Powdery Mildew
Powdery mildew on barley
Pathogen
Fungus
Hosts
Wheat, barley, oats, grasses.
Symptoms
White cotton wool pustules appear on the leaves, the stem and on the ear. The pustules can easily be rubbed off and the tissue is discoloured brown underneath.
With time the pustule itself turns brown and black spore cases (cleistothecia) can be seen embedded in the fungus. Occasionally a reaction called hypersensitivity occurs on barley varieties with a high disease resistance rating. The symptoms are small brown flecks with no evidence of mildew pustules. The reason for this is that the infective mildew spores germinate on the leaf and begin to infect the plant. The host tissue dies and the mildew no longer has living material to grow in and a small, brown lesion results. It is also possible that fungicidal phenolics are generated by the dying tissue.
Development
Powdery mildew can only grow on green plants. It survives the harvest period on late tillers releasing spores that are spread by the wind to infect the new crop.
The usual life-cycle duration is 5 – 7 days.
Favourable Factors
Ideal growth conditions are temperatures of between 15 – 22°C and humid (but not raining / free water) weather. Wind aids spread. Temperatures above 25°C check disease spread. Rapidly growing thick crops provided with high levels of nitrogen are more susceptible, particularly some susceptible varieties as described by recommended list ratings.
Importance
Mildew diverts nutrients from the plant to the fungus and increases respiration and transpiration. Severe early season infections, although very conspicuous, don`t impact on yields on a comparable scale to Septoria (on wheat) or rusts.
10% to 15% yield loss in barley is common but can be as much as 40%. Yield loss wheat is not as high, but can be up to 20 % where disease pressure is severe.
Grain filling is reduced leading to shrivelled grain and a poor sample.
Control
Correct fungicide treatments
Varietal choice
Removal of volunteers which provide a `bridge` to early sown crops
Close up of pustules
Powdery mildew on wheat
Close up of mycelium
Mildew on wheat ear
Close up, note the black spore cases (cleistothecia)
