By Fiona Burnett, Head of crop and soil systems, SRUC

THE DRY spring has helped to reduce septoria leaf blotch pressure a bit in wheat crops this year, but there is a surprising amount still there in crops. Scottish wheat growers are well aware that this is their key disease risk – it’s very much a ‘Scottish disease’ – but sadly they are also all too aware how much harder this disease has become to manage in the last three or four years. 

So, this article looks at the basic biology of this pathogen, some of the reasons it has become so tricky to manage and concludes with how to make the best of current control measures.

Septoria (known to the boffins as mycosphaerella graminicola) is a pathogen with many tricks up its sleeve. Once in plant, it can cycle rapidly in a clonal manner and infect new leaves either by splashing upwards or simply by moving in surface moisture where leaves make direct contact. 

It is also a pathogen which can adapt rapidly because, in addition to reproducing clonally, it also introduces sex – which always complicates things!

This means that when a new mutation appears by accident, such as one conferring fungicide resistance, it can spread rapidly through the population and we’ve seen that often – and to our cost – with this disease. 

The classic life cycle is that airborne ascospores are produced in the old crop and on crop debris from late summer onwards. These are carried over wide distances and land in the new season crop. 

Early drilled crops will be exposed to this influx for longer and so tend to have more disease as we move in to winter than later drilled crops do. 
Warm autumns and winters and early drilling over recent years have helped to load the dice in the disease’s favour. Once in the crop, it moves into new leaves and lies latent for a while before visible symptoms appear and new spores are formed in the classic black dot-like sporing structures. 

The time it takes for symptoms to appear is driven by thermal time so where the temperature is only 10°C symptoms might emerge in four weeks and at 20°C in only two weeks (broadly it takes about 220-day degrees for symptoms to emerge). 

That’s important in a practical sense as we have only limited eradication or kick-back from our fungicides. Even the best of these will struggle to kick back more than 100-day degrees which, when it is warm, is worth remembering might be less than a week – which is why we are so obsessed with targeting each leaf layer and not stretching out timings.  

We’ve really done everything wrong when it comes to managing septoria, as we have historically favoured high yielding but susceptible varieties of wheat. 

That was all fine when fungicides worked well, but relying on fungicides to manage the problem drove the septoria population towards resistance. 

In a Darwinian way, any mutation isolates more resistant to the fungicides in use. And then it has much more of the crop to run riot in and so reproduce and spread rapidly in the population. 

We’ve seen this with several groups of chemistry. The strobilurins were initially fantastic against septoria, so were widely used.

But the population fought back after just a few seasons’ use, when a simple one-step mutation conferred complete resistance. 

We’ve seen it again in slow motion with the azole group of chemistry.

Because resistance to that group is controlled by multiple genes, septoria has had to accumulate more and more and more mutations, so efficacy slowly eroded. 

We have compensated a bit by introducing new azoles and by increasing the dose, but the ‘arms race’ there continues. 

SDHIs were a welcome addition and have helped reduce pressure on the azoles but they, too, are under pressure, with increasing numbers of mutated septoria strains being picked up in 2016 and 2017.

The good news is that the worst of these mutants don’t seem to be very fit, so aren’t increasing rapidly. But minor mutations can add up, as we’ve learnt with the azoles, and some have been detected at all our trial sites. 

There has been a move towards more resistant varieties in recent seasons, which is really to be encouraged. 

Many current varieties are pretty susceptible, for example Leeds and Viscount are both rated 4.6. Revelation is a little better at 6.4 and LG Sundance is rated as a promising 7.3. It isn’t the highest yielding in the north and not widely grown, but it is hopefully a sign of better things to come. 

Given the moving situation with fungicide resistance then using balanced mixtures of chemistry, is really key in managing the disease. That means not over relying on either the SDHI or azole partners. 

Mixing in multi-site chemistry, like chlorothalonil, is a win:win in managing the disease, maintaining yield and stewarding fungicides.