HRZ growers urged to be prepared for STB threat
The stubble borne disease is impacting growers throughout South Australia’s South East, southern Victoria and Tasmania.
STB has become more problematic because of the increased intensity of wheat cultivation, wetter and cooler springs, the cultivation of susceptible varieties and a shift to earlier sowings.
FAR Australia managing director Nick Poole is recommending an integrated disease management (IDM) approach in the early stages of the growing season as the best way to reduce the disease risk.
Mr Poole says rusts are a “puppy” compared to STB because fungicides do not achieve the same level of control as seen with stripe rust in Australia, and because the disease has a longer latent period which makes it more difficult to determine its progress. In addition, the pathogen has developed the early stages of resistance or reduced sensitivity to triazole fungicides in southern Australia.
“The risk of further fungicide resistance development places much greater emphasis on an IDM approach,” Mr Poole said. “It also means we have to make sure we have as many fungicide tools in our tool box as possible. The introduction of more fungicides with different modes of action will help extend the lives of those materials we already have."
Mr Poole is working on a Grains Research and Development Corporation (GRDC) funded project led by Curtin University to generate efficacy data which, combined with manufacturers’ data, might lead to the registration of new fungicides with new modes of action.
However, with the STB pathogen becoming resistant to strobilurins and triazoles in other parts of world and with evidence that the pathogen in southern Australia has some of the early stage resistance mutations to triazoles, Mr Poole is urging growers to use an IDM strategy and not base control on fungicides alone. He recommends:
- Using more resistant cultivars, particularly where growers are considering planting earlier in April.
- Reducing disease inoculum by reducing stubble on the surface (burning, grazing, stubble incorporation).
- If susceptible cultivars have to be grown, sow them at the end of their safe sowing window.
- Grazing pre-growth stage (GS) 30 (start of stem elongation) has been shown to reduce disease pressure.
- Fungicide management, taking account of recently discovered resistant strains of STB.
Mr Poole says these management strategies have to be balanced against other factors, such as loss of yield potential with later sowing or soil erosion with stubble management measures.
It is important for growers to try and get an idea of how effective these measures are in an IDM approach to controlling STB on their farm. For example, effectiveness of stubble management may be reduced if the farm is surrounded by infected stubbles since the initial spores released from stubble can travel long distances.
Sowing and grazing
Mr Poole says sowing date is crucial to manage STB risk.
“The earlier you sow, the more you expose yourself to the STB pathogen,” he said. “This is primarily because of the long distances that ascospores can travel when released early in the season and only crops that are out of the ground get infected by them. This means the crop gets infected earlier in the autumn and allows more disease cycles during the course of the season. But that doesn’t mean that we now want to go to late sowing.
“Most of the work we’ve done with Southern Farming Systems and Dr James Hunt at CSIRO has shown there are clear benefits to earlier sowing. An effective IDM approach would be to plan sowing windows taking into account each variety’s susceptibility, aiming to plant the more resistant cultivars in the earlier sowing windows, assuming they are adapted for early sowing.”
The primary inoculum spread of the disease is through windblown spores from the stubble which means infection can spread from neighbouring paddocks even if there has been some form of stubble management to reduce inoculum in a newly planted paddock. Stubble management may be of limited value if a grower’s paddocks are surrounded by other paddocks with wheat stubbles from the previous year.
Grazing is a good method of STB control that could be useful in early-sown susceptible wheat crops. In trials conducted by CSIRO, FAR and SFS at Inverleigh, Victoria, last year, grazing before GS31 was as effective at controlling STB as applying a fungicide at GS31 in early April sown Revenue wheat.
Mr Poole says one of the “best fungicides” for controlling this disease was dry weather. Growers should monitor rain between key fungicide timings, most particularly between the start of stem elongation (GS30-32) and flag leaf emergence (GS39).
Cultural practices, such as grazing and later sowing, can be used in conjunction with fungicides to reduce the impact of STB in susceptible varieties. The most effective fungicide strategy for controlling STB in susceptible varieties has been found to be a two-input approach with the first input either flutriafol at sowing or a foliar fungicide at first node (GS31) and a follow up at flag leaf (GS39).
However, Mr Poole says if conditions are very dry after GS31, the second spray may not be needed unless a return to wet conditions occurs later. In addition, where grazing wheat is part of the farming system the need for the first spray at the start of stem elongation could be reviewed, depending on the quality of the grazing (research results were achieved with simulated hard grazing).
“In looking at fungicide options, bear in mind the recent discovery by Dr Andrew Milgate at New South Wales Department of Primary Industries of strains of the disease that show mutations conferring reduced sensitivity (low level resistance) to a number of triazole fungicides,” Mr Poole said. “From experiences in Europe, this form of resistance has become more severe due to an over-reliance on fungicides alone to control the disease.
To extend the useful life of the fungicides, Dr Milgate says growers must change the way they are currently being used.
“Do not use the same active ingredient more than once within any crop for the control of STB,” Dr Milgate said. “Where possible, apply fungicides that contain a mix of actives with different modes of action, such as a triazole/strobilurin mixture. Though formulated triazole mixtures have the same mode of action, the complex nature of triazole resistance means that there may also be a benefit to using triazole/triazole mixtures.”
Mr Poole agreed that repeated applications of the same active ingredient to a crop will accelerate the development of fungicide resistance.
Nick Poole, FAR Australia, 03 5265 1290, firstname.lastname@example.org
Dr Grant Hollaway, Department of Economic Development, Jobs, Transport and Resources Victoria, 03 5362 2111, email@example.com
Dr Hugh Wallwork, South Australian Research & Development Institute, 08 8303 9382 firstname.lastname@example.org
Dr Andrew Milgate, NSW DPI, 02 6938 1990, email@example.com
ResourcesDownload a GRDC STB fact sheet at www.grdc.com.au/GRDC-FS-Septoria-Tritici-Blotch-Wheat
GRDC Project Code CUR00019, DAN000177