Powdery Mildew

Pathogen

  • Blumeria. graminis f. sp tritici affects wheat and triticale
  • Blumeria. graminis f. sp. hordei affects barley
  • Blumeria. graminis f. sp. avenae affects oats
  • Blumeria. graminis f. sp. secalis affects rye

Hosts

All cereals can be infected by mildew although there are several forms of the disease which are specific to individual crops and do not cross-infect.

Symptoms

Symptoms of powdery mildew can be found on leaves, stems and ears, but leaves are most commonly infected. Typically, white pustules appear which produce masses of spores with a powdery appearance . Eventually, black spore cases (cleistothecia) can be found embedded in the mildew pustules - usually towards the end of the season

Life Cycle

Powdery_Mildew-life-cycle.gif

Mildew overwinters primarily as mycelium on volunteers and autumn-sown crops. The cleistothecia produced during late summer are resistant to low temperatures and drying out, and they allow the fungus to survive for a time in the absence of a host. In humid weather, cleistothecia release the sexually produced ascospores which can initiate autumn infections. As temperatures rise in the spring, dormant mycelium starts to grow and spores are quickly produced. These germinate over a wide range of temperatures, from 5OC to 30OC, although 15OC is optimal with relative humidity above 95%. Free water inhibits spore germination. Under dry conditions, fresh spores can be formed in about 7 days. At the end of the season, volunteers and early autumn-sown crops may become infected, providing inoculum for the following crop.

Importance

Late-sown winter wheat crops are often particularly prone to attack, especially when growing rapidly in the spring. Excessive nitrogen fertiliser also encourages the disease and mildew can be particularly severe in dense crops. The visual appearance of the disease usually outweighs its damage potential especially during the autumn and winter. In susceptible varieties yield losses can be high (up to 20%) and early control can be very important. However, the disease generally causes much smaller yield losses and late attacks (after flowering) on the flag-leaf and ear rarely cause significant losses.

Cultural control

The eradication of volunteer cereals which act as overwintering sources of inoculum and disposal of stubble and debris which may be infested with cleistothecia can be important in mildew control. High levels of nitrogen fertiliser should be avoided. Later drillings of wheat tend to suffer more from mildew attack.

Wheat varieties exhibit a wide range of resistance to the disease and many current varieties have good disease resistance. However the range of sources of resistance used in most current wheat varieties is relatively narrow. Some varieties from other European countries can be very prone to mildew attack.

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