Treating spot form blotch in barley
In Australia spot form net blotch (SFNB) is the most common foliar disease of barley.
In a season where rainfall is sufficient to grow a crop with a 7t/ha yield potential, crop yield can be reduced by up to 25 per cent.
Factors affecting loss
The extent of the loss is related to:
- seasonal conditions (seasons with mild weather (15-25°C) and extended periods of leaf wetness)
- paddock selection
- barley variety selection.
SFNB commonly appears as dark brown spots or streaks, surrounded by yellow tissue. As the disease continues to develop the lesions continue to elongate, following the veins, and are often surrounded by a yellow margin.
Severe infection can result in premature dieback of large areas of leaf commencing at the tip. It is important to note that the disease survives between seasons on stubble.
Resistance to fungicidal treatment
The application of fungicides and the use of disease tolerant varieties are, along with some key cultural practices, the main control strategies available for the management of SFNB.
Due to the lack of adequate genetic resistance in our main barley varieties, fungicides are frequently solely relied upon for the control of many fungal diseases.
Similar to an antibiotic resistance, when fungicides are overused, fungal pathogens adapt and develop resistance. This can result in cross-resistance where other fungicides that share the same mode of action are also at risk. Unfortunately this is happening faster than new fungicides are being released, so the number of effective treatments are progressively reduced.
This fungicide failure has been reported in-field across Western Australia, with STNB resistance to Group 3 DMI’s fungicides of epoxiconazole, propiconazole and tebuconazole and Group 7 (SDHI’s) fungicide of fluxapyroxad. Reduced sensitivity to fungicides are also increasing with these DMI and SDHI fungicides.
Data collated from WA barley leaf collection surveys from 2020, and analysed by the Curtin Centre for Disease Management, showed that one quarter of the samples were resistance to some Group 3 (DMI) fungicides.
See diagram attached.
The paddocks that resulted in reduced sensitivity shouldn’t be impacting fungicide performance, but this is an indication that the use of DMI fungicides is very high and the paddock is at risk of developing resistance. The good news is that 50 per cent of the paddock tested negative for resistance.
Plan of attack
This latest SDHI resistance discovery is a timely reminder to growers and advisers of the need to be vigilant in monitoring for disease and to implement a resistance management strategy that has an integrated approach.
- avoid growing consecutive barley crops – look for suitable alternate break crops
- choose fungicide mixtures with different modes of action and develop good fungicide rotations to mitigate the risk of further resistance development
- Group 7 fungicides (seed dressing and foliar) should not be used more than once per season in any crop rotation – alternate them instead with other fungicides with different modes of action
- do not apply the same Group 3 fungicides twice in a row – look at alternate sprays and avoid using Group 3 fungicides stand-alone
- use fungicides as early as possible after symptoms develop if conditions are highly conducive for disease development
- do not spray below label rates.
Article written by Laura Archer, Elders agronomist based in Albany and Mt Barker, Western Australia.
Header image courtesy DPIRD.
For more information or advice specific to your needs contact your local Elders agronomist.