Cereal and pulse disease update: 2023

Take home messages

• Disease pressure in the 2023 season will likely be extreme with carry-over of inoculum from 2022 crops on stubble (for example Septoria, Botrytis), volunteers (for example rusts) and sclerotes (for example Sclerotinia).
• Monitor for disease in all crops this season and have a plan to prevent and control disease.
• Disease severity and associated yield loss will depend heavily on conditions in spring.
• Fungicide strategies should reflect varietal resistance, and seasonal conditions.

Seasonal update

Early sowing and April rainfall in parts of Victoria meant early plant establishment in some regions. The combination of early plant establishment and substantial inoculum carryover from last season, increases the disease risk for 2023. Specifically for rust, most regions had summer rainfall and a green bridge, allowing for rust to carryover between seasons. The first rust report for 2023 was on 24 April 2023 in the Wimmera, highlighting the potential risks if the season becomes conducive for rust. Early plant establishment potentially results in more disease lifecycles and a chance of greater disease severity. This disease can also move onto later sown crops. Therefore, proactive disease management is required through monitoring crops, ensuring the management strategy matches the varietal resistance, and applying appropriate fungicides at the required time.

Stripe rust in wheat

Rust has already been observed during 2023. This highlights that there is a potential risk if the season becomes conducive for rust. The early rust on-set during 2022, along with the favourable conditions for disease development, resulted in a damaging outbreak of stripe rust across Victoria. Where control was inadequate, large yield losses due to stripe rust in wheat occurred.

AgVic trials in the Mallee demonstrated yield losses of up to 50% or ~1.81t/ha in the susceptible varieties when not controlled (Table 1). The severity was significantly (P <0.001) lower in resistant or better rated varieties and the yield losses accounted for only 0.42–0.86t/ha, highlighting the importance of using resistant varieties even during a high-pressure season such as 2022. Severe stripe rust pressure also led to head infection later in the season, resulting in grain quality losses in most varieties (data not shown).

Table 1: Stripe rust severity (% leaf area infected), head infection and associated yield loss of six wheat varieties with and without disease at Nullawil (BCG), Victoria during 2022.

#Within a column, means with one letter in common are not significantly different (P=0.05). ** = statistically significant at 5% Lsd. ^AHollaway and McLean (2022) Cereal Disease Guide 2022. Agriculture Victoria.^BDate of assessment made and Zadoks growth stages: Z51, Ear emergence; Z75, Milk development,according to Zadoks et al. (1974). ^C Max. disease = Maximum disease treatment; Min. disease = Minimum disease treatment. ^D Yield loss per cent for each variety was presented as percentage yield decrease vs the minimum disease treatment.

Varieties are generally susceptible as seedlings and increase their resistance as they develop, and adult plant resistance (APR) gene(s) become active. APR genes become effective at various growth stages and differ between varieties and their resistant rating. Depending on the existing paddock conditions, APR genes are effective as early as Z14 (fourth leaf) in resistant varieties whereas delayed until Z59 (flowering) in susceptible varieties. Fungicides may be required at earlier growth stages to minimise infection levels until APR genes are expressed within varieties.

During 2022, wet conditions delayed expression of APR and higher infection levels prompted application of fungicides particularly in susceptible varieties at earlier growth stages than usual. Severe yield losses were found where fungicides were not applied. In the Mallee, fungicide applications significantly (P < 0.001) reduced both stripe rust (leaf) severity and head infection in wheat variety Scepter (MSS) compared to untreated control (Table 2). Dual spray combination at stem elongation (Z31) and flag leaf emergence (Z39) following stripe rust detection in the trial resulted in yield increase by ~32% or 1.09t/ha. Single foliar spray at Z39 was the next best treatment with a ~25% yield increase.

Table 2: Stripe rust severity (% leaf area infected) and associated yield loss in wheat variety Scepter (MSS) in response to different fungicide treatments at Nullawil (BCG), Victoria during 2022.

#Within a column, means with one letter in common are not significantly different (P=0.05). ^ADate of assessment made. ^BYield gain per cent for each variety was presented as percentage yield increase vs the minimum disease treatment.

From these results, it is apparent that resistant varieties should be preferred over susceptible varieties during 2023 given that rust risk is likely to be extreme. Increased area sown to varieties with better resistance has several benefits. These include less reliance on multiple fungicide applications, less inoculum load, less yield penalties, and reduced opportunity for development of fungicide resistance.

Septoria tritici blotch in wheat

Septoria tritici blotch (STB) has been reported in the early sown crops in the Wimmera. It was the second most damaging disease of wheat after stripe rust during 2022. STB is a stubble borne disease and stubble is the source of primary inoculum. Above average rainfall during spring 2022 resulted in high biomass of cereal crops in Victoria which left higher stubble loads. Therefore, the risk in 2023 will be very high where wheat is sown into wheat stubble or close to wheat stubble. AgVic trials showed incredible levels of inoculum following 2022 harvest demonstrating the heightened risk of the disease in 2023 (Table 3). The inoculum levels did not vary significantly (P = 0.74) between varieties with different resistance/susceptibility and reflected STB infection levels at the end of a very wet 2022 season. Hence, a minimum of at least one year break should be allowed between two wheat crops to reduce STB risk.

Septoria severity in-crop also varied with the amount of rainfall received and varietal resistance used. Above average rainfall during spring months at Hamilton (high rainfall zone) and Longerenong (medium rainfall zone) enabled STB to progress to the upper canopy resulting in significant (P <0.001) impacts on grain yield and quality (Figure 1). AgVic trials demonstrated losses of ~35–43% in the Wimmera (medium rainfall zone) during 2022 compared with <10% during 2021, clearly demonstrating the role of rainfall in the damage caused by STB (Table 3). Yield losses were reduced when resistant varieties were grown in preference to susceptible varieties and when two foliar fungicide applications at Z31 and Z39 with a seed treatment were used (Table 3).

Figure 1. Septoria tritici blotch severity (% leaf area affected) across time in wheat (cv. Razor CL Plus, susceptible to very susceptible to STB) at three different rainfall zones in Victoria, during 2022.

Table 3: Septoria tritici blotch severity (% leaf area affected) and yield loss of six wheat varieties treated with and without disease and corresponding pre-sowing inoculum levels at Longerenong, Victoria, 2022.

^AWithin a column, means with one letter in common are not significantly different (P=0.05).** = statistically significant at 5% Lsd. ^BDate of assessment made. First two assessments were average of single plot assessments while the third assessment was average of the top three leaves of ten tillers per plot.^CMax. disease = Maximum disease treatment (No disease control with 1kg STB infected wheat stubble); Min. disease = Minimum disease treatment (No stubble, Seed (Fluquinconazole 167g/L @ 300mL/100 kg seed) + Foliar applied fungicide at Z31 (Benzovindiflupyr 40g/L + Propiconazole 250g/L @ 500mL/ha) + Z39 (Epoxiconazole 500g/L @ 125mL/ha)). ^DAnalysis was conducted using log-transformed data to linearise the relationship between dependent and independent variables but non-transformed data is presented for reader benefit.

Botrytis disease management in pulse crops

General

Botrytis is a serious risk to 2023 susceptible pulse crops with substantial inoculum (infected stubble) carryover from 2022. In areas where crops established early, early canopy closure can occur, making fungicide penetration into the canopy difficult. Botrytis affects most pulse crops (faba bean, lentil, vetch, chickpea and lupin). The disease is called chocolate spot in faba bean and has been previously in vetch. It is caused by two pathogens, Botrytis cinerea and B. fabae which are both found across faba bean, lentil, vetch, and lupin, with chickpea only affected by Botrytis cinerea. Therefore, disease can spread readily between susceptible pulse crops or from previously infected stubble. The pathogens are necrotrophic fungi, which means they kill plant cells and then feed off those dead cells. This infection process places stress on the plant which makes plants more susceptible to further infection. Therefore, it becomes more difficult to control the disease once it is established and can cause greater disease severity further leading to yield loss.

Botrytis development can occur at most growing season temperatures, but disease development is quickest when canopy humidity is high (> 70%) and temperatures are warm (15–25°C).

Faba bean management

Chocolate spot management is centred around a canopy closure preventative fungicide application. Follow up fungicides may be required if rainfall results in high canopy humidity after canopy closure. In an experiment conducted during 2020 at Lake Bolac, several fungicide treatments (Table 4) were compared to identify the best fungicide strategy that is cost-effective to manage chocolate spot in four common faba bean varieties.

Table 4: Fungicide treatments and timings in faba bean experiments conducted at Lake Bolac and Gymbowen during 2020.

^AThese fungicides are additional to all treatments receiving a tebuconazole application at the 4 to 6 node growth stage (PER13752 until 30/6/2024) and multiple carbendazim and procymidone. ^B Proymidone is applied under PER92791 until 31/10/2025. ^CThe full control treatment is a rotation of fungicides applied to ensure minimal to no disease as a control in the experiment.

PBA Bendoc and Fiesta VF consistently showed higher levels of chocolate spot compared to PBA Samira and PBA Amberley, with disease symptoms observed and progressing under all treatments (data not shown). These results highlight the requirement for fungicides to be applied to all varieties, to prevent severe disease. Although there was no variety x treatment interaction in grain yield, there is a trend to reduced grain yield in PBA Bendoc and Fiesta VF, compared to PBA Samira and PBA Amberley . The treatment effects showed greater fungicide efficacy in the dual active chemistries, with fludioxonil + pydiflumetofen at early flowering, providing higher yield gains compared to the other fungicide strategies. However, the gross margins showed all treatments had good gross margin increases (Table 5).

Table 5: Grain yield of four varieties with contrasting resistance/susceptibility, with seven different fungicide strategies applied at Lake Bolac during 2020. Percentage yield increase relative to untreated and gross margin ($/ha) of each fungicide treatment is also presented.

^ASee Table 4. ^BResistance rating of the variety. ^CDifferent letters indicate pairwise significance (P<0.05); ^DGross margin was calculated as the grain yield gains minus the cost of the fungicide treatments. Chemical prices were an average of three chemical resellers prices provided, grain price was assumed to be $400/ton, and an application cost of $10/ha.

Sclerotinia white mould

Sclerotinia white mould (SWM) is another damaging disease that can infect many pulse crops including lentil, chickpea, faba bean, vetch, field pea and lupin. It can also affect canola, pasture legumes and many weeds. This disease poses its greatest risk during seasons with prolonged damp conditions. Currently, there is limited knowledge on its control, but several fungicides currently registered or under permit for other diseases in pulses and could be used in Victoria. Sclerotinia was widespread in Victoria during 2022 and sclerotes can last in the soil for over 15 years. Research into management strategies is in the preliminary stages, with a focus on determining whether varietal resistance exists, and research on the incorporation of integrated disease management strategies.

The following article provides more detail for Victorian growers : Field Crop Diseases Victoria

Fungicide resistance

Resistance to fungicides is becoming an increasing threat to crops across Australia. There are five fungicide strategies, with two applicable in-season that growers can adopt to slow the development of resistance in pathogen populations and therefore extend the longevity of the limited range of fungicides available:

• spray only if necessary and apply strategically. Avoid prophylactic spraying and spray before disease gets out of control
• rotate and mix fungicides/modes of action. Use fungicide mixtures formulated with more than one mode of action, do not use the same active ingredient more than once within a season and always adhere to label recommendations.

For more information on the management of fungicide resistance, consult the ‘Fungicide Resistance Management Guide’ available from AFREN.

Conclusion

The inoculum sources are present in paddocks and therefore, in the absence of proactive disease control, the risk of yield losses is likely if conducive conditions eventuate. Proactive in-season disease management plans should consider monitoring crops, ensuring the management strategy matches the varietal resistance, and applying appropriate fungicides at the required time.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. Co-investment for this work was provided by the Victorian Government and the GRDC. Thanks to Agriculture Victoria’s Field Crops Pathology team, Southern Pulse Agronomy team, Birchip Cropping Group, Southern Farming Systems, NSWDPI, The University of Adelaide and SARDI.

Useful resources

Current Victorian cereal disease guide

Current Victorian pulse disease guide

Australian cereal rust survey

Identification and management of field crop diseases in Victoria

Wheat stripe rust management 2023

Australian Pesticides and Veterinary Medicines Authority (APVMA)  – for crop protection products details including minor use permits.

Contact details

Hari Dadu
Agriculture Victoria
Private Bag 260, Horsham VIC 3401
03 5450 8301
Hari.Dadu@agriculture.vic.gov.au
@Imharidadu

Joshua Fanning
110 Natimuk Road, Horsham VIC 3400
0419 272 075
Joshua.fanning@agriculture.vic.gov.au
@FanningJosh_