Disease biology

Septoria

Summary

Septoria leaf blotch is the dominant species in the early spring. Glume blotch appears later. Attacks by Septoria leaf blotch show in the early spring as light leaf spots on the oldest leaves, in which the black pin-sized fruit bodies (pycnidia) can be seen with the naked eye. Glume blotch shows on the leaves as small brown spots that gradually bleach in the middle. In case of severe attacks the spots grow and merge into large areas. Both fungi are highly epidemic under favourable conditions. Control should be carried out preventively after growth stage 33 and after a period with precipitation. When severe attacks have set in, control will often be useless.

Mycology

In the case of both Septoria leaf blotch and glume blotch the pycnidia are round, 100-200 µm in diameter and have a wrinkled surface. The pycnidia of Septoria leaf blotch are black, whereas those of glume blotch are cinnamon-coloured. The pycnidia spores of Septoria leaf blotch are long and thin (1.7-3.4 x 39-86 µm) with 3-7 partitions, whereas those of glume blotch are 2-4 x 15-32 µm with 1, 2 or 3 partitions and with rounded ends. Systematics: Ascomycotina, Loculoascomycetes, Pleosporales, Pleosporaceae.

Symptoms

In the early spring Septoria leaf blotch is the completely dominant species. The symptom of a Septoria leaf blotch attack is light, irregular leaf spots on the oldest leaves, in which the black, pin-sized fruit bodies (pycnidia) can be seen with the naked eye. Later in the spring the leaf spots turn browner and become more longitudinal and linear in shape. The attacks gradually spread to larger areas of the leaf and are at this time very difficult to distinguish from glume blotch. During prolonged damp conditions the Septoria leaf blotch fungus spreads to the top leaves, whereas the ears are very rarely attacked. Glume blotch shows on the leaves as small brown spots that gradually bleach in the middle. The spots grow and merge into larger areas in case of severe attacks. All above-ground plant parts can be attacked. Gradually, cinnamon-coloured, pin-sized fruit bodies (pycnidia) appear in the attacked tissue. The attacks begin as with Septoria leaf blotch on the lower leaves and spread upwards. Glume blotch can also attack the ear giving the top part of the glumes a brown colour. Glume blotch is seed-borne and can be the cause of reduced germination capacity or discolouration of the germinated plants. The fungus moreover spreads from nearby infected plant debris. BARLEY can be attacked by glume blotch. It shows as lens-shaped leaf spots that often are surrounded by a yellow zone that continues along the ribs. It is characteristic that the spots are often seen close to the transition from leaf sheath to leaf blade. Can be mistaken for: Glume blotch attacks on ears can be mistaken for grey mould attacks. The attacks can be distinguished by grey mould attacks showing a distinct, dark edge round the symptom. Symptoms in barley can be mistaken for attacks by scald or net blotch.

Time of attack

Attacks by Septoria leaf blotch in the early spring are very common but are not of great importance at this time. A damp spell in May can result in well-established attacks in the field. Damp conditions around earing (stage 51-59), at the end of May and beginning of June, can result in the infection spreading to the upper leaves and ears.

Biology

After harvest the fungi survive on straw and stubble debris. The time of survival on plant debris is 5-18 months. Glume blotch and possibly also Septoria leaf blotch are seed-borne. The fungal spores are spread via rain splash and wind over short distances. The attacks spread from one tier of leaves to the next. An open plant population improves the possibility of the rain drops spreading. Attacks therefore often occur in tramlines. Climatic requirements: Both species thrive best under humid conditions, with Septoria leaf blotch especially needing constant humidity. Thus leaf blotch is found mainly in early spring when both the microclimate and the air humidity are high. Humidity is necessary for spore spreading (rain splash), just as spore germination, infection and sporulation are best under humid conditions. This applies to both fungi. Spore germination is possible at 4-37°C with an optimum at 20-25°C. The duration of the leaf humidity and the temperature are the factors most important to reproduction. Epidemics occur especially at frequent and short humid periods, whereas the total amount of rain is of secondary importance. The period from infection to formation of pycnidia in Septoria leaf blotch can vary from 3 to 5 weeks. The shortest latent periods in the field are in hot periods for which latent periods of 11-15 days have been reported. Under Danish summer conditions the latent period is usually about 3 weeks. The latent period is shorter in susceptible cultivars than in less susceptible cultivars. Glume blotch has a shorter latent period (approx. 7-14 days) and thrives best at slightly higher temperatures.

Control measures

Prevention: Plough in plant debris and volunteers. Use healthy or fungicide-treated seed (max. 5% infected grains) and resistant cultivars, even though effective resistance rarely occurs. Most attacks hit short-strawed cultivars because the spores are more easily transported up the plant from leaf to leaf. The risk of attack is highest in late ripening cultivars and in case of heavy N fertilisation. The Danish Plant Directorate carries out analyses of the need for fungicide treatment against the glume 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: Both fungi are highly epidemic under favourable climatic conditions; for this reason no damage threshold is related to the degree of the attack. Therefore the need for control is based on a risk assessment. This includes first and foremost the number of days with precipitation and the cultivar (straw length and susceptibility). Chemical control: Ongoing attacks are difficult to stop. A preventive treatment should therefore be carried out at growth stages 33-55 after a prolonged period with precipitation.

Names and distribution

English: Septoria leaf blotch / Glume blotch. Other names: Wheat leaf blotch / wheat leaf spot, Septoria leaf and glume blotch. Distribution: The fungi are distributed all over the world but are found especially in areas with a cool and humid climate. The first time glume blotch showed major attacks in Denmark was in 1963. Septoria leaf blotch was not found to a greater extent until 1983.

Hosts and importance

Importance: Major leaf attacks of both glume blotch and Septoria leaf blotch can lead to major yield losses, especially in cases of attack at the beginning of elongation (stages 31-33) or at the beginning of flowering (stage 61). The earlier the attack, the higher the yield loss. When the ear is attacked by glume blotch, yield losses of up to 30% can occur. The grains shrivel and the quality is reduced. Attacks after the end of flowering have only little effect on the yield. Attacks in barley are less frequent and are considered to be of minor importance in Denmark. Attacks in rye are rare. Hosts: Wheat and barley as well as rye and a number of wild and cultivated grasses, for instance smooth meadow-grass, annual meadow-grass, perennial ryegrass and couch-grass. The importance of alternative hosts is considered to be minor. Infection from wheat to barley and vice versa is assumed not be possible in practice.

Names in different languages

Danish :	Septoria (Hvedegråplet)
German :	Blattflechenkrankheit / Braunfleckigkeit
English :	Septoria
Latin :	Septoria tritici / Stagonospora nodorum
Norwegian :	Hvetebladprikk / Hveteaksprikk
Swedish :	Svartpricksjuka / Brunfläcksjuka

Early attack of Septoria tritici/Septoria leaf blotch

Septoria tritici/Septoria leaf blotch

Septoria nodorum/Glume blotch

Drechslera tritici-repentis (DTR) can be mistaken for Septoria nodorum

Septoria nodorum/Glume blotch in the ear

Grey mould attacks in the ear can be mistaken for attacks by Septoria nodorum

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