The emergence of a new strain of potato late blight, which has shown resistance to one of the most widely used blight fungicides fluazinam, is raising concern among agronomists.
Five cases of the 37_A2 (Dark Green 37) were confirmed in the UK last year. This year there have been further cases throughout the country from North Yorkshire to as far south as Kent.
Darryl Shailes, Hutchinsons potato agronomist, says: “From only a handful of cases in 2016 we have seen close to 20 confirmed cases this year.
“In what was a relatively low-pressure year this is an alarming rate of expansion, with incidents so far in Kent, Shropshire, North Yorkshire, Cheshire and, most recently, Derbyshire."
Although little is known about the strain, it has proved to be highly aggressive. At the Eurofins fungicide demonstration site near Derby, Dark Green 37 was detected in Maris Piper and Melody crops, despite not being released. Pink 6 was also detected in a crop of Melody, but Blue 13 was absent from all samples.
For many years fluazinam has been a mainstay of most programmes and against Blue 13 it has been among the most effective actives. However, it is possible such frequent use has increased the selection pressure on fluazinam. This raises questions about how best to manage blight at the start and end of the programme – the main fluazinam usage periods, says Mr Shailes.
John Keer, agronomist for Richard Austin Agriculture Services, suggests bringing forward applications of the fungicide Infinito (propamocarb + fluopicolide) to reduce reliance on fluazinam.
He says: “It is the start of the season which concerns me most. The only time I have had trouble controlling blight is when it has got into the crop early and got away. I regard the first three sprays of the programme as being very important, so will look to use Infinito here, where possible,” he adds.
Both agree, however, tuber blight protection remains a priority.“In recent years we have not seen much tuber blight even though we have had seasons with high pressure. It seems Dark Green 37 is equally threatening to tubers as it is foliage,” says Mr Shailes.
“We still have Cyazofamid and Zoxamide to use for tuber blight, but it could be more of a threat with Dark Green 37 than we have seen with either Pink 6 or Blue 13, so the fluopicolide in Infinito will remain important here too,” he adds.