Disease biology

Tan spot

Summary

Tan spot is a relatively new disease in Denmark. It can be difficult to distinguish the symptoms from especially those of glume blotch (Stagonospora nodorum), but they can be recognised by their being more eye-shaped (elliptical) and defined than the lesions from glume blotch. Usually, there will also be a dark spot in the middle of the tan spot lesion. Under favourable climatic conditions the fungus is highly epidemic. Hot, wet summers promote disease development.

Mycology

The sexual spore cases, the pseudothecia, formed on straw debris in the autumn/winter are black, 200-350 µm in diameter. The ascospores (the sexual spores) are oval or round, brown and 18-28 x 45-70 µm with 3 partitions. The spores (typically 8) are placed in a sacklike cell in the spore cases. The conidia (asexual spores) are cylindrical, 12-21 x 45-200 µm with 4-7 partitions.

Symptoms

The fungus can attack stems, leaves and grains. 2-3 days after infection small, brown dots are visible on the leaves. 4-6 days after infection the lesions have grown (1-2 mm), and a yellow zone is visible round the brown spot. The dark brown/black spot most often remains in the centre of the lesion. Older tan spot lesions are large, elliptical or eye-shaped. Older infected leaves will begin to die down from the leaf tip as the lesions merge. At times, lesions like those on the leaves are visible on the stems. Ear symptoms are rare, but at times brown stripes are visible on the glumes. The fungus often appears together with the Septoria diseases and can be difficult to distinguish from glume blotch in particular.

Time of attack

Most ascospores are released in early April; then the spore release decreases until mid-June. The secondary conidia infection is increased until mid-June and remains high until the crop is ripe. The greatest release of conidia occurs during the last part of the growing season in July. Spore germination is favoured by high temperatures (20-28°C) and high air humidity. As the disease depends very much on the climate, it will be most significant in very hot and humid summers.

Biology

The fungus survives as a saprophyte on straw and stubble debris on which spore cases with sexual spores (ascospores) are formed during the autumn and winter. The ascospores are released in the spring in rainy weather and act as primary inoculum. Other primary sources of infection are mycelium from infected seed or conidia formed on infected straw, other grass hosts and volunteers. As the growing season progresses conidia from the primary lesions are formed in humid weather. The conidia are spread by the wind over large distances and form secondary infections. The largest release of conidia occurs late in the growing season. High humidity and heat promote infection. The optimum temperature for infection is between 20 and 28ºC. The period between infection and first appearance of symptoms varies between 3 and 8 days. Tan spot can under optimum conditions produce a new disease cycle every 8 days. Compared with the Septoria diseases tan spot develops more epidemically under optimum conditions due to its short latent period.

Control measures

Prevention: The wheat cultivars differ with respect to susceptibility to tan spot, but so far no completely resistant cultivars are known. As the fungus survives on straw and stubble debris, cultivation measures such as ploughing and a crop rotation with two years between wheat growing will reduce the amount of infectious matter. However, infection will still be possible from other fields. Oats, barley, rye and triticale can maintain the amount of infectious matter without suffering major damage. The fungus is highly epidemic under favourable conditions. 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). Chemical control: Fungicide treatment can control seed-borne infection. One application at the optimum time is usually sufficient to prevent major yield losses in case of severe disease pressure.

Names and distribution

English: Tan spot. Other names: DTR, yellow leaf spot. Latin: Asexual stage: Drechslera tritici-repentis. Sexual stage: Pyrenophora tritici-repentis. Distribution: Tan spot is found in all important cereal growing areas all over the world, but it has not become a major problem for wheat growing in several countries until the last few decades.

Hosts and importance

Importance: The disease was not found for certain in Denmark until 1999 and is now considered a common disease. Intensified growing of wheat, ploughless tillage and a ban on burning straw on the fields are some of the circumstances that have created conditions favourable to tan spot in Denmark. Severe leaf attacks can lead to diminished yields. There are reports from other countries of yield losses of up to 20-40% depending on cultivar susceptibility and infection pressure. Hosts: Wheat, rye, triticale and a number of grass species, including couch-grass, are susceptible. Barley can in rare cases host the disease.

Names in different languages

Danish :	Hvedebladplet(DTR)
German :	DTR-Blattfleckenkrankheit / DTR-Blattdürre
English :	Tan spot
Latin :	Drechslera tritici-repentis
Norwegian :	DTR
Swedish :	Vetets bladfläcksjuka

Tan spot: Symptoms of tan spot. At the beginning dark brown spots encircled by a yellow zone (chlorosis) are visible on the leaves. Inside the lesion a distinct dark spot (the site of infection) is often visible

Symptoms of tan spot. At the beginning dark brown spots encircled by a yellow zone (chlorosis) are visible on the leaves. Inside the lesion a distinct dark spot (the site of infection) is often visible

Attack of tan spot

Severe attack of tan spot. Older infected leaves will begin to die down from the tip of the leaf onwards

Tan spot attack on the stem

Black, patent spore cases (pseudothecia) on straws, produced by the fungus in the autumn

Crop Protection Online is published by Department of Agroecology, Aarhus University and SEGES Innovation.
Informations for which license is not needed may be used with reference of source.

E-mail to planteit@seges.dk