University of Saskatchewan

Winter Cereal Production

Powdery Mildew


Powdery mildew is a fungal disease that attacks a wide range of plants. There are a number of different host specific forms of powdery mildew and, except in special circumstances, only Erysiphe grammis f. sp. tritici infects wheat. Within the tritici subspecies of powdery mildew, there are a large number of races that are classified according to their ability to infect different wheat cultivars.

[ Images | Causes and Symptons | Losses | Control ]


a) Causes and Symptoms:

Powdery mildew is spread by airborne ascospores (sexual spores) or conidia (asexual spores). The powdery mildew fungus overwinters as mycelium on winter wheat plants or as cleistothecia (structures that contain ascospores) on wheat straw and stubble. It can infect wheat at any growth stage between seedling emergence and plant maturity.

New powdery mildew infections are produced by spores that blow onto uninfected plants. The spores germinate on the plant and produce superficial patches (colonies) of greyish-white cottony mycelium and conidia on the upper leaf surface. Yellow chlorotic spots appear on the underside of the leaf opposite the developing colonies of mycelium. Under favorable conditions, colony size and numbers may increase until the whole leaf and sheath is covered. Eventually the mycelium patches turn a grey-brown color and start producing small, black, fruiting bodies (cleistothecia). While powdery mildew is usually found on the leaves, it can also attack the stems and immature heads of the plant. Plants that have been severely infected at an early stage are often stunted and may fail to head.

The generation time (time from spore germination to spore production) of the powdery mildew fungus can be as short as 7 to 10 days and a large number of spores may be produced each generation. This allows powdery mildew epidemics to build up very quickly under favorable conditions. Unlike rust spores that can be blown for hundreds of kilometres, spread of powdery mildew spores is usually restricted to a few kilometres. However, the differences in spread patterns are partially compensated for by the fact that rusts must be blown into western Canada from the U.S.A. each year while powdery mildew overwinters on the Canadian prairies.

Once they have been dispersed by the wind, powdery mildew spores can only survive a few days without their wheat host. A relative humidity level between 85 and 100% is optimal for spore germination while periods of dry weather or heavy rain restrict disease development. The most favorable conditions for the development of powdery mildew epidemics occur in lush stands of rapidly growing wheat when the temperature is cool and the humidity is high.

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b) Losses:

Powdery mildew is a common disease in the wheat growing regions of the world. Severe yield losses (>35%) are normally associated with early infections of highly susceptible cultivars.

Powdery mildew outbreaks regularly occur on the Canadian prairies, but disease development is usually restricted to the lower leaves of the plants where it rarely causes significant yield losses. The most severe outbreaks have been reported on wheat grown in the higher rainfall, more humid regions and under irrigation.

In Saskatchewan, powdery mildew outbreaks on winter wheat have been reported most frequently in the extreme northeast section of the crop production area. Symptoms are usually restricted to the lower leaves, but the flag leaves and heads of winter wheat plants may be infected when conditions are especially favorable for powdery mildew development.

[ Images | Causes and Symptons | Losses | Control ]


c) Control:

1. Crop rotation, elimination of volunteer winter wheat, and destruction of wheat residues will reduce overwintering inoculum. However, because powdery mildew inoculum can build up very quickly and spores can be blown in from neighbouring fields, these practices do not provide a very effective means of controlling this disease.

2. When available, resistant cultivars provide the most cost effective means of minimizing losses due to powdery mildew. However, none of the winter wheat cultivars registered for production in western Canada have very good resistance to this disease. Resistance to prevalent races of this fungus often breaks down very quickly and "slow-mildewing" cultivars may be more effective in providing long term control.

3. Foliar fungicides can be used to control severe powdery mildew outbreaks. Depending on the weather, more than one application of fungicide may be required for effective control.

4. Powdery mildew is favored by conditions that promote lush plant growth. Therefore, fertilizer applications should provide a proper nutrient balance and excessive nitrogen levels should be avoided.

[ Images | Causes and Symptons | Losses | Control ]

 

powdery-mildew1

 

Figure 14. Powdery mildew produces greyish-white patches of cottony mycelium on the upper surface of the leaf. (Image Size = 62k)
inoculum can build up very quickly and spores can be blown in from neighbouring fields, these practices do not provide a very effective means of controlling this disease.

2. When available, resistant cultivars provide the most cost effective means of minimizing losses due to powdery mildew. However, none of the winter wheat cultivars registered for production in western Canada have very good resistance to this disease. Resistance to prevalent races of this fungus often breaks down very quickly and "slow-mildewing" cultivars may be more effective in providing long term control.

3. Foliar fungicides can be used to control severe powdery mildew outbreaks. Depending on the weather, more than one application of fungicide may be required for effective control.

4. Powdery mildew is favored by conditions that promote lush plant growth. Therefore, fertilizer applications should provide a proper nutrient balance and excessive nitrogen levels should be avoided.

[ Images | Causes and Symptons | Losses | Control ]

 

powdery-mildew1

 

Figure 14. Powdery mildew produces greyish-white patches of cottony mycelium on the upper surface of the leaf. (Image Size = 62k)