By Fiona Burnett 
(SRUC) for the Farm Advisory Service

Plant disease risks associated with a changing climate and with the global movement of seeds and plants have had a lot of press coverage in recent years with examples such as ash die back catching the public’s attention. 

Statutory controls do much to keep out many of the notifiable diseases of concern to agriculture that would erode Scotland high health status.

These controls, such as plant passporting and inspections at points of import, are important, but, plant diseases don’t just come in on infected plants but can travel in other ways such as by wind or via insect vectors. 

In addition, diseases which appear only rarely because our climate does not favour them, are increasing as temperatures warm so that a changing climate carries a risk that other disease will establish. 

Tan spot is an example of this – it has appeared in Scottish crop surveys intermittently – but never at high levels until recently. In 2016 and again in 2017 it has been very common in SRUC trial sites and coming into the clinic from commercial crops. 

Tan spot, as it sounds, produces obvious tan shaped lesions in wheat and barley, often with a characterised dark centre, and we see it most often in treated crops where other diseases have been managed well so it stands out on otherwise green leaves. 

The disease, caused by the fungal pathogen Pyrenophora tritici-repentis, is common in Scandinavian countries and generally enjoys warmer temperatures and colder winters than are usually seen in Scotland although high moisture levels are also important too. Hence, it can cling on in Scotland in what are maybe suboptimal temperature conditions for it. 

Potentially as our temperature climbs it will become one to watch. It is mainly trash borne over the winter and then airborne, blowing into new crops in the spring where it establishes and splashes upwards on to new leaves. It can affect ears too, causing a glume botch and as such can also be carried as a seed infection. 

Because of the very obvious symptoms it causes a lot of concern, and also frustration because of its habit of appearing in crops where control has otherwise been good. As yet it is treated as minor pathogen and is still relatively weak in terms of robbing yield or competing for space against our more established diseases problems. 

Control is not specifically recommended but one possible reason we are seeing more of it is because of fungicide resistance issues. It has developed a partial resistance to strobilurins and it also carries mutations conferring reduced sensitivity to SDHI fungicides which is why it is often so visible in treated crops where other diseases have been checked by fungicides, leaving it a clear green space to capitalise on as it has been less affected by the fungicides applied. 

A common misapprehension is that it is exclusively a disease of wheat. We have actually seen far more of it in barley over the last two years.

This may be due to the traditionally lower doses used in barley crops and the use of QoI and SDHI partners in lower dose azole mixes.

Observations from SRUC trials suggest that azole fungicides are still effective and also suggest that although very visible it doesn’t yet warrant specific control measures. Superficially in barley and wheat it might be confused with rhynchosporium or septoria but the dark centre shown in these photographs makes it quite distinctive.