With disease management in cereals focused on control of septoria, rusts and mildew, are we leaving the door open for tan spot? Abby Kellett finds out more.
Current trends in agronomic practice, a lack of varietal resistance and the threat of withdrawal of key fungicide active ingredients all indicate heightened risks of tan spot, according to Defra.
Having remained at low levels for decades, more recently tan spot incidence has been on the rise, so much so that in 2015, it was the second most common disease among UK wheat, affecting 18% of crops, according to Defra’s disease survey.
This figure is significantly higher than the 10-year average of 7% and the highest incidence since 2011 when 23% of crops were affected.
Although Defra’s 2016 wheat disease survey suggests tan spot incidence was closer to the long-term average, Dr Neil Havis, of the Scotland’s Rural College, identified the disease in several cereal crops throughout the country.
“My job is to score all spring barley and wheat in variety trials across the UK and I noticed tan spot in most sites last year. Symptoms were particularly noticeable further north,” he says.
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“It’s one of those diseases which is bubbling under the surface and whether people are noticing it and targeting their fungicide programmes towards it is unlikely.”
Since tan spot was first identified in the UK in the mid-1980s there has been a continuous increase in the proportion of wheat crops grown after minimum tillage, according to Defra.
Its research suggests the more extensive use of min-till systems is directly linked to the increased severity of tan spot, with crops established following minimal tillage suffering 90% greater disease incidence than crops following the plough.
As of yet, the disease has not had a major impact on crop yields in the UK. However, across the world yield losses of up to 30% have been reported.
Because of its threat to cereal yields, several European countries have developed resistant varieties to help control it, yet there are no official variety resistance ratings for tan spot in the UK and new varieties are not assessed for tan spot susceptibility in AHDB Recommended List trials.
Source: Adapted from AHDB cereal disease encyclopaedia
Defra survey data shows that in the UK, three of the most commonly grown varieties in 2014, JB Diego, Solstice and Gallant, were also in the top six for susceptibility to tan spot. Dr Havis says this is likely to favour further development of the disease.
“Control programmes are generally aimed at other pathogens as tan spot is not considered a major problem, but in the EU people are very concerned about the disease and about resistance.
“Equally, breeding programmes are targeted towards producing varieties which are resistant to only one or two pathogens. This could inadvertently allow tan spot to become a much bigger problem for UK growers.
“Just look at ramularia, it came from what was considered a minor pathogen and now it is very much a major one.”
Its similarity to other common cereal diseases such as septoria and rhynchosporium, means there is a danger tan spot is misdiagnosed and not targeted for fungicide treatment.
“Peoples’ first reaction when they see that yellow-type lesion on the leaf is that it’s rhynchosporium or septoria and it’s easy to get confused between the three,” says Dr Havis.
He urges growers and agronomists to assess the colouring of the lesion carefully in order to distinguish between diseases.
“Tan spot has a dark centre, whereas rhynchosporium has a grey-green centre. With septoria you don’t get that dark centre, but you get black pycnidia, which appear as small black spots within the lesion.
While tan spot incidence seems to be on the increase, severe infections are still uncommon in the UK. NIABTAG technical director Bill Clark suggests this is a result of intensive fungicide programmes brought about by other disease pressures.
“Tan spot is very common in Scandinavia and parts of France but severe outbreaks are still rare in the UK – probably as most intensive fungicide programmes give reasonable control – only ‘failing’ in very high disease pressure situations.
However, the time between plants becoming infected and tan spot symptoms appearing is very short, so the timing of a T2 application may not be effective, he says.
“Most of the triazole fungicides applied against septoria will normally control tan spot, although timings are more critical as tan spot has a very short latent period of only five to seven days.
“This means a typical flag-leaf spray wouldn’t control tan spot on leaf 2 because it would have already been emerged for 10 to 14 days by the time the flag-leaf spray is applied.
“If tan spot became a regular issue, we’d probably have to change our fungicide timings to include a T1.5 [GS33-37] to control the disease on leaf 2,” says Mr Clark.
With tan spot levels still considered moderate, he advises growers to apply an appropriate fungicide when wet weather occurs between GS32 and GS39, in order to protect the upper leaves.
Where high levels of tan spot have been identified, growers should consider ploughing ahead of the following crop in order to avoid inoculum build up, he advises.
At present, strobilurin and triazole fungicides are effective against tan spot, but isolates with resistance to strobilurins have been identified in a number of countries across Europe, including the UK.
Moreover, the threat of effective active ingredients being withdrawn is likely to leave growers with few options to control the disease, should it become more prevalent, according to Defra.