Wheat powdery mildew (WPM) infection and development is favoured by high humidity (85-100%) and temperatures of 15-22°C, but tends to diminish in spring as temperatures rise above 25°C and as humidity declines. Disease may persist longer in high production situations (such as high rainfall or humid coastal areas) where high seed rates or heavy application of nitrogen fertilizer have promoted dense crops that stay humid over extended periods of time.
WPM can be a constraint to production when susceptible wheat varieties are grown in higher rainfall areas of the Western Australian wheatbelt. It can also be found over greater geographical range in years with high disease pressure and unusually widespread humid and wet weather conditions during the growing season. Historically in WA, epidemics of wheat powdery mildew causing widespread significant yield loss have been sporadic, particularly in northern and eastern growing regions which have typically shorter periods with weather conditions conducive to infection and spread of the disease. However in 2015, with summer storms causing an autumn greenbridge followed by a humid and moist growing season, it was a widespread issue in the northern and central WA wheatbelt and around Esperance, particularly in widely grown susceptible varieties such as Mace, Wyalkatchem and Corack. With wheat area dominated by susceptible varieties (eg. Mace, Scepter), powdery mildew has remained a concern for growers statewide and has been particularly persistent and damaging in the Esperance port zone.
Powdery mildew appears as fluffy, white powdery growths of fungal spores on the leaf surface and in conducive conditions on awns and glumes of the head. Early symptoms can appear as yellow flecks on leaves before mycelial growth occurs. Symptoms typically progress from lower to upper leaves, though infection can occur at any stage through the season where WPM spores are present and conditions allow. Rapidly growing tissue is more susceptible to infection and as such plants at early growth stages and following nitrogen application are typically more susceptible to greater severity of infection. Under severe disease pressure as the season progresses, the fluffy, white powdery growth can also affect stems and heads. Fungal colonies eventually enlarge and merge together. The area surrounding the lesion and on the reverse side of the leaf turns yellow to brown. Older infections on leaves and heads turn grey and can develop black fruiting bodies, called chasmothecia (previously named cleistothecia), which appear as black specks. Moderate to severe infections can result in leaf death. Young, succulent fast growing plant tissue such as new leaves and tillers are more susceptible than older plant tissues. A powdery mildew infected crop will appear yellow from a distance, similar to a crop suffering from water logging or nutrient deficiency. Therefore, close examination is needed, especially if growing a susceptible variety. Details and more photos of the symptoms of powdery mildew are available on the MyCrop page.
Managing the disease in 2020
WPM epidemics are generally favoured by the presence of disease in the preceding season, presence of a green bridge leading into the current season and widespread deployment of susceptible varieties. WPM infection was not widely reported in 2019, with spring infections in the Esperance region being the primary occurrence. Presence of green bridge is a significant contributor to regional risk, this is dependant on summer and autumn rainfall (see Seasonal climate information page). With widespread deployment of susceptible varieties, monitoring of crops remains a high priority, particularly in the Esperance port zone where disease conducive weather persists well into spring.
Infection early in the season can significantly reduce yield (by up to 25%) by reducing photosynthetic leaf area and crop available nutrients, but more crucially by affecting yield potential through stimulating over production of tillers which do not produce heads. Severe infection of powdery mildew at early growth stages can also cause stunting of plants and delayed maturity, which increases the chances of reinfection. Infection later in the season, such as between stem elongation and flowering stages, can reduce photosynthetic leaf area and cause reduced grain size, lower yields and quality downgrades. Severe infection can also cause crop lodging through weakened stems. The earlier the infection and the higher up the plant it spreads, the larger the potential yield loss.
Severe infection at later stages (after Z39) can cause 5-25% yield loss. In DPIRD field trials conducted in the 1990s and early 2000s a single spray application gave a yield response ranging from <5% to 15-17% (when fungicide was applied from flag leaf to head emergence). In 2015 trials, an average yield response of 10% (range of 3-26%) was achieved from a single fungicide spray in four out of six trials conducted by DPIRD and industry partners Landmark, Imtrade, Liebe Group and Northampton Agri Services. Greater yield responses can occur where powdery mildew is present with other diseases. In trials where disease established at an earlier growth stage (stem extension) and two sprays were applied, responses up to 20-25% have been recorded in high potential areas such as the southern region of WA. DPIRD trial work has found that where mildew is the dominant disease, yield responses are not guaranteed so this is important to keep in mind. For example, in 2015, two out of six trials across the wheatbelt had no significant response to foliar fungicide application. At one of these sites the crop was late sown and hot dry spring conditions hastened disease demise, reduced fungicide impact and limited crop yield potential. If disease is not severe or diminishes naturally (due to warm dry conditions for example), then fungicide is unlikely to provide significant yield benefit. At the second non-responsive site, fungicide was applied too late; the flag leaf was already infected with 17% at time of application and fungicides are more efficient as protectants than eradicants.
Results of DPIRD trial research into wheat powdery mildew are housed at Fungicides for Managing Powdery mildew in wheat historical trial report.
Monitoring crops is essential
Control of powdery mildew is more difficult when infection is already strongly established in crop canopies, so growers are encouraged to monitor their crops regularly from early tillering when crops are most susceptible, particularly in susceptible varieties, to detect early symptoms. Particular attention should be given to crops that were early sown, have dense canopies, high nitrogen status and a good soil moisture profile. Weather conditions are key to powdery mildew development, if the outlook is for hot (above 25°C) drying conditions; it is likely that disease will diminish quickly reducing the need for fungicide intervention. Keep up with disease reports in your area through subscribing to PestFax.
How does the disease carry over between seasons?
Wheat powdery mildew can survive between seasons on volunteer wheat plants (green bridge) and on wheat stubble. The presence of green wheat regrowth leading into a cropping season provides an opportunity for biotrophic pathogens such as rusts and powdery mildew to generate infection, meaning greater levels of disease inoculum (spores) to spread onto emerging crops at the start of the season. Favourable summer/autumn weather may allow development/persistence of regrowth in some areas, as occurred in 2015 (see soil moisture maps on the Seasonal climate information page - accept the disclaimer, then see the Soil Water tab). Presence of disease on this regrowth is largely dependent on having significant disease levels in or near that location in the previous season.
The fungus also survives as fruiting bodies on wheat stubbles (from previously infected crops) that release spores after autumn rains. Infection can spread over long distances by light, airborne spores from the white fluffy infections on leaves once a crop is infected.
What conditions favour disease spread?
Powdery mildew of wheat is specific only to wheat and will not infect barley and vice versa with barley powdery mildew but they have very similar symptoms and biology. Powdery mildew is generally favoured by:
- a susceptible host,
- early growth stages e.g. tillering and elongation where growth rates are high
- mild temperatures (15–22°C) and high humidity (in excess of 70%),
- dense crop canopies, where air circulation is poor and in damp, shaded areas,
- high seeding rate leading to dense canopy
- high nitrogen nutrition leading to rapid growth rate and dense canopy
- potassium deficient soil,
- good soil moisture profile which promotes canopy humidity.
Weather - Powdery mildew has a short infection cycle and produces millions of spores (conidia) so can develop rapidly in the crop. In favourable conditions (optimum temperature and high relative humidity) the cycle of spore germination, infection and subsequent spore production can be completed in as little as five days. DPIRD experiments with WA isolates have shown considerable variation in latent period (time between infection and spore production) and spore production with changing temperatures. Ideal temperatures for epidemic development are 15-22°C. Lower temperatures (5-10°C) significantly delay the infection cycle, taking 2-3 times longer than at 20°C, with both delayed symptom expression and diminished spore production. At temperatures above 25°C, infection and sporulation are halted.
Once infection has established, the white fluffy powdery mildew conidia are spread as a secondary infection by the wind throughout the crop. Humid, mild weather and a damp canopy favour development of the disease. The fungus does not require leaf wetness for infection to occur, therefore rain is not required for disease spread but does encourage canopy humidity. Incidence of infection increases as relative humidity rises to 90%, but it does not occur when leaf surfaces are wet (for example, in a rain shower). Heavy rain may actually wash spores off the leaf surface and temporarily slow disease progress.
Infection development diminishes when environmental conditions are unfavourable. These include dry and warm weather conditions that result in periods of low canopy humidity and temperatures above 25°C. DPIRD experiments have shown that 6-12 hours exposure to 25°C will delay disease development for 4-6 days and reduce severity by 30-50%, more than 24 hours exposure to 25°C effectively halts disease development. Consequently, the disease can disappear rapidly in the higher temperatures and lower relative humidity of the spring months.
Region - Regionally the influence of climatic conditions can influence periods of greatest risk and need for fungicide intervention. In northern cropping zones epidemics will develop rapidly during milder winter conditions and can diminish suddenly in warmer dry springs while in central and southern regions epidemics may be slower during cooler winters but accelerate and persist in longer milder springs.
Growth stage - Plants are more susceptible to wheat powdery mildew at early growth stages i.e. seedling to stem elongation, where growth rates and carbohydrate levels in crop leaves (food for WPM) are higher and genetic resistance generally lower.