New fungicides and disease management strategies for wheat and barley
Author: N. F. Poole and T. Wylie, Foundation for Arable Research (FAR) Australia | Date: 01 Mar 2017
Take home messages
- Results with the new Succinate Dehydrogenase Inhibitors (SDHI’s) have been very promising against a range of barley and wheat diseases including net blotches and scald in barley and in wheat yellow leaf spot and Septoria tritici blotch (STB).
- The wet spring encouraged greater issues with wet weather stubble borne diseases such as STB and net form of net blotch which has been more widely reported in 2016.
- The SDHI fungicides are at a moderate to high risk of pathogen resistance development so it is imperative that we don’t overuse these products and adhere to anti resistance guidelines.
- As more evidence of fungicide resistance (or insensitivity) in triazoles emerges it emphasises the need to use fungicides as part of an Integrated Disease Management (IDM) approach that capitalises on cultivar resistance and other cultural control measures.
- Combining two Adult Plant Resistance (APR) genes in Avocet Near Isogenic Lines (NILs) reduced the maximum yield response to stripe rust control from (significant responses) 0.98 and 0.4 t/ha where the single APR genes were used alone down to a non-significant response of 0.22 t/ha where the APR genes were combined.
- Controlled environment studies have shown that the curative activity of fungicides such as epoxiconazole against stripe rust is approximately 7-14 days depending on the mean temperature.
- 2016 research work shows that multiple APR genes reduce the need for fungicide applications.
New products for foliar disease control
Research into new fungicide active ingredients
The GRDC New Fungicide Actives project led by Curtin University (Project CUR 00019 and the new bilateral between Curtin/GRDC Program 9) has been working with different target diseases in cereals to generate efficacy data that combined with manufacturers’ data might lead to the registration of new fungicides with new modes of action in Australia. These research projects have and continue to assist with new product registrations that have good activity on important diseases such as powdery mildew, yellow leaf spot (YLS), net blotch and Septoria Tritici blotch (STB). FAR Australia has led the field research and have already identified a number of new fungicide candidates, which are at various stages of development and registration. Though the work has been conducted on a wide range of diseases the results presented here represent just some of the new products experimented upon. To recognise commercial sensitivities, where products have not been registered or where permission has not been given to reveal their code, fungicides have been given a treatment number (e.g. ‘1’) in the graphs.
New products for the control of Yellow leaf spot and STB
In research work conducted on the new fungicide active ingredients, some of the new succinate dehydrogenase inhibitors (SDHI) fungicides have performed extremely well against the STB pathogen. In studies conducted on the SDHI fungicides containing bixafen and fluxapyroxad, results have been very promising against STB (Figure 1). The performance of Aviator® Xpro (containing prothioconazole & bixafen which at present is registered only for the control of blackleg in canola) and the foliar fungicide proposed to be called Ceriax (based on three active ingredients from three different modes of action epoxiconazole, pyraclostrobin and fluxapyroxad) which are currently undergoing registration, have been independently tested since 2013 in GRDC New Actives and CCDM Programme 9 field trials. (Figure 2). The same is true for independent testing carried out on YLS, where trials have shown that if infections persist through stem elongation, the new combination of SDHI with triazoles and strobilurins appear to confer a good level of disease control, although it is noticeable that the level of YLS control with fungicides does not match other diseases.
Figure 1. Influence of new SDHI fungicides on % STB infection on flag -2 assessed during early grain fill (26-27 days after application of flag leaf spray in a two spray programme) – cv Espada 2013 & SQP Revenue 2014 , Southern Victoria.
Figure 2. % Yellow leaf Spot infection on flag-1 28 days after two spray programmes of 14 different foliar fungicides and subsequent yield response – Yorke WA, cv Scout 2013
In barley there are a number of new and future fungicide products that offer a higher level of powdery mildew, NFNB, SFNB and scald control than the existing standards such as propiconazole (e.g. Tilt®). These new actives take the form of both currently registered seed treatments and foliar fungicides. The future introduction of the SDHI bixafen (a component active in Aviator Xpro) and fluxapyroxad (as the currently registered seed treatment Systiva® and the component in the proposed foliar product Ceriax) will provide a step forward for the control of wet weather stubble diseases such spot form of net blotch (Figure 1 & 2). The new active quinoxyfen (available as LegendTM) is a new mode of action for powdery mildew control in barley, which whilst less problematic in the eastern states, it does provide a new option for controlling this disease in barley without depending on the triazoles, SDHI’s and strobilurins, however unfortunately its range of activity is limited outside of powdery mildew so it would need to be mixed with a triazole for broad spectrum activity (Figure 3). Note that SDHI’s and actives such as quinoxyfen may improve our ability to control diseases that are more prone to resistance development, however it should be emphasised that these new actives may themselves be more prone to pathogens developing resistance and therefore need to be mixed with other modes of action if not already formulated with a mixer partner. If the SDHI seed treatment is used, make sure follow up fungicides are of a different mode of action. Do not repeat use of Systiva year after year in barley to control disease. It should also be remembered that Systiva’s control of powdery mildew and leaf rust is less effective than with net blotches and scald.
Figure 3. Spot form net blotch (SFNB) control using the new SDHI’s Fluxapyroxad (Systiva® seed treatment) and the new SDHI foliar fungicide active bixafen mixed with prothioconazole (Aviator Xpro 225EC®) compared to propiconazole applied as a foliar two spray programme in barley cv Hindmarsh – Meckering, WA 2013.
Yield performance and screenings in these trials correlated with disease control assessments (Figure 3), indicating that products containing new active ingredients offer us new and potentially alternative management tools for barley disease control.
Figure 4. Influence of Spot form of net blotch (SFNB) control on barley yield and quality cv Hindmarsh – Meckering, WA 2013
Notes: Tilt 500mL/ha and Aviator Xpro 300mL/ha were applied as foliar sprays twice at GS31 & GS49. Systiva was applied as a seed treatment alone and was not followed up with a later foliar fungicide.
Figure 5. Powdery mildew control in barley using the new active ingredient quinoxyfen (now available as Legend) cv Baudin– Kojonup, WA 2013.
Adjustments to cereal management strategies
The proposed introduction of the SDHI’s, bixafen and fluxapyroxad actives, provide better opportunities to control wet weather diseases such as STB and YLS in wheat and net blotch and scald in barley. Quinoxyfen gives growers better protectant activity against powdery mildew but will need to be mixed with a triazole in order to control other diseases and act as an anti-resistance strategy.
Fungicide resistance risk with SDHI’s
SDHI’s are at a moderate to high risk of fungicide resistance. Ensure that they are not over used particularly as a seed treatment in successive years with no follow up foliar fungicide. Instead consider using fungicides with a different mode of action in alternating seasons or as a follow up to the seed treatment in the same season.
Curative activity of new fungicide actives against stripe rust
One of the most frequently asked questions with foliar fungicide strategies is how much curative activity do the fungicides exhibit, in other words, how long after a crop becomes infected can control of the disease still be achieved with a systemic fungicide? New research conducted under the ACRCP research programme has shed light on this with experiments performed under controlled environment conditions. Work on stem, stripe and leaf rust is just being completed by FAR working in collaboration with Sydney University. Initial indications for stripe rust suggest disease control can be achieved up to seven days after infection with triazole fungicides such as epoxiconazole under controlled conditions. The results indicated that whilst fungicides applied seven days after infection substantially reduced active pustulation (85-95% control), there was still a level of leaf necrosis associated with the disease that the fungicide could not prevent (Figure 4). Since temperature is a key driver for the development of both the crop and the disease, it is important to describe the seven days curative activity in terms of average daily temperatures in the glasshouse. This was 200C for the stripe rust experiment, or 1400C. day degrees. Clearly if average daily temperatures were cooler, the curative activity would be longer in calendar days. For example at an average daily temperature of 150C might be equivalent to nearer 10 days (1400C. days /150C = 9.33 days). At eleven days under these temperatures (11 days x 200C = 2200C. days), the level of active rust infection evident increased, reducing the level of control to 37 – 82% control. Again the majority of leaf damage was as necrosis associated with the disease, rather than leaf area affected by active pustulation.
Figure 6. The influence of three fungicides (an experimental SDHI compound FAR F1-14, Opus® 125 and Radial®) applied 1 day before, 3, 7 and 11 days after inoculation with stripe rust - cv Elmore CL Plus
Value of combining more than one APR gene against stripe rust in wheat
One of the most frequently asked questions with newer foliar fungicides that confer greater green leaf retention is “do they have a value with cultivars protected by Adult Plant Resistance (APR)?” FAR Australia in collaboration with The University of Sydney have been attempting to answer this question using both commercial cultivars and Near Isogenic Lines (NILs; genetically uniform lines that differ only in the presence of a rust resistance gene). Using NILs allows the influence of the APR gene to be assessed in a common genetic background.
In work conducted at Wagga Wagga on the AGT trial site, NILs carrying different APR genes were treated with a combination of a triazole & strobilurin fungicide (Radial® - epoxiconazole and azoxystrobin) at different development timings. The work funded under the Australian Cereal Rust Control Programme (ACRCP) revealed that the single APR genes Yr18, Yr29 and Yr46 significantly reduced a late infection (GS55-59) of stripe rust compared with the fully susceptible Avocet, which is known to have no APR genes. The NILs with Yr29 and Yr46 single genes were more effective than Yr18 alone at preventing stripe rust necrosis (Figure 5). The combination of two APR genes in an Avocet background had significantly less stripe rust necrosis on the flag leaf than any of the single APR genes. The late onset of infection gave the APR genes the best opportunity to express their resistance to the disease given that APR genes are usually considered to be fully active by the flag leaf – ear emergence stage (GS39-59). With the late onset of infection, the most effective spray timing was the single flag leaf emergence application, though this was only statistically superior to the head spray with NIL+Yr18 and susceptible Avocet. The early GS31 spray gave poor results because the infection onset was late and the leaves protected were flag-3 and flag-2 and not the flag leaf and flag-1.
Figure 7. Influence of different APR genes Yr18, Yr29 & Yr46 in a common Avocet background on stripe rust necrosis during grain fill GS72-75.
The differences in stripe rust necrosis lead to the fungicide Radial® giving significant yield increases with all Near Isogenic lines except NIL Yr18+29 where two APR genes were incorporated into the genome (Figure 6). With NIL Yr18+Yr29, a single fungicide application (Radial at GS39), significantly increased green leaf retention of the flag leaf at the doughy ripe stage (GS78-83), from 9% to 39% and increased yield by 0.22t/ha, however this increase was not statistically significant. With the exception of the fully susceptible Avocet, there was no significant difference in fungicide response amongst the GS39, GS59 and two spray fungicide approaches. The early GS31 spray did not give a significant yield response over the untreated with any of the germplasm lines tested.
Figure 8. Influence of different APR genes Yr18, Yr29 & Yr46 in a common Avocet background on fungicide response from different application timings.
New GRDC research being conducted on integrated disease management (IDM) and new fungicides in conjunction with the agrichemical manufacturers shows great promise that will result in better disease management in Australian cereal and pulse crops. However new products with new modes of action are not immune from resistance development, therefore to prolong their activity and that of our existing triazole products, we need to use them judiciously and in combination with other IDM control options.
Controlled environment studies suggest that the curative activity of fungicides such as epoxiconazole against stripe rust is 7 days when the mean daily temperature is 20oC, meaning that at lower temperature the curative activity could be extended. Near Isogenic Lines with an Avocet background illustrated that two APR genes Yr18 + Yr 29 when combined, significantly reduced stripe rust necrosis compared to single APR gene effects of Yr18, Yr29 and Yr46. The differences in stripe rust necrosis led to the fungicide Radial® giving significant yield increases with all Near Isogenic lines except NIL Yr18+29 where two APR genes were incorporated into the genome.
N. B. Please note that reference to an agrichemical fungicide in this paper does not constitute a recommendation or that the active ingredient or product referenced carries an approval for control of a specific disease.
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 author would like to thank them for their continued support.
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