Powdery Mildew

Erysiphe graminis

Biology

Powdery mildew is one of the most common plant diseases. This disease can be very destructive to wheat and barley and, to a lesser extent, other cereals and grasses. The powdery mildew fungus is made up of different races and forms that are highly specialized, even down to the cultivar of wheat or barley they can attack. Wheat cultivars might be resistant to a certain race of the mildew fungus, but susceptible to another race.

This fungus overwinters on crop residue. Spores are dispersed in early spring by wind and infect susceptible plants. Secondary infections within the crop result from spores produced abundantly on the white mildew that covers the surface of infected plant parts.

Development is rapid in warm humid weather. This fungus is unique in that spore production and infection can take place in the absence of free moisture. Heavy rains are not favorable to spore production or fungal growth over the leaf surface. Rapid growth and dense foliar canopies caused by high nitrogen fertilizer application encourage disease development.

Damage Description

Powdery mildew attacks the leaves, but stems and heads are also affected. The fungus grows primarily on the surface of the host and feeds on the living green cells of the cereal plant. Small, white or gray tufts of spore-producing fungus are most prevalent early in the growing season on the upper surface of the lower leaves. Tissue on the opposite sides of the leaf turns pale green to yellow. The fungal tufts enlarge, join and may turn a reddish brown. Later, this fungal growth becomes dotted with black pepper-like sexual bodies that enable the fungus to survive between growing seasons.

Damage occurs from reduced photosynthetic ability when green surfaces are shaded and the host is robbed of moisture and food by fungal growth. Yields may be reduced by 20 per cent or more. Cereals affected by mildew produce fewer tillers and grains per head and the grains may be poorly filled. Barley and spring wheat, other than soft white wheat, are seldom affected at economic levels on the prairies. Winter wheat is affected to a greater degree. Considerable losses have occurred in some seasons on soft white wheat. The disease will seriously reduce yields if the flag and second leaf are affected.

Diagnosis

Examine the crops at the milky ripe stage (see Feekes scale). Assess no less than 25 main tillers selected at random along two diagonals from one corner to the opposite corner of the field. Assess the percentage of first and second leaves affected. Take the average of the first leaf and the second leaf and apply it to the following formula.
This will give you a reasonable estimate of expected crop loss.

% yield loss = 2/3 x % area of flag leaf infected + 1/2 x % area of second leaf infected / 2

• Use resistant varieties.
• Follow a crop rotation of at least one to two years, or grow other types of wheat.
• Bury crop residue and destroy volunteer cereals.
• Use balanced applications of nitrogen and phosphorus. Heavy nitrogen applications generally produce conditions favorable to outbreaks of this disease.
• Apply a foliar fungicide.