French field trials suggest the strobilurin fungicide, pyraclostrobin, is unaffected by the most commonly found mutation in the barley disease net blotch.

But that doesn’t appear to hold true for all strobilurin fungicides.

Net blotch is affected by a different mutation to the G143A one commonly found in septoria tritici. The first piece of good news is that, unlike G143A, this mutation, F129L, is developing quite slowly and does not appear to confer anywhere near the same amount of resistance to strobilurin fungicides, Martin Semar of BASF says.

For example, the amount of pyraclostrobin need to kill 50% of the mutated population in a laboratory test is about 50 times the amount needed for it to kill 50% of the wild, sensitive population in septoria. With net blotch it is only five to eight times more, he says.

Binding

The reason is probably in how the change in the coding affects the binding site structure. The switch from a glycine to alanine at the 143 position – hence G143A – in the code effectively introduces a methyl group into the binding site and makes it difficult for the strobilurin to bind to its target site in the fungus.

In contrast, the switch from phenylalanine (F) to leucine (L) in the F129L mutation removes three methylene groups from the binding site, probably having less effect on the strobilurin’s ability to bind, Dr Semar explains.

About 30% of the UK net blotch population on average carries the F129L mutation, Epilogic and BASF sampling has discovered. That is similar to France, and in both countries there are areas where the resistant proportion is much higher – more than 75% – and places where it has not been detected.

Mutation

Perhaps surprisingly the mutation has been only found very rarely in Germany, Dr Semar says. “It is difficult to explain why that is the case. If it is a net blotch year it is all over Europe, not just in France or the UK, and the products and programmes used do not differ that much between countries.”

The key question is how the mutation affects field performance, he acknowledges. The good news is that for pyraclostrobin a series of trials in France suggests performance is not altered significantly.

In the trials different rates of Comet were tested on fields with known levels of F129L net blotch, ranging from 0 to 100%. Amistar, Opus and Opera were also trialled. “Field performance was always at a high level with pyraclostrobin and the results suggested it was unaffected by the level of F129L.”

That was true for both the full label rate of 250g ai/ha of pyraclostrobin and a lower 100g ai/ha rate, more typically used on farm, albeit with a drop off in control related to dose for the latter.

Resistance levels

But a second strobilurin, Amistar, did appear to be affected by increasing resistance levels, Dr Semar noted.

It is something Syngenta‘s Matt Pickard doesn’t deny. “We would not argue that there isn’t a differential between strobilurins, and as pyraclostrobin is intrinsically more active it appears to be affected less than azoxystrobin.

“However, it is a crazy resistance strategy to use something that is still affected. In our trials work if you cut pyraclostrobin rates back to what is used in the field its activity falls back.

“So the best way to approach net blotch control is to use other products, such as prothioconazole and cyprodinil from a resistance point of view. And in mixtures we don’t see any difference between Amistar and pyraclostrobin.”

Dr Semar agrees strobilurins should only be applied in mixtures. “But pyraclostrobin has a strong place in the programme. It gives good control of net blotch, even in the presence of high levels of F129L, and has full activity on rust and rhynchosporium.”

Net blotch resistance to strobilurins

• Slow build up
• Strobilurins still effective
• Differential in product performance
• Always use in mixture