Disease biology

Net blotch

Summary

Net blotch can in case of severe attacks result in a decrease in yield of 10-15%. Symptoms are either well-defined round or oval spots (necroses) (spot type) or distinct, longitudinal and transverse dark spots that form a net (net type). Eventually, yellow tissue (cloroses) is formed round the spots, often ending with the entire leaf turning yellow and then dying. Under favourable climatic conditions the net blotch fungus is highly epidemic. The need for control is determined on the basis of a risk assessment. Use healthy seed and resistant cultivars. For choice of fungicides: see the control guidelines.

Mycology

In the leaf spots conidiophores are formed from which isolated cylindrical conidia with 4-6 partitions appear. Their size is 90-120 x 19-21 µm. Dark pseudothesia (200-400 µm) can be found in the spring on overwintering straw and plant debris. These contain double-walled asci with oval ascospores (38-60 x 20-25 µm). The ascospores all have transverse partitions and one or more longitudinal ones. Systematics: Ascomycotina, Loculoascomycetes, Pleosporales, Pleosporaceae.

Symptoms

Symptoms of attack show as either well-defined round or oval spots (spot type) or distinct, longitudinal and transverse, dark spots that form a net (net type). In time, yellow tissue (chloroses) will form around the spots, often with the final result that the entire leaf turns yellow and dies. For a short time before the symptoms show as necroses or chloroses the infected tissue may show as water-soaked areas. The fungus secretes toxins, which means that the leaves may die quickly. Attacks in the ear only result in minor discolourations.

Time of attack

The first symptoms will show in winter barley as early as October. Major attacks are usually not visible until spring. The symptoms in spring barley will in damp years show from the elongation stage (stage 30-31) onwards.

Biology

The fungus is found on both living and dead tissue. Its capacity for infection can last 1-1.5 years. Large amounts of plant debris result in a high infection pressure but not necessarily in major attacks. Under favourable climatic conditions even a low infection pressure can result in major attacks. In the spring fruit bodies show clearly as small pin-sized dots (pseudothecia) on attacked plant debris. The fungus is also seed-borne; a small amount of seed infection can under favourable climatic conditions be sufficient to cause severe attacks. Spreading of infection occurs only over short distances through rain splash and wind. Climatic requirements: The fungus thrives best at relatively high temperatures and humidity. Spore germination is at its optimum at 10-15°C. 10-30 hours of leaf humidity is necessary to achieve a high rate of infection.

Control measures

Prevention: Let several years pass between the growing of barley (both spring and winter barley). Plough in infected plant debris and volunteers. Use healthy seed and resistant cultivars. If the seed contains more than 5% infected grains, it should be treated with a fungicide. Seed infection develops most under cool conditions; therefore late sowing of spring crops restricts infection. Attacks increase with increasing N level. The Danish Plant Directorate carries out analyses of the need for treatment with fungicides against the net blotch fungus. 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). Damage threshold: The net blotch fungus is highly epidemic under favourable climatic conditions. The need for control is based on a risk assessment in which the number of days with precipitation is the most important factor. The risk of attack is highest from stage 31 (the elongation phase) until the last part of the growing season.

Names and distribution

English: Net blotch. Other names: Barley net blotch. Distribution: The disease is widespread in temperate humid climates. In Mid-Europe barley net blotch only causes minor damage.

Hosts and importance

Importance: The fungus mainly attacks barley. In Denmark the yield losses in winter barley are approx. 10-15% in cases of major attacks. Host: Barley.

Names in different languages

Danish :	Bygbladplet
German :	Netzfleckenkrankheit
English :	Net blotch
Latin :	Drechslera teres
Norwegian :	Bygbrunnflekk
Swedish :	Bladfläcksjuka

Net blotch (net type) and cereal leaf beetle larva on barley

Net blotch – net type

Net blotch – spot type

Net blotch – net type

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