Management of barley powdery mildew in 2018

Page last updated: Tuesday, 1 May 2018 - 1:15pm

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Barley powdery mildew remains a threat to production of susceptible barley varieties, particularly in higher rainfall favourable environments. Since 2012, Western Australian barley growers have noticed a decline in control from some triazole based (DMI) fungicides and subsequently barley powdery mildew populations resistant to tebuconazole based products have been identified across the wheatbelt. Integrated management strategies to manage barley powdery mildew in 2018 inlude varieties with improved resistance and wider choice of effective fungicides.

What happened in the 2017 season?

Barley powdery mildew was generally observed at lower levels in the wheatbelt in 2017. This is mainly due to reduced usage of susceptible barley varieties such as Baudin and environmental conditions which were not as conducive for powdery mildew build up in many parts of southern and central regions. Many popular current varieties are ranked moderatly susceptible (MS) or better for powdery mildew and as such extreme levels of infection are less common. Wheat powdery mildew has become more prevalent in recent seasons but while wheat and barley powdery mildew have similar life cycles they are different diseases and do not cross-infect.

Integrated management of powdery mildew in 2018

Implementing an integrated disease management approach, including strategic use of fungicides, will minimise losses due to powdery mildew and reduce reliance on fungicides and prevent further development of fungicide resistance.

  • Reduce the proportion of area sown to barley varieties rated very susceptible (VS) or susceptible (S) to powdery mildew. Where possible utilise varieties with better resistance which may not require fungicides for powdery mildew management. The barley disease resistance profiles and ratings in WA is available from the 2018 Barley variety sowing guide for Western Australia page. Current malting varieties have a range of disease ratings for powdery mildew, from very susceptible (VS) through to resistant (R) at seedling stage and adult plant stages. Therefore the decision to grow a VS or S rated malt or feed variety should be based on: market price and demand, availability and cost of fungicides which are still effective against powdery mildew, the seasonal outlook and the capacity to implement other in-crop integrated management strategies. Several varieties which are rated MR/R to powdery mildew are susceptible to leaf rust, therefore barley varieties for 2018 should be selected based on the disease situations experiance in 2017 and the seasonal outlook for 2018.
  • Control volunteer barley plants prior to seeding, particularly of S varieties, this will reduce inoculum of powdery mildew (and leaf rust) carried into the season.
  • Avoid sowing back into barley stubble from highly infected crops, mildew is carried as fruiting bodies on infested stubble.
  • Avoid growing extremely dense canopies. Dense canopies make it difficult to get adequate penetration of the fungicide and foster ideal conditions for powdery mildew development. Management practices which enhance canopy size include high rates of nitrogen at or just after seeding. Grazing crops before stem elongation can reduce canopy size and may reduce disease pressure without affecting crop yield.
  • Balance crop nutrition, particularly ensuring adequate potassium on deficient soils and avoid excess use of nitrogen fertiliser.

Fungicide management of powdery mildew in 2018

High disease pressure regions: high-medium rainfall, VS-S varieties, historically frequent fungicide usage required for powdery mildew management (for example, the south coast).

Seed or fertiliser applied fungicides

If reduced efficacy of flutriafol in-furrow has been observed locally, then the length of protection provided by this product is likely to continue to decline, requiring alternative approaches or increased rates (observe label instructions) to maintain length of protection.

For susceptible varieties in these areas, using fluquinconazole seed dressing or a foliar spray at disease onset is worth considering.Up to now fluquinconazole seed dressing efficacy does not appear to have been compromised. The newly registered seed dressing product Systiva® (fluxapyroxad) is from a different fungicide active group (SDHI) and is also registered for powdery mildew suppression. Uniform® (azoxystrobin+metalaxyl-M) also now been registerd to use as in-furrow in suppression of powdery mildew.

Moderately susceptible (for example, Bass) or more resistant varieties are unlikely to need specific fungicide protection from powdery mildew at sowing but may require foliar fungicide protection during the season. Seed dressing or in-furrow products applied to manage other diseases may provide some degree of protection from powdery mildew.

Foliar fungicides

The compromised triazole ingredients tebuconazole and triadimefon will have reduced efficacy and are not recommended for powdery mildew control. Any use of tebuconazole increases the selection pressure on the the fungicide resistant strains of powdery mildew.

Products containing other triazole fungicide active ingredients like epoxiconazole (for example, Opus®), prothioconazole + tebuconazole (for example, Prosaro®) or propiconazole (for example, Tilt®) , should have uncompromised activity against powdery mildew.

Fungicides containing both a strobilurin and an uncompromised triazole active ingredient such as azoxystrobin + cyproconazole (for example, Amistar Xtra®), azoxystrobin + epoxiconazole (for example, Radial®, Tazer Xpert®) or pyraclostrobin + epoxiconazole (for example, Opera®) should have uncompromised activity against powdery mildew. These strobilurin based fungicide mixtures should use one application per season as part of a fungicide resistance management strategy within the disease control program.

For greatest efficacy, fungicides should be applied before significant levels of disease establish in crop.

Use the fungicide to protect the crop rather than attempt to recover the crop. Monitor crops regularly for presence of disease and utilise fungicides to allow maximum disease control and maintain effective green leaf area.

Strobilurin products are protectant chemicals and should not be applied to heavily infected crops. In multiple spray programs use strobilurins in rotation with one of the uncompromised triazole, to minimise potential pressure on this chemistry.

Product choice, rate and water volume used should reflect the range and time of onset of the diseases to be managed and the density of the canopy. It is important to read the product label. When repeated applications are required observe restrictions on maximum dose rates and applications.

Lower disease pressure regions: medium-low rainfall, less frequent fungicide use for powdery mildew.

If mildew has not been a regular focus of disease management in previous seasons then requirement for prophylactic seed dressing or in-furrow fungicide application for powdery mildew should not be required:

  • Use appropriate fungicides to manage disease threats present within the cropping system.
  • If repeated applications of fungicide are required within the season then utilise uncompromised fungicides as part of rotation of products.
  • Apply fungicides according to the label directions and restrictions.

Management of other barley diseases with special reference to net blotches

Selecting a barley variety with good resistance profile is the cheapest option in managing many foliar diseases. In the absence of resistance, common practices in managing many diseases include adopting crop rotation, managing weeds and regrowth barley, maintaining good plant health and use of fungicide. Use of fungicide has become the most popular option among the growers, particularly given the resistance profiles of some popular varieties. Lack of new fungicide chemistry and continued use of a few triazole based products has resulted in development of fungicide insensitivity among some pathogen populations. In 2015, a net-type net blotch infected leaf sample from the Kojonup area was tested by CCDM, Curtin University and triazole resistance was detected. Similarly two spot-type net blotch pathogen populations resistant to some triazoles base poducts have been detected in Albany and Esperance Port Zones in 2017. The extent of the distribution of triazole resistant net-type and spot-type net blotch populations across the wheatbelt is currently not known.

Submission of infected leaf samples to Curtin University for resistance testing is important. Powdery mildew samples should be sent in a paper envielop address to Fran Lopez, CCDM, Department of Environment & Agriculture, Curtin University, Bently, WA 6845.

Selection of fungicides should suit the management of disease threat/s present (according to label recommendations) while limiting the risk of development of fungicide resistance.

Smut and bunt seed dressing fungicides are predominantly triazole, and whilst historical benefits of delayed mildew infection are less likely, they should still be used for management of loose and covered smut.

For more information refer to Seed dressing and in-furrow fungicides for cereals 2018.

Succinate dehydrogenase inhibitor (SDHI) based actives such as fluxapyroxad (Systiva®) have recently been registered as seed dressing against many diseases such as powdery mildew, net-blotchs, scald, leaf rust, smuts and rhizoctonia. Like other fungicide products, SDHI based products are also vulnerable to resistance development and if chosen should be used wisely and in rotation with other fungicide groups, as part of an integrated management package to prevent resistance development.

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