Septoria resistance is in the headlines again.
Irish farmers are being advised to only use the key two azole fungicides, prothioconazole and epoxiconazole, a maximum of twice in fungicide programmes this season. Other azole fungicides should be used at T3, while no triazole should have been used at T0.
The reason? Researchers at Teagasc Crops Research Centre at Oak Park, Carlow have twice – last summer and again this spring – found strains of Septoria tritici that are significantly less sensitive to prothioconazole in the laboratory than any previously tested. The strains also show a reduction in sensitivity to epoxiconazole.
So worried by these findings and what they could mean in the medium term for septoria control in the country, Teagasc advisers have put in place these relatively strict guidelines.
They are not that concerned that growers will be unable to control septoria this season. The strains have been found at a low level, and used in mixture with other modes of action, particularly boscalid and chlorothalonil, the fungicides should work effectively, it is believed.
Instead the advice is aiming to apply the precautionary principle to try to avoid selecting less sensitive strains.
But the advice, particularly limiting the numbers of applications, is contentious. As Andy Doyle said writing in the Irish Farmers Journal in late April, it makes sense from an epidemiological perspective – ie reducing selection pressure – but it could result in hardship and yield loss at farm level.
UK researchers and, less surprisingly, the pesticide manufacturers, BASF and Bayer CropScience, have reacted with more caution than the Irish advisers. No such restrictions are being issued in the UK for this season, not least because monitoring in the UK has failed to pick up such results. Indeed recent monitoring has suggested that any previous shifts in sensitivity had stabilised.
But there is a chance the strain is already in the UK septoria population, says Neil Paveley of ADAS, because of ability of ascospores, which are released from infections on the remains of previous crops, to travel long distances.
But the likelihood is it would only be at very low levels, he stresses, and the risk of any change in performance this season is small.
“To cause an effect all of the following must apply: One, laboratory insensitivity translates to the field, which we don’t know. Two, there is no or very low fitness cost for the pathogen – it might be fit enough in petri dish tests, but not in the field. And three, whether it has risen independently in the UK or from Ireland, it will need to have come to dominate the population in a relatively small number of generations. A generation takes about three weeks, so we are only one generation from T2, so in all likelihood it would have been picked up by now if it was dominating.”
But that doesn’t mean it couldn’t become a problem in the future, he stresses. “If those things are true, it will start to dominate, so we could see problems by the start of the following season.”
The good news is that we will have a lot more information by then. “The Irish are trying to identify if it is a specific mutation, which if it is, means we will be able to screen all samples for it and get a picture of how it is spreading. And the HGCA’s fungicide performance trials should pick up change in performance in the field at low rates.”
For this season he and other experts in the UK are advising no change to fungicide programmes.
Any change to try to protect triazoles from increasing resistance has to be balanced against disease control needs, he says.
“If the worst happened it would mean we would be more dependent on chlorothalonil and boscalid, and some of the new actives coming through.
“But I don’t think the picture is that bleak. It is unlikely to be a complete loss of control, and we have better disease resistance coming through in varieties, so yield penalties from septoria, hopefully, should get smaller.”
But there’s no doubt the finding has put everyone on red alert. Rothamsted Research and both manufacturers have asked Teagasc for the strains to confirm the results in their own laboratory, and there will be a significant ramping-up trials work this spring to investigate field performance.
The results from those trials and the monitoring this season should help shape whether there’s any need for fungicide programme changes next season – and how desperately growers should be for approval of the new carboxamide fungicides.