- Losses from crown rot in affected paddocks are usually ~5%, but can be >20%. In addition to its effects on grain yield, crown rot can also reduce grain size
- Whilst all of these diseases are common in any system, their incidence and potential impact if not managed will be increased under stubble retained systems.
The adoption of No-Till, stubble retained farming systems has favoured the development of stubble borne diseases within the cropping system, especially where cereals dominate the rotation. Many of the stubble borne diseases are necrotrophic and do not need a living host to survive. The most common of these diseases in the Mallee and Wimmera include yellow leaf spot (YLS), spot form net blotch (SFNB) and crown rot. While YLS in wheat and SFNB in barley are commonly seen each year, their impact on grain yield is greater in wet seasons with losses greater than 20% possible.
Root diseases such as rhizoctonia have also become more prevalent due to the reduction in cultivation. Each of these diseases can cause significant loss of production in seasons favourable for their development. Crown rot can reduce grain yields in wheat by more than 30% in seasons with a dry finish if disease levels are allowed to build up. Rhizoctonia, also influenced by seasonal conditions, has been known to cause yield loss of up to 50% in extreme cases.
For nearly all of these diseases, there are management practices that can be implemented to reduce their severity. However in the initial years of a stubble retained system, especially when rotations are still cereal dominated, there may be challenges with disease control. An integrated management practice of these diseases commonly involves the following:
- Selection of paddocks with low levels of inoculum present
- Avoiding susceptible and very susceptible cereals in paddocks where disease is present
- If disease levels are high include more break crops in the rotation (which must be kept grass free)
- Timely fungicide applications for some diseases
Challenging diseases in stubble retained systems
Yellow leaf spot (yls)
- ID: Tan coloured lesions surrounded with yellow halos on leaves and black fruiting bodies on stubble (Figure 1). Infection of new leaves occurs when rain splashes off the stubbles and lands on the crop leaves. Once infected, the disease is particularly difficult to control with commonly used fungicides.
- When to monitor: Mostly occurs in early growth stages in winter. Optimal conditions are temperatures of 15-28oC and up to 12 hours of leaf wetness.
- Crops affected: Wheat however, it will remain present in barley stubble but won’t impact barley in-crop.
- Host: All cereal stubbles will carry the disease (including barley and stubbles of resistant wheat varieties).
- Fungicide treatments: Fungicide can provide protection for up to three weeks, not generally economical but may provide some control. If a susceptible crop has YLS and conditions are likely to be favourable (prolonged leaf wetting during temperatures of 15-28oC) then fungicide could provide some protection. Apply according to foliar application timings (two treatments at ~GS31 and GS39). Registered products for foliar application for YLS include products containing Azoxystrobin, Propiconazole or some Tebuconazole products. For a full list of the registered fungicides for YLS see page 128 of the winter crop variety sowing guide.
- Management: YLS can persist for two years on stubble and any practice that accelerates stubble breakdown will decrease disease survival. Avoiding susceptible and very susceptible varieties will reduce disease development and yield loss from yellow spot. If paddocks have wheat stubble present, either avoid planting wheat in that paddock, or grow the most resistant wheat in that paddock.
- Recent research: BCG has done many articles on stubble borne diseases. A trial conducted in 2011 in conjunction with FAR (Nick Poole) identified which disease control method was the best: Prosaro® at 300ml/ha at flag leaf emergence (GS39) was the most effective fungicide timing for yellow leaf spot control and 50% ear emergence (GS55) for stem rust control)
Spot Form Net Blotch (SFNB)
- ID: Foliar disease, characterised by dark brown spots or lesions on leaf blades and sheaths often with a yellow surround. Severely affected leaves may die. Stem is weakened and lodging may occur.
- When to monitor: Late tillering to GS30
- Crops affected: Barley, and yield loss can be up to 25%. Most malt barley varieties are susceptible to SFNB.
- Hosts: Barley is the primary host.
- Fungicide treatments: There are several fungicides registered for the control of SFNB including Azoxystrobin, Epoxiconazole or Propiconazole products. More details including commercial trade names can be found on page 128 of the Winter crop variety sowing guide. Apply fungicide during growth stages 31-39 if necessary (>10% leaf affected). Monitor and apply a second foliar fungicide up to ear emergence (GS49). The seed treatment Systiva® is effective at suppressing SFNB during tillering and can replace foliar application at GS31. It is also good at suppressing some other foliar diseases (scald, NFNB). In a dry year such as 2015 it may not be economical to apply fungicide.
- Management: Plant resistant varieties and rotate crop types, apply a foliar fungicide if necessary according to foliar application timings. Dry seasons mean a low level of stubble breakdown so more spores will be present, although sowing early can increase disease pressure as the crop can mature earlier when disease levels are highest having the greatest impact on crop growth and yield.
- Recent research: In 2014 Agriculture Victoria undertook trials to compare several management practices on commonly grown barley varieties with different susceptibility ratings to SFNB. Severity ratings were found to decrease throughout the season (after GS33) but evidence of disease was still much higher in those susceptible varieties (Figure 2).
In terms of grain yield penalty, only the very susceptible varieties showed significant yield penalty, while those varieties with better resistance were not penalised. Differences were observed in grain quality with increased screenings of up to 3% and reduced retention of 3-10%. While the impact on grain quality will be a potential issue for the Malt varieties, the impact will vary under different seasonal conditions. Subsequently, growing a variety that performs well in your environment with a susceptible or better disease rating, will reduce the impact of this disease on yield and quality.
While not a direct issue from stubble retention it occurs as a result of minimum disturbance through reduced tillage.
- Symptoms: Shortened, brown, spear tipped roots. Bare or stunted patches in the paddock, from just a few plants to metres across.
- When to monitor: Before sowing and 2-3 leaf stage
- Crops affected: All cereal and broadleaf crops. Although it is mainly a disease of cereals it also causes losses in pulses, oilseeds and pastures.
- Fungicide treatments: Some seed treatments offer suppression of rhizoctonia during early growth stages of the plant. There are several up-front fungicide products available for suppression of rhizoctonia (Dividend®, Evergol Prime® and Uniform®).
- Management: Difficult to control as it has a broad host range. There are no known cereal cultivars with resistance to this disease. Different crop types such as canola, can reduce inoculum levels in low rainfall, light textured soils. Rhizoctonia can survive and feed on organic matter in the soil, therefore it does not require a living host. Encouraging faster early growth of crops can help the roots avoid the inoculum. This can be done through:
- Adequate nutrition, particularly phosphorus (P), nitrogen (N) and zinc (Zn). Apply nitrogen at sowing or soon after will promote faster growth early and help roots grow through the Rhizoctonia hyphae. It is important to ensure sufficient seed separation from the urea to prevent seed burn.
- Keeping the paddock grass free (dead) at least 10-14 days prior to sowing.
- Soil disturbance 50mm below seeding depth. This breaks up the fungal hyphal networks. Tyne seeders perform this more effectively than disc seeders.
- Early sowing into warm soils which will promote early crop vigour enabling the plant to grow through the disease.
- Avoiding sulfonylurea (SU) herbicides (eg. Ally®) and higher rates of trifluralin as residues may negatively affect early root growth.
Crown rot has become a common disease in Victoria due to the intensification of cereals and stubble retention practices, resulting in losses during seasons with below average spring rainfall (Hollaway and Exell 2010). Losses from crown rot in affected paddocks are usually ~5%, but can be >20%. In addition to its effects on grain yield, crown rot can also reduce grain size.
- Symptom: 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 those 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.
- When to monitor: Crown rot must be controlled prior to sowing as there is no in-crop control available. Inspection of previous cereals for 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.
- To estimate the risk of growing a cereal next season, monitor in a W pattern and collect five plants at 10 locations and examine the base for basal browning. As a general rule, the risk for a cereal the next season will be LOW if less than 10% of plants are infected, MEDIUM if 10-25% of plants are infected or HIGH if more than 25% of plants are infected
- Crops affected: Durum wheat is most affected, followed by wheat and then barley. Being an earlier maturing crop, barley tends to escape yield loss. Crown rot can cause yield loss up to 50% in bread wheat and 90% in durum wheat if disease levels are high and there is moisture stress during grain fill.
- Host: Crown rot is hosted by ALL winter cereals and many grassy weeds. Some other crop types can also host the disease over seasons and also grass weeds if present during the “break” phase.
- Fungicide treatments: The seed treatment Rancona Dimension has noted suppression of crown rot.
- Management: 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. In paddocks with high levels of crown rot it is best to avoid growing cereals. The best management of crown rot is through rotations and break down of the stubble. Any crop break crop must be grass free otherwise the disease levels will remain or increase overtime. The inoculum levels can survive several years and only reduce as stubble decomposes. Practices such as slashing, cultivation, spreading and grazing can increase stubble breakdown but can also spread inoculum and may increase infection if conditions are poor for stubble breakdown. Burning is not particularly effective as inoculum is often below ground. If a cereal must be sown then barley is least affected followed by bread wheat and durum.
Recent research: Inter-row sowing using accurate (±2cm) GPS auto steer has decreased infected plants by 50%, which resulted in a 5-10% yield increase.
Some other potentially important diseases in a stubble retention cropping system include septoria and net form of net blotch in wheat, and scald in barley.
Challenges of managing disease in stubble retained systems
Whilst all of these diseases are common in any system, their incidence and potential impact if not managed will be increased under stubble retained systems.
The most effective way of reducing inoculum levels of any disease is through resistant varieties and breaks from cereals. Carry out a Predicta B test prior to sowing to identify diseases before their level gets too high, monitor diseases in-season and manage where necessary. Predicta B detects several soil borne diseases including crown rot, CCN, take-all, Rhizoctonia, cereal cyst nematode, root lesion nematode, stem nematode and blackspot of peas.
Reducing stubble may assist in reducing inoculum levels however can be a risky practice. Practices such as slashing, cultivation, spreading and grazing can increase stubble breakdown but can also spread inoculum and may increase infection if conditions are poor for stubble breakdown. Burning may prove effective for some diseases but is ineffective for diseases where the inoculum is below ground, such as crown rot. Reducing stubble also comes with an increased risk of erosion due to reduced ground cover. Reducing ground cover below 70% leads to an increased risk of erosion and reduced moisture retention.
Stubble decomposition and use of fungicides both have action that decreases inoculum levels in a system. However, fungicides can actually reduce the rate of stubble decomposition by damaging microbe communities that assist in its breakdown.
By Jessica Lemon Research Officer (BCG) email@example.com
This research is being conducted by BCG as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 Maintaining profitable farming systems with retained stubble in Victoria and Tasmania).
Disclaimer: Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of the Birchip Cropping Group (BCG) or the Grains Research and Development Corporation (GRDC). No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. BCG and GRDC and contributors to these guidelines may identify products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to. Other products may perform as well as or better than those specifically referred to. BCG and GRDC will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication.
Stubble project overview: This guideline has been developed for BCG Farming Systems Group as part of the Maintaining Profitable Farming Systems with Retained Stubble initiative, funded by the Grains Research and Development Corporation (GRDC). The initiative involves farming systems groups in Victoria, South Australia, southern and central New South Wales and Tasmania collaborating with research organisation’s and agribusiness to explore and address issues for growers that impact the profitability of cropping systems with stubble, including pests, diseases, weeds, nutrition and the physical aspects of sowing and establishing crops in heavy residues.