Pathogen 

• Pyrenophora tritici-repentis (Drechslera tritici-repentis)

Hosts

The disease affects wheat but can also attack barley, rye and some grasses.

Symptoms

P. tritici-repentis can be seed borne and infect seedlings, resulting in small tan to light brown flecks on young leaves. However, symptoms are generally seen later in the season in the middle and upper canopy. Early symptoms of infection are small tan to light brown flecks, with a chlorotic halo, often with a dark spot at the centre. Later these develop into light brown oval lesions with slightly darker margins with a light coloured spot at the centre. Under wet conditions the lesions produce spores which can make lesions darker in colour. Under ideal conditions these lesions coalesce to produce large areas of dead tissue.

Life cycle

P. tritici-repentis survives mainly as dormant mycelium on stubble and crop debris. This produces pseudothecia on stubble which produce ascospores for long distance spread. Under warm, wet conditions, leaf spots produce dark conidia which are spread up the plant. The disease can infect the ear and cause discoloration of the glumes and the grain. Symptoms on the head are indistinct but can cause brownish glumes. Infected grains can have a reddish appearance, similar to fusarium infection. The disease develops over a wide range of temperatures but has quite a high optimum (20-28^O C) and is favoured by long periods (18 hours or more) of dew or rain.

Importance

Tan spot is very common in Scandinavia and parts of France but is still rare in the UK. Although it is occasionally recorded in disease surveys it rarely causes serious losses on individual crops grown under minimum tillage systems. However is a few isolated cases the disease has caused serious losses.