Causal organism - Pyrenophora tritici-repentis
Wheat but can also attack barley, rye and some grasses.
Tan spot can be seed borne and infect seedlings, resulting in small tan to light brown flecks on young leaves. Symptoms are normally seen later in the season in the middle and upper canopy. First 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.
The disease is greatly favoured my minimum tillage systems as the disease survives mainly on crop debris left on the soil surface. Disposal of crop debris by ploughing can help prevent early infection. Many varieties are susceptible.
Seed infection is controlled by most seed treatments. It is essential to protect the upper leaves from disease by appropriate fungicide sprays when wet weather occurs between GS32 and GS39. Sprays applied against Septoria will normally control tan spot although timings are more critical as tan spot has a very short latent period (5-7 days).
Fig: Pseudothecia developed in wheat stem
Tan Spot survives mainly as dormant mycelium on stubble and crop debris. This produces pseudothecia on stubble which produce ascospores for long distance spread. In the absence of crop debris, initial infections in the autumn or spring may result from seed borne infection but this is not thought to be a major source of inoculum. Under warm, wet conditions, leaf spots produce dark conidia which are spread up the plant under wet conditions. 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 (20-28oC) and is favoured by long periods (18 hours or more) of dew or rain.