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Tackling the evolving fungicide resistance threat

Broadacre
10.05.2016

Currently, only two fungicide mode of action (MOA) groups are commercially available for foliar use in Australian cereal crops; Triazoles (Group 3) and Strobilurins (Group 11).

The lack of MOA choice puts the broadacre industry in a precarious position with regards to the development of fungicide resistance. Alarmingly, a popular practice for cereal disease management currently is applying a Group 3 fungicide in-furrow and also at seed dressing, then followed by extensive use of Group 3 fungicides for foliar disease control.

Following a history of overwhelming over-reliance on Group 3 fungicides, it’s not surprising the industry is now witnessing a sensitivity shift to diseases such as net blotch, powdery mildew and septoria.

Net Blotch

Net blotch causes leaf death, poor grain development and increased screenings.

Researchers have identified that the barley disease Net-Type Net Blotch (NTNB) in Western Australia is resistant to the fungicide tebuconazole, as well as other types of triazole fungicides.

This discovery delivered a warning to growers to avoid spraying barley crops with straight tebuconazole products. Various tebuconazole products are registered to control powdery mildew and scald in barley. Its usage on these diseases is considered high risk because it encourages further development of NTNB-resistant populations.

WA growers have been encouraged to use alternative fungicide mixes that contain a quinone outside inhibitor (Qol), as NTNB is unlikely to develop significant resistance to Qol.

The best example of a broad spectrum Qol is Amistar Xtra®. Certain other Qol products are registered for suppression only of net blotch on barley, which may in itself contribute to the future selection of Qol resistant populations.

Powdery mildew

Powdery Mildew causes major yield losses in barley if infection occurs early in the season.

Western Australian growers have noticed a decline in control with triazole-based fungicides on powdery mildew in barley crops.

Powdery mildew populations resistant to tebuconazole based products have now been identified across the Western Australia wheat belt. The current recommendation is for growers to implement integrated management strategies to manage powdery mildew in 2016, given the presence of this fungicide resistance.

According to DAFWA tebuconazole will have reduced efficacy and is not recommended for powdery mildew control in barley. Use of tebuconazole at the rate for scald in barley would increase the selection pressure on the fungicide resistant strains of powdery mildew.

DAFWA note that fungicides containing both a strobilurin and a triazole product, such as Amistar Xtra, have uncompromised activity against powdery mildew. Strobilurin-based fungicide mixtures should be used once per season as part of a fungicide resistance strategy, within the disease management program. For greatest efficacy, Amistar Xtra should be applied before significant disease levels in-crop.

SDHI seed treatment

Group 7 fungicide based actives, such as Systiva®, have recently been registered for seed dressing against a number of diseases. Like other fungicide products, Group 7 fungicide based products are also vulnerable to resistance development. If a seed treatment with foliar disease activity is chosen this season, it should be used wisely. Where a foliar fungicide application is required, it should be with a product from a different MOA, as part of an integrated management package to prevent resistance development.