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 Crop Variety Sowing Guide has more information on individual variety resistance profiles.
- 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 that 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
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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.
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Variety ratings
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.
See the latest Crop 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, Crop Variety Sowing Guide ratings are applicable across all regions of the wheatbelt.
Fungicide management Strategies
Fungicides at sowing
A range of seed dressing and in-furrow products are registered in wheat for diseases including smuts and bunts, rhizoctonia root rot, wheat rusts, yellow spot and septoria tritici but none are currently registered for powdery mildew. However, some fungicides registered in wheat such as in-furrow products containing flutriafol, triadimefon or azoxystrobin or seed dressings containing fluquinconazole, triadimenol or fluxapyroxad can effectively delay or reduce early build up in risk prone environments where susceptible varieties are grown. Information on registered fungicides at seeding in cereals is available at https://www.agric.wa.gov.au/barley/seed-dressing-and-furrow-fungicides-c....
Foliar fungicides
It is crucial to control the disease before it becomes too severe and develops in the upper canopy and on heads as then it is very difficult to control. A registered foliar fungicide can reduce the disease impact, but growers need to consider the following when deciding whether an economic response to fungicide application is likely:
- presence of disease symptoms
- weather outlook (humid mild conditions favour disease, hotter drying conditions do not),
- variety susceptibility- powdery mildew is more likely to advance rapidly on more susceptible wheat varieties such as Scepter, Vixen, Wyalkatchem, Zen and Corack.
- growth stage and
- crop yield potential.
Very susceptible and susceptible varieties with infection present in the crop canopy and favourable weather over coming weeks will be at greatest risk. 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. Leaves or heads that have not emerged at the time of fungicide application will not be directly protected from disease and may be a source to reinfect the crop. 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. Spraying prior to the end of flowering is usually the best strategy to achieve an economic return but it depends heavily on weather outlook.
Prior to head emergence: If wheat powdery mildew is present and increasing in the canopy of a susceptible variety and the weather outlook is favourable (humid and mild), it is recommended that growers intervene with a well-timed application of registered foliar fungicide to stop disease reaching damaging levels and moving onto the flag leaf and head.
After head emergence: Fungicide applied soon after full head emergence and prior to severe infection can reduce risk and impact of head infection, however in terms of yield responses, trial results have been variable and it should be noted that a period of warm dry weather can quickly eliminate powdery mildew infection on wheat heads and eliminate value of fungicide. Spraying after heads have emerged, prior to the end of flowering and before significant head infection is more likely to achieve an economic return if a wet spring or longer season is expected.
Research has shown that optimal fungicide application is: 1) before disease becomes severe in lower canopy and before flag leaves and particularly heads are infected and 2) where disease onset is later, once all leaves have emerged (i.e. after Z39) so maximum canopy area is protected.
Value of fungicide application after head emergence is variable and depends on level of infection (better to apply before significant disease present in heads) and a soft finish to the growing season (response to fungicide more likely if there’s a wet spring) and is more likely to be economic in a longer season environment. The value of applying a second fungicide has been found to be variable and in the few trials where it was tested by DPIRD was not justified by a significant yield response, but most trials were conducted in a season with a hot dry hot spring and a wetter finish could provide a different result.
A large range of foliar fungicide products are registered for powdery mildew in wheat. Choice of foliar fungicide to apply should be determined by product label recommendation for powdery mildew, previous fungicide history in crop and other diseases present.
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 crop 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.
Important points to consider:
- The varieties most at risk are those rated moderately susceptible, or lower.
- 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.
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.