Managing powdery mildew in wheat

Page last updated: Wednesday, 22 June 2022 - 3:32pm

Please note: This content may be out of date and is currently under review.

Integrated disease management strategies

  • Variety choice – The cheapest option would be to select resistant wheat varieties and reduce the proportion of area sown to wheat varieties rated very susceptible (VS) or susceptible (S) to powdery mildew in powdery mildew prone areas. The Wheat Variety Sowing Guide has more information on individual variety resistance profiles at seedling and adult plant stages.
  • Seed dressing and in-furrow fungicides registered for the control of other wheat leaf diseases can assist with reducing powdery mildew infection but are not registered for this use.Young plants are most susceptible to WPM, in high disease risk situations reducing risk of early disease onset can be beneficial.
  • Foliar fungicides can be used to control powdery mildew infection, application early in disease development is most effective.
  • Avoid sowing back into wheat stubble from highly infected crops, mildew is carried as fruiting bodies on infested stubble. Wheat powdery mildew only infects wheat.
  • Control/destroy volunteer wheat plants at least four weeks prior to seeding, this will reduce inoculum of powdery mildew (and leaf rust) carried into the season on green bridge.
  • Avoid growing extremely dense canopies. Dense canopies make it difficult to get adequate penetration of fungicides 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
    • Nitrogen. Avoid excess nitrogen application beyond crop yield potential, manage nitrogen inputs to maximise return on investment and be prepared for fungicide applications in high nitrogen status crops. Plants with high nitrogen status grow more rapidly and are inherently more susceptible to greater levels of WPM infection. Consider split applications of nitrogen, with lower rates at early growth stages and remaining applications depending on yield potential, seasonal conditions and soil type.

    • Potassium. Ensure adequate potassium on deficient soils (potassium deficiency can make crops more vulnerable to infection) but be aware that potassium application above plant needs will not further reduce disease risk.
Figure 3: Impact of rates of applied nitrogen on powdery mildew in three varieties, Magenta (blue), Mace (orange) and Scepter (grey).
Figure 3: Impact of rates of applied nitrogen on powdery mildew severity in three varieties, Magenta (MRMS, Mace (MSS), Scepter (VS).

Variety ratings

Disease development, yield impact and potential fungicide responses are usually greatest in more susceptible varieties, such as those rated VS or S, Corack, Trojan, Wyalkatchem, Scepter. Varieties such as Mace and Calingiri are moderately susceptible to susceptible and can still suffer severe infection. DS Pascal, which is rated R, has the best level of resistance among current varieties, followed by Arrino (MR), Illabo (MRp) and the following MRMS varieties: Binnu, Longsword, LRPB Kittyhawk, Magenta and Yitpi. See the latest Wheat Variety sowing guide for ratings. During 2016 – 2017, testing of WPM isolates from across the wheatbelt showed no regional variation in response of commercial varieties, Wheat Variety sowing guide ratings are applicable across all regions of the wheatbelt.

Disease severity and incidence is closely related to the presence of inoculum and also to the susceptibility of the variety chosen. Susceptible (S) and very susceptible (VS) varieties are most likely to suffer early infections and require fungicide management. Regrowth of these varieties is also most likely to multiply inoculum in a green bridge.

Fungicide management Strategies

Seed dressings and in-furrow fungicides

Currently no seed dressing or in-furrow fungicides are registered for powdery mildew in wheat but several, such as flutiafol in-furrow and fluquinconazole seed dressing, are registered for powdery mildew in barley. DPIRD trials conducted in 2016 showed that fungicides at seeding show potential for providing powdery mildew management. A list of registered seed dressings and in-furrow fungicides is available.

Foliar fungicide application

Early infection such as during tillering

In situations where varieties susceptible to powdery mildew are sown without seed dressings/in-furrow fungicides, it is important to apply foliar fungicide to the crop if there are early signs of mildew, especially during the tillering stage to protect emerging tillers. In high powdery mildew prone environments, the first foliar fungicide spray may coincide with post-emergent herbicide application at tillering. A second spray may be warranted before head emergence, three to four weeks after the first spray, if active infections are visible on leaves which have emerged subsequent to the first fungicide spray and seasonal conditions favour the persistence of the disease.

Infection from stem extension onwards

Applying registered fungicides can limit the progress of infection into the upper canopy and heads and is recommended in the more susceptible varieties, although it should be noted that responses from a single fungicide in trials have varied from zero to up to 25% yield increase, but on average around 10%. When favourable conditions continue a second fungicide application may be required if the disease symptoms return after 2-3 weeks. A period of warmer drying weather can rapidly reduce progress of infection, limiting response to fungicide.

Economic considerations

Application of fungicide sprays can reduce the impact of disease, however yield responses and positive economic benefits from fungicide application are never guaranteed. Factors which favour a positive response from fungicide application include:

  • having powdery mildew present, and presence of other diseases,
  • a more susceptible variety (see wheat variety sowing guide),
  • weather (high humidity and mild temperatures) favourable for disease development and crop growth,
  • high-risk areas or in regions with high yield or quality expectations,
  • younger growth stage – early uncontrolled infection during crop tillering can have a large yield impact, fungicide sprays should be applied prior to crop flowering for optimal outcome,
  • good yield potential,
  • fungicide rate sufficient to reduce need for follow-up treatment.

Consequently fungicide responses from fungicide applications later in the year are more likely in early sown crops and in higher rainfall environments. DPIRD MyCrop pages Powdery mildew – economic considerations for management and MyEconomic Tool have information to assist assessing the economic risk and financial impact of treatment options.

Important points to consider:

  • The varieties most at risk are those rated moderately susceptible, or lower, such as Mace, Scepter, Calingiri, Zen, Corack, Trojan, Cobra and Wyalkatchem.
  • Early sown crops are most at risk as they reach flag leaf and heading earlier in the season.
  • Fungicides are best used as protectants rather than eradicants, that is, aim to stop disease progress by protecting uninfected leaves rather than trying to eliminate a severe infection already present. Fungicides can be applied as protectants before powdery mildew becomes severe, regardless of growth stage, if powdery mildew is detected in crops of moderately susceptible to susceptible, or lower. Applying fungicides before infection reaches approximately 5% is particularly important for very susceptible and susceptible varieties.
  • Fungicides will only protect the leaves that are out at the time of application. Fungicide application should protect sprayed leaves for a period of weeks and slow disease development in the crop, however it is unlikely to totally eradicate disease and infection of unprotected foliage can still occur. Later germinating plants/tillers that do not have all their leaves out at time of fungicide application may harbour disease and be a source of ongoing infection for the crop if not controlled by a second fungicide application.
  • Consider other diseases present when choosing a fungicide product.
  • Fungicides should be applied before the canopy becomes too thick or infection progresses to the flag leaf and heads; managing high levels at these stages is difficult and often too late. Apply fungicide just before or at head emergence if active infections are present on middle canopy leaves. If head emergence spray was missed, spraying prior to the end of flowering is usually the best strategy, as sprays after flowering are less likely to give an economic return.
  • Economic responses to sprays during and after flowering are less likely, unless the crop is reaching these growth stages very early in the season or is also affected by stem rust.
  • Yield response to fungicide application is not guaranteed, if disease is not severe or diminishes naturally, then fungicide is unlikely to provide significant yield benefit.

See Registered foliar fungicides for cereals in Western Australia for more information.

A recording of the highly informative webinar Wheat powdery mildew presented by Geoff Thomas (DPIRD) is available on the Training Growers YouTube channel which may assist with decision making on foliar fungicide application.

Factors to consider when making fungicide application decisions:

  • the susceptibility of the variety
  • the growth stage and the level of disease present in the crop
  • weather outlook (humid mild conditions favour disease, hotter drying conditions do not)
  • yield potential and presence of other diseases
  • control of powdery mildew is difficult if infection is advanced and fungicides are best used as protectants rather than eradicants.

Fungicides can reduce the disease severity but yield responses are variable, from no response to around 10%, from a single spray. If the disease is present and increasing in the canopy and weather conditions are favourable, it is recommended growers intervene in susceptible varieties to stop disease reaching damaging levels and moving onto the flag leaf and head.

Timely application more important than which fungicide

All fungicides registered for powdery mildew control in cereals should provide significant protection. Several DAFWA and industry trials with a range of active ingredients have given significant yield responses in wheat (refer to data summarised at Fungicides for managing powdery mildew in wheat historical trial report, more recent collaborative trial work presented at the 2016 crop updates, and at-seeding fungicide trial work conducted by DPIRD in 2016 however yield responses were not guaranteed. Where responses occurred from a single spray application the yield response ranged from <5% to 20% (when fungicide was applied from flag leaf to head emergence). Greater responses can occur where powdery mildew is present with other diseases. In DPIRD trials in the 1990s and early 2000s where disease established earlier (stem extension) and two sprays were applied, responses up to 20% were recorded in the southern region of WA.

Contact information

+61 (0)8 9956 8504
Jason Bradley
+61 (0)8 9368 3982

Authors

Ciara Beard
Jason Bradley

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