Disease biology

Mildew

Summary

Mildew on cereals shows as white colonies on leaves and stems. In case of severe attacks they can merge into entire coatings. Attacks often begin in places with a dense plant population, for example near hedges. The lower plant parts are attacked first. Attacks can occur throughout the growing season. The yield loss from mildew attacks is often 10%, but losses of up to 25% are known. There are several approved fungicides – use the control guidelines to calculate control needs and dosage.

Mycology

Close-set, club-shaped conidiophores are formed on the above-ground hyphae. On these are formed conidia chains, in which the conidia are fastened to each other. The conidia are 8-10 µm x 25-30 µm in size. Under Danish conditions cleistothecia are formed in June-July; these are the results of the sexual cycle of the fungus. Immature cleistothecia are small, light and round. Mature cleistothecia are visible without a magnifying glass. In old colonies they show as brown-black spots. Systematics: Ascomycotina, Plectomycetes, Erysiphales, Erysiphaceae.

Symptoms

Attacks often begin in dense plant populations. The lower plant parts are attacked first. White, mealy colonies show on stems and leaves; in case of severe attacks they merge into entire coatings. Especially root shoots are attacked because young plant tissue is more susceptible than old. In case of severe attacks the ear can also come under attack. Old mildew pustules turn grey or brown, and small black pin-sized fruit bodies (cleistothecia) are formed, which indicates that the fungus has passed through its sexual stage. In connection with the plant’s resistance reactions to mildew attacks yellow (chlorotic) spots or brown spots (necroses) can appear on the leaves in barley; with a magnifying glass it is possible to see mildew mycelium. Can be mistaken for: The mildew resistance symptom can be mistaken for a net blotch attack (spot type).

Time of attack

Infection occurs throughout the growing period. In winter crops attacks can occur in the autumn, first and foremost in winter barley. Attacks are especially found in early sown winter crop fields that are infected with mildew from volunteers. In spring crops the first symptoms do not show until approx. 3 weeks after germination.

Biology

The fungus can only live on living plant material. The sources of infection in spring crops are therefore volunteers and winter crops. The fungus is spread by the wind over large distances – up to 600-800 km. Infection thus comes from for example both the United Kingdom and Germany. Volunteer plants sustain infectious matter after harvest, thereby infecting germinating winter crops. Winter crops can also be infected from cleistothecia (ascospores), but the extent of this is not certain. Mildew can be transferred in the spring from winter to spring crops. The distant infection arrives approx. 10-14 days later than the infection from the Danish winter crops. Climatic requirements: Mildew thrives best under changeable weather conditions. The fungus develops most rapidly at a temperature of 20-25°C. Spore production is promoted by moisture, whereas spore dispersal is best in dry and windy weather. The period from infection until symptoms are visible (the incubation period) is shortest at high temperatures; at 20°C 5-6 days, whereas it is 14 days at 5°C. The spore production can continue during periods without frost. Heavy precipitation removes the spores from the air and damages the mycelium. Periods with frost in the early spring has a vastly inhibitory effect on the fungus.

Control measures

Prevention: Remove volunteers and use mildew resistant cultivars. Cultivar mixtures consisting of cultivars with different resistance to mildew can also be used. Early sowing of winter crops and late sowing of spring crops increase the risk of attacks. Avoid high nitrogen level and dense plant populations. Application of slurry during the growing season can inhibit the fungus. Control severe attacks in winter crops to avoid infection of spring crops. Forecast service: Aarhus University, Faculty of Agricultural Sciences and the Danish Agricultural Advisory Service issue forecasts. Keep up to date on the Internet under: Varsling/Registreringsnet (= Warning/registration network developed). In Denmark virulence in mildew is monitored continuously. The virulence monitoring records changes in the genetic composition of the mildew population and ensures early information on mildew resistance for each cultivar. Keep up to date on the Internet under: www.planteinfo.dk/sortinfo . Chemical control: Several approved sprays exist – use the control guidelines for mildew to calculate control needs and dosage.

Names and distribution

English: Wheat mildew, barley mildew, rye mildew and oat mildew. Other names: Mildew in cereals, powdery mildew. Distribution: Mildew is widespread all over the world but is found especially in the temperate areas of Europe, in the coastal areas of Canada, in the USA along the east and west coast, in Asia and in northern India.

Hosts and importance

Importance: Yield losses of up to 25% can occur depending on the time of attack and the course of the epidemic. In Denmark the yield loss through mildew attacks in barley and wheat is often 5-10% in the field trials. Hosts: In addition to the cereals, most wild and cultivated grasses can be attacked. No infection can occur between the plant species but only between winter and spring forms, for example from winter barley to spring barley. Among the cereals attacks occur especially in barley and wheat, then in rye and to a lesser extent in oats.

Names in different languages

Danish :	Meldug
German :	Mehltau
English :	Mildew
Latin :	Erysiphe graminis
Norwegian :	Mjøldogg
Swedish :	Mjöldagg

Severe mildew attack in wheat

Mildew on the base of a straw

Left: older or controlled mildew; right: fresh attack

Mildew seen through a powerful magnifying glass

Mildew resistance reactions

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