Management of barley powdery mildew in the face of fungicide resistance

Page last updated: Wednesday, 29 July 2020 - 3:37pm

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Barley powdery mildew populations resistant to some fungicide active ingredients are present in the WA wheatbelt. Integrated disease management strategies to manage barley powdery mildew include growing varieties with improved resistance and strategic use of fungicides. It is important for the whole industry to take a united approach to minimise losses due to powdery mildew, reduce reliance on fungicides and prevent further resistance development to fungicides.

Early powdery mildew infection in barley can cause yield loss in susceptible barley varieties of up to 25 per cent and late infection up to 10 per cent. Resurgence of powdery mildew every year is mainly due to growing susceptible barley in disease prone environments. So disease is primarily managed by use of resistant varieties and foliar fungicides.

Barley powdery mildew (Blumeria graminis f. sp. hordei) was a threat to production of susceptible barley varieties, particularly in higher rainfall favourable environments until barley variety Baudin was phased out. Since 2012, Western Australian barley growers have noticed a decline in control of powdery mildew for some triazole based (DMI) fungicides such as tebuconazole, propiconazole, prothiconazole, epoxiconazole, flutriafol, cyproconazole and fluquinconazole due to mutations that occurred in CYP51 gene in the mildew-causing pathogen populations across the wheat-belt. Therefore, managing the disease should be based on integrated disease management to reduce further fungicide resistance development.

Integrated management of powdery mildew

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 resistance development to fungicides.

Grow a variety with powdery mildew 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 carrying the mlo gene which generally do not require fungicides for powdery mildew management. The mlo gene is more stable than other mildew resistance genes currently possessed by Australian barley varieties. The barley disease resistance profiles and ratings in WA are available from the Western Australian Crop Sowing Guide. Current popular malting varieties, as shown in Table 1 except for Rosalind (feed), have a range of disease ratings for powdery mildew, from moderately susceptible (MS) through to resistant (R) at seedling stage and adult plant stages. Therefore, the decision to grow malt or feed variety should be based on: market price and demand, the seasonal outlook and, if choosing to grow susceptible varieties, the capacity to implement other in-crop integrated management strategies. Variety selection should be based on the disease situation experienced in the previous season and the seasonal outlook. For example, RGT Planet possesses the mlo gene and resistance to powdery mildew but is susceptible to both net blotches.  In 2019, a Rosalind attacking pathogen population was found in the lower Great Southern (Albany Port Zone) which is a reminder of the need to be vigilant. Continuous Rosalind cropping will increase this MlLa attacking pathogen population and reduce the mildew resistance of other varieties which possess the MILa resistance gene such as La Trobe (Table 1).

Table 1. Popular varieties grown in Western Australia with resistance genes and powdery mildew susceptibility (Source: 2020 Western Australia Crop Sowing Guide)
Variety Resistance genes Powdery mildew susceptibility
    Seedling Adult Plant
Bass Mla8 MSS MSS
Flinders Mla1 R R
La Trobe MILa MSS MS
RGT Planet mlo R R
Rosalind MILa MS MRMS
Scope CL MILa, Mla7 R R
Spartacus CL MILa, Mla7 MS MRMS

 

If powdery mildew susceptible barley is grown in the region, then

  • 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 on powdery mildew susceptible varieties

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 and fluquinconazole seed dressing 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.

The seed dressing product Systiva® (fluxapyroxad) is from a different fungicide active group (SDHI) and is also registered for powdery mildew suppression; use of this as a seed dressing precludes the application of Aviator®Xpro® or Elatus® Ace later in the season as those products also contain a SDHI. Uniform® (azoxystrobin+metalaxyl-M) is also registered 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 but it is best to use a registered product to get effective mildew control. A list of the products registered for use on barley is available from the Registered seed dressing and in-furrow fungicides for cereals page.

Foliar fungicides

Thirteen compounds from four modes of action (FRAC Code 3, 5, 7 and 11) are registered in Australia for powdery mildew control in barley. A list of these is available from the Foliar fungicides registered for cereals page. The compromised triazole ingredients tebuconazole (Orius®), triadimefon (Triadimefon 125 EC), epoxiconazole (Opus®), prothioconazole + tebuconazole (Prosaro®) or propiconazole (Propiconazole 500 EC), will have reduced efficacy and are not recommended for powdery mildew control. Any use of these actives indicated above will increase the selection pressure on the fungicide resistant strains of powdery mildew.

One way to reduce the resistance development of the pathogen is to use fungicide mixtures with different modes of action. Fungicides containing both a strobilurin (Group 11) and an uncompromised triazole (Group 3) active ingredient such as azoxystrobin + cyproconazole (Amistar Xtra®), azoxystrobin + epoxiconazole (Radial®, Tazer Xpert®), azoxystrobin + propiconazole (TopnotchTM), pyraclostrobin + epoxiconazole (Opera®) or succinate dehydrogenase (SDHI; Group 7) base, benzovindiflupyr + propiconazole (Elatus® Ace) or prothioconazole + bixafen (Aviator® Xpro®) should have uncompromised activity against powdery mildew. Use one application per season of each of these strobilurin or SDHI based fungicide mixtures as part of a fungicide resistance management strategy within the disease control program. A powdery mildew specific multisite amine (Group 5), spiroxamine (Prosper 500® EC) is also registered.

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 strobilurin in rotation with one of the uncompromised triazoles, 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 foliar 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. See detail in ‘Foliar fungicides’ above.
  • Apply fungicides according to the label directions and restrictions.

 

Disclaimer: Mention of trade names does not imply endorsement or preference of any company’s product by Department of Primary Industries and Regional Development. Any omission of a trade name is unintentional. Recommendations are current at the time of publication.

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