Tackling Septoria requires sophisticated resistance management

The fungus is genetically highly variable and can easily adjust itself during the season to circumstances in the field. Growers can therefore benefit from sophisticated resistance management that includes growing more resistant varieties, adding multi-site fungicides and alternating modes-of-action.

Septoria is one of the most destructive leaf diseases in wheat, according to Danish researcher Thies Marten Heick at Aarhus University. And, since the fungal disease was commonly acknowledged as the most important cereal disease, fungicide-resistance has developed in several ways in Europe, both in different fungicide-groups and in different regions.

But Heick points out that triazole-resistance has evolved slowly and resistance against this group of fungicides is linked to mutations of a specific gene in the fungus: CYP51. “We see that more and more of these CYP51-mutations occur and that they also appear in combination,” says Heick.

SDHI-fungicides

Resistance against the fungicide group strobilurins is caused by a single mutation in a gene of the fungus. As soon as this mutation (G143A) occurred in the Septoria population it spread rapidly which made strobilurins, as a group, no longer effective to control Septoria. With the SDHI-fungicides, a relatively new group of fungicides, resistance is developing slowly in certain parts of Europe, but so far has not been seen in other parts of the world, according to Heick.

He sees a clear gradient in Europe running both north-south and west-east with regard to fungicide resistance. “We see more Septoria in the west and south. Especially in the United Kingdom and Ireland the resistance problem is huge, because weather circumstances are very favorable for the fungus over there. We also see multiple infection periods per season and the spray-intensity is higher than in the rest of Europe.”

Heick claims that the situation in Scandinavia is less severe than in the UK and Ireland. However, this region has a lack of sufficiently effective fungicides although, in 2017, the situation is improving. In Norway, Sweden and Finland, registration was obtained for two SDHI-products and another will possibly become available in Denmark. Strobilurins are not recommended for Septoria control as these products are not effective enough.

Yield loss

"When Septoria hits your crop early and is not controlled effectively, a wheat grower can lose 20-30 per cent yield," says Terry van Loon, crop manager at ADAMA. He adds that crop protection should be used in a smart way, because the fungus gets better and better at adjusting itself to the products that are used. “In the past, strobilurins were a group of products that were commonly used to control Septoria,” he adds. “Nowadays, mostly triazoles are used, but you can see that the fungus is adjusting itself in this group of products too. Ten years ago, the preventive efficacy of triazoles was above 90 per cent, but that is now down to 60-70 per cent. In a relatively short period the efficacy has decreased by 20-30 per cent.”

One of the reasons for the lower efficacy of fungicides, also known as "shifting", is that the products are used too much in a curative way. The grower only starts spraying if the disease appears. “Curative treatments are stimulated in Integrated Pest Management (IPM), but for cereal fungicides this strategy is not very sustainable. Research in the UK and Denmark has proved that the curative action of triazoles has decreased by 60 per cent in the last 10 years. Preventive spraying is often not considered as an option, but with curative sprays the selection towards a less-sensitive Septoria population is much stronger than with preventive sprays. Septoria tritici has a lifecycle of about 3-4 weeks, depending on weather circumstances. If a grower sees the disease in the field and applies fungicides, it is often too late. He invests a lot of money in fungicides and the result is poor - a waste of money.”

Multi-site approach

When using crop protection, timing is vital, but a good combination of products is very important too. In a single-site approach, different single-site products (like triazoles and SDHIs) are mixed. In this way, it is more difficult for the fungus to adapt and the efficacy of the products is maintained. But it is a short-term approach even though it is better than just applying one product, a mix of single-site products is still giving the fungus the opportunity to adapt. Therefore, van Loon is a big fan of using a multi-site approach with products like mancozeb, chlorothalonil or folpet.

Multi-sites are unique in the way that they work on multiple points in the lifecycle of Septoria. This makes it almost impossible for the fungus to adapt. A disadvantage of multi-site products is that they are less effective than single-site products, so they should always be mixed with triazoles and/or SDHIs. By mixing a single-site with a multi-site product you can protect the crop in a very efficient way and provide optimal resistance management.

T0-treatment

According to researcher Jaap van ’t Westeinde of research station SPNA in Nieuw Beerta (in Northeast of the Netherlands) Septoria has been the key-disease for research topics at his research station for many years. “In trials, we see that the fungus is almost always present in cereal fields. As soon as the crop starts to elongate a grower should protect the newly formed leaf layers,” he recommends. “What we see more and more is that growers use a T0-spray at the beginning of elongation, mainly to control yellow rust. This has a good influence on the Septoria that is already present at that time as well. You can start the rest of the season with a ‘clean slate’ because you are reducing the disease pressure towards the new leaf layers.”

Picking the right variety is also important to control Septoria, Van ‘t Westeinde adds. There is a clear difference in variety sensitivity. “Current varieties are much stronger than the ones of a few years ago, but in every variety Septoria can pop-up.” In variety trials at SPNA there is a clear difference in Septoria sensitivity, but there are no completely Septoria-resistant varieties. “Research on fungicides and spray strategies is often done in sensitive varieties. In this way, the chances for infection are high, so different strategies and products can show their strength.”

Van ’t Westeinde emphasises that growers should look at how long the products will work after application of a T0 spray. “An early spray offers a good start, but if you leave a 5-6-week interval after the first spray you take a big risk of a new infection in your field. It’s important to come back and protect the crop at the right times. It’s better to come back more often with 75% dose rate than doing three full-rate sprays with too wide an interval. This is a commonly-known pitfall. Regarding product registrations, these lower rates are OK, as long as a grower makes good use of alternating products and modes-of-action.”

Effective resistance management

Thies Marten Heick fears that when the trend of less efficacy of active ingredients continues it will be more difficult to control Septoria. New active ingredients should help to fight the disease and add efficacy, but also to maintain the lifespan of current products. “What we have learned from previous experiences is that fungicide resistance can develop and spread very rapidly. To delay this process as much as possible is of vital importance to implement an effective resistance management strategy,” he adds.