, SQP Revenue and Wallup) are now two or more rating levels more susceptible. Should conditions be suitable for leaf rust development, growers will need to be more vigilant with its control. Consult a current cereal disease guide for current ratings.

Table 1: Changes in wheat leaf rust rating following the detection of a new strain.

Cultivar

Wheat Leaf Rust Rating Changes

2014 2016A
SQP Revenue R SVS
Beaufort, Derrimut, Livingston R MSS
Axe MR SVS
Lincoln MR S
Corack, Wallup MS SVS
Bolac, Chara MS S

AAlways consult a current disease guide.

Septoria tritici blotch (STB)

Septoria tritici blotch (STB), an important stubble borne foliar disease of wheat, is now the most important wheat disease in Victoria’s high rainfall cropping zones. The increase in STB has been favoured by stubble retention, intensive wheat production, susceptible cultivars and favourable conditions (cool and moist) for disease. Also, strains of STB with reduced sensitivity (partial resistance) to common fungicides have been detected.

Septoria tritici blotch control

An integrated approach that incorporates crop rotation/stubble avoidance, variety selection and fungicides can provide effective suppression of STB. Identification of strains with partial resistance to common fungicides highlights the need to adopt an integrated control approach to slow the further development of resistance to fungicides (see below: Managing STB resistance to fungicides).

Since STB is primarily a stubble borne disease, both crop rotation and stubble management contribute to disease control. In most instances, a one-year break from wheat is effective in reducing early disease occurrence, but during dry seasons a two season break may be required. Any tillage that reduces stubble on the surface (such as burial, burning or grazing) can reduce inoculum levels, but these practices need to be balanced with the risk of soil erosion. Stubble management will not reduce disease caused by spores blown in from other paddocks.

Avoiding susceptible and very susceptible varieties (ratings of S, SVS or VS) is an effective strategy to reduce in-crop disease severity and historically has provided long term disease control. Since STB is pathogenically diverse (that is different strains can attack different varieties), and resistance breakdown is known to occur, it is important to consult a current disease guide each year. With support from GRDC and Agriculture Victoria, a new STB screening nursery was established at Hamilton to screen Australian NVT entries and pre-breeding lines. These results contribute to the ratings published in the cereal disease guide.

Fungicides can contribute to STB control, especially during wet seasons. In high risk areas the timing of fungicides is important to achieve adequate disease control. In early sown susceptible varieties, where infection is established during autumn, an early fungicide application at Z31-32 may be required to suppress the disease and protect emerging leaves. Another fungicide application may be required once the flag leaf has fully emerged at Z39 to protect the upper canopy. Since STB is developing resistance to fungicides, it is critical that strategies are implemented that reduce the likelihood of further resistance developing.

Managing STB resistance to fungicides

Since partial resistance to some fungicides has been identified in the Victorian STB population an integrated STB management approach is essential to minimise further resistances developing in the population. Should integrated strategies not be adopted, currently available fungicides will become less effective and we may not be able to prevent STB losses.

Partial resistance in STB to some azole fungicides reduces their effectiveness, rather than making them completely ineffective. However, if they are continued to be used, further selection pressure will result in development of more serious mutations. The two mutations identified reduce the efficacy of a number of azole fungicides (Group 3) commonly used in Australia: triadimfon, triadimenol, tebuconazole, propiconazole and epoxiconazole (not registered for STB in Australia).

Growers must adopt strategies to extend the useful life of currently available fungicides. One strategy is to mix or alternate different azoles. This is because not all azole fungicides are affected equally by mutations of the STB fungus. Products that combine azoles such as Tilt® Xtra (propiconazole and cyproconazole) or Impact Topguard® (tebuconazole and flutriafol) which have a STB registration could be used in this way. Also, in crops where two fungicide applications are to occur (e.g. at Z31 and Z39) the same active should not be used at both applications. Growers MUST always follow label guidelines and ensure maximum residue limits (MRLs) are adhered to.

In Australia, there are limited choices of fungicides with different modes of action for use on wheat. A number of products combine a strobilurin with an azole and these may provide some benefits in delaying or reducing the risk of resistance development. However, the strobilurins on their own are considered to be at high risk of developing resistance due to their single site mode of action. In some countries, resistance to strobilurins is so widespread they are no longer recommended as effective, even in mixtures.

In addition to mixing or rotation of fungicide actives, an integrated approach to disease control is required to reduce the likelihood of further fungicide resistance developing, by keeping inoculum loads low. The approach will include crop rotation, avoiding susceptible cultivars and delay sowing.

Barley disease

Stubble-borne diseases will need to be managed during 2016, due to the abundance of inoculum in the cropping system and the dry conditions that have reduced stubble breakdown. Paddocks with barley stubble from 2014 and 2015 will provide an inoculum source and should be avoided where possible. Most malt accredited barley varieties are susceptible to spot form of net blotch (SFNB) and scald and will provide the greatest risk of causing yield loss.

Leaf rust of barley is unlikely to be an issue during 2016 unless there is significant growth of volunteer cereals following summer rain and favourable in-crop conditions. Compass now has a rating of very susceptible (VS) to leaf rust and the majority of other varieties are also rated susceptible (S). They should therefore be monitored during the spring as significant yield loss is possible if conditions become favourable.

Spot form of net blotch (SFNB)

Spot form of net blotch provides the greatest risk of causing loss during 2016 due to abundant inoculum from 2014 and 2015 seasons, and insufficient rainfall for stubble to break down. Susceptible varieties sown into paddocks with infected stubble will need to be monitored during the season and fungicide applied if necessary.

The risk of loss is largely dependent on the frequency of growing season rainfall events and yield potential. In general, crops should be managed during average or above average rainfall seasons and where yield potential is greater than 2.5t/ha.

Monitor susceptible crops during late tillering stages of crop development and apply fungicide during growth stages Z31-39 if necessary. This is generally when greater than 10 per cent of leaf area is affected by disease. Crops should continue to be monitored and a second foliar fungicide applied if necessary up until ear emergence (Z49), should favourable climatic conditions occur.

The new seed treatment Systiva® is effective at suppressing SFNB during the tillering stages of crop development and can be used instead of foliar application at Z31. Our experiments have shown almost complete suppression of SFNB early in the crop’s development in most seasons and good suppression of other foliar diseases such as scald and net form of net blotch. However, it will need to be used in an integrated management strategy and in rotation with other fungicide chemistries as it has the SDHI mode of action which the pathogens may develop resistance to.

Experiments conducted during 2015 demonstrated losses due to SFNB were generally minimal and fungicide application was not economical. This was due primarily to the dry season in which SFNB infection was less than 10 per cent and grain yields were also low. An experiment conducted on fallow at Horsham demonstrated that SFNB developed during the winter months but had reduced to very low levels by grain fill (Table 2). This experiment also demonstrated the benefit of growing moderately susceptible or more resistant varieties, which had very low SFNB all season, compared to Susceptible or Very Susceptible rated varieties which had potentially yield reducing SFNB levels during winter.

Table 2: Spot form of net blotch severity was low and grain yield loss insignificant in eight barley varieties grown at Horsham during 2015 due to dry seasonal conditions.

Variety

SFNB (% leaf area affected)
Grain Yield (t/ha)
24/8 (Z32) 29/9 (Z69) Average LossA 
Fathom (MR) 1.3 0.3 3.2 -0.1ns 
Scope (MS) 4.5 1.7 2.2 -0.2ns
Compass (MS) 3.3 1.2 3.1 0.1ns
GrangeR (S) 6.8 2.4 1.8 0ns 
La Trobe (S) 8.7 2.1 3.0 0.3ns 
SY Rattler (VS) 9.3 2.6 2.1 0.2ns
Hindmarsh (VS) 9.0 3.0 3.0 0.2ns 
Dash (VS) 10.0 2.3 1.5 0.1ns
LSD (0.05)=
P=
1.166
<0.001
0.406
<0.001
   

ALoss determined by comparing a Diseased treatment (infected stubble added and no fungicide) to a Fungicide treatment (Systiva® + Prosaro® @ 150 ml/ha @Z32-4 and Z39-49). ns = not statistically significant at 0.05.

Scald

Scald can be damaging when favourable climatic conditions consisting of cool, wet weather during the winter and early spring occurs. Experiments at Horsham during 2013 showed that up to 27 per cent grain yield loss can occur as well as significant reductions to grain plumpness and weight (Table 3). Avoiding growing very susceptible rated varieties reduces the potential for yield loss.

Fungicides are effective at suppressing scald. Using the same approach as for SFNB management, with the application of foliar fungicide at Z31-39 being most effective. Two applications may be necessary if the season is favourable for disease development. Seed (triadamenol, Systiva®) and fertiliser (flutriafol) fungicides are effective at suppressing scald during the early stages of crop development and can be used instead of foliar application at Z31.  However, a follow up application of foliar fungicide is often required.

Table 3: Scald severity increased and grain yield and quality decreased significantly according to susceptibility rating of five barley varieties grown at Horsham during 2013.

        Grain Quality Loss (%)B
Variety RatingA Scald severity (Leaf area affected) Grain yield lossB (t/ha) Retention (% grain >2.5 mm in width) Screenings (% grain <2.2 mm widith) Grain weight (g) 
Yagan VS 62%* 1.2 (27%) 23%* 6%* 20%*
Barque SVS 24%* 0.9 (19%)* 20%* 5%* 12%*
Skipper S 18%* 0.8 (16%)* 12%* 4%* 4%ns
Flagship MS 13%* 0.7 (14%)* 11%* 3%* 2%ns
Hindmarsh MR 0ns 0.1 (2%)ns 4%* 1%* 4%ns 

AAlways consult a current cereal disease guide. Hindmarsh is susceptible to some pathotypes not present in this experiment. BGrain yield and quality loss determined by comparing a disease treatment (infected stubble added and no fungicide) to a fungicide treatment (Flutriafol + Prosaro® @ 300ml/ha @Z32-4 and Z39-49). * = statistically significant; ns = not statistically significant at 0.05.

Net form of net blotch (NFNB)

Net form of net blotch is potentially very damaging, but is of low risk due to good levels of resistance in the majority of varieties grown in Victoria. Monitor varieties with an MS, S or VS rating and apply foliar fungicide at Z31-39 if necessary.

Bunts and smuts

Seed treatments provide cheap and effective control of bunt and smut diseases. Seed should be treated every year with a fungicide. Without treatment bunt and smut can increase rapidly, resulting in unsaleable grain. Good product coverage of seed is essential and clean seed should be sourced if a seed lot is infected. Note that fertiliser treatments do not control bunt and smuts, so seed treatments are still required.

Crown rot

Crown rot is favoured by the intensive cultivation of cereals and stubble retention practices, which contribute to losses during seasons with below average spring rainfall. Losses from crown rot are usually approximately 5 per cent, but can be greater than 20 per cent where abundant inoculum is present. In addition to its effects on grain yield, crown rot can also reduce grain size.

The distinctive symptom of crown rot is the presence of whiteheads in the crop at early grain fill. These heads mature early and contain shrivelled grain or no grain. Whitehead expression is more common in seasons with a dry spring. The other, and more reliable, symptom of crown rot is browning of the stem bases. Inspections of stems for browning is best performed from mid to late grain fill through to harvest. To see the browning, leaf sheaths should be pulled back and in some cases the pink of the causal fungus may be observed.

Crown rot must be controlled prior to sowing as there is no in-crop control available. Inspection of previous cereals for the presence of crown rot symptoms provides an indication of potential risk from crown rot. A PreDicta B soil test is a good option to identify crown rot risk in paddocks. A field trial conducted in the Wimmera during 2015 shows the strong relationship between pre-planting crown rot inoculum level and resulting grain yield in durum and bread wheat (Figure 1).

Figure 1: Effect of increasing crown rot inoculum levels at sowing on the grain yield (t/ha) of one durum wheat (WID802) and four bread wheat cultivars grown at Dooen during 2015 (study conducted in collaboration with Dr S. Simpfendorfer, NSWDPI; DAW00245).

Figure 1: Effect of increasing crown rot inoculum levels at sowing on the grain yield (t/ha) of one durum wheat (WID802) and four bread wheat cultivars grown at Dooen during 2015 (study conducted in collaboration with Dr S. Simpfendorfer, NSWDPI; DAW00245).

In paddocks with medium to low crown rot levels, yield losses can be minimised by avoiding durum wheat and growing barley in preference to bread wheat (Figure 2). Even though barley is a good option for reducing yield loss from crown rot it will increase inoculum levels for the next crop.

In paddocks with high levels of crown rot it is best to avoid growing cereals. Previous work showed that cereals increased inoculum levels while broadleafs and fallow decreased inoculum levels of crown rot. In general, a two year break from cereals is required to reduce medium to high inoculum levels to a low level. A three year break may be required following the dry season of 2015.

Figure 2: Grain yield (and grain yield loss) in the presence of low (nil treatment) and high crown rot levels at sowing at Dooen during 2015.

Figure 2: Grain yield (and grain yield loss) in the presence of low (nil treatment) and high crown rot levels at sowing at Dooen during 2015.

Nematodes

A survey of 385 paddocks in western Victoria, during 2014 and 2015, found that root lesion nematodes (Pratylenchus spp.) are widespread, being present in up to 98 per cent of paddocks with yield losses possible in many paddocks (Table 4). Using this data we estimated annual losses due to root lesion nematodes at two to 13 million dollars, depending on the season.

Cereal cyst nematode (CCN) was identified in four and 12 per cent of paddocks in the Wimmera and Mallee, respectively, showing the effect of good control through the cultivation of resistant cereal varieties and crop rotation. However, if CCN levels increase large yield losses are possible.

The yield loss caused by nematodes is directly related to the number of nematodes present in a paddock. A pre-sowing root disease test is the best way to identify paddocks at risk of damage. Should damaging numbers be identified then appropriate control strategies, including rotation to resistant crops, can be implemented to minimise loss.

Table 4: Percentage of paddocks (n = 385) surveyed in three regions in western Victorian during 2014 and 2015 within each nematode risk category and the corresponding potential yield loss (%).

PreDicta B Risk Categories
Victorian Region (%)
Potential Yield Loss (%)

Risk Category Number / g soil Mallee (n=173) Wimmera (n=182) Western District (n=30)
Roost lesion nematode (Pratylenchus neglectus)
BDL <0.1 2 5 37 <2
Low 0.1-20 77 75 60 0-10
Medium 20-60 18 20 3 0-20
High >60 3 0 0 0-40
Root lesion nematode (Pratylenchus thornei
BDL <0.1 72 47 73 <2
Low 01-20 27 46 27 0-10
Medium 20-60 0 6 0 0-20
High >60 1 1 0 0-40
Cereal Cyst Nematode (Heterodera avenae)
BDL <0.05 96 88 100 <5
Low 0.05-5 4 9 0 5-25
Medium 5-10 0 2 0 10-50
High >10 0 1 0 15-70

Conclusion

Cereal root and foliar diseases can cause grain yield and quality loss where inoculum and favourable climatic conditions are present. Despite the low foliar disease pressure during the 2015 season, there is still a need to manage disease during 2016 as there is abundant inoculum in the cropping system. It is important that disease risk is identified for each crop on a paddock by paddock basis and appropriate disease management plans developed and implemented where required. Integrated disease management strategies that include resistant varieties, crop rotation and fungicide application should be implemented to minimise potential losses.

Acknowledgements

Funding for this work was provided through the GRDC Projects DAV00129, DAV00128, DAV00144, DAN00175, DAV00136, DAW00245, DAS00137 and the Victorian Government and their support is gratefully acknowledged.

Thanks to the Cereal Pathology Technical Team (Graham Exell, Jordan McDonald, Luise Sigel, Tom Pritchett and Melissa Cook).

Useful resources

Cereal Seed Treatments 2016

GRDC Septoria tritici blotch fact sheet

GRDC Root Lesion Nematodes fact sheet

PreDicta B website

Contact details

Grant Hollaway
Agriculture Victoria, 
Private Bag 260, Horsham 3401, Victoria
03 5362 2111
grant.hollaway@ecodev.vic.gov.au

GRDC Project Code: DAV00129, DAV00128, DAV00144, DAN00175, DAV00136, DAW00245, DAS00137,