Dark Green 37: coming to a field near you
The emergence of a new strain of potato late blight (Phytophthora infestans) with resistance to fluazinam, one of the most commonly used blight fungicides, is raining concern among agronomists.
After five cases of 37_A2 (Dark Green 37) were confirmed in the UK last year it has spread, mainly to neighbouring counties around the midlands, but also as far as Kent.
“From only a handful of cases in 2016 we have seen close to 20 confirmed cases this year,” says Darryl Shailes, Hutchinsons potato agronomist.
“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.
“We don’t know near where it will go next, but we do know that it is at least as competitive as the dominant strains Blue 13 and Pink 6,” he adds.
Although little is known about this 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.
“It appears that our 13_A2 (Blue 13) has been outcompeted as it was not present in any of the samples,” said Eurofins in a note to manufacturers.
For many years fluazinam has been a mainstay of most programmes and against Blue 13 it has been amongst the most effective actives. It’s possible however, that 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 period, says Darryl Shailes.
John Keer, agronomist for Richard Austin Agriculture Services, shares this concern.
“We need to look at not using fluazinam at the start and the end of the programmes. I don’t see that we can risk it when there is a new strain which we don’t know how dominant it will be. Fortunately, we have not seen any in East Anglia yet, but that can change,” he says.
Both Darryl and John have a similar response, to make more strategic use of Infinito (propamocarb + fluopicolide) by bringing forward applications to the start of the season.
“It is the start of the season which concerns me most,” says John Keer. “The only time I’ve ever had trouble controlling blight is when it’s got in 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 says.
Both agree however, that tuber blight protection remains a priority.
“In recent years we haven’t seen much tuber blight even though we have had seasons with high pressure. It seems that Dark Green 37 is equally threatening to tubers as it is foliage,” says Darryl 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 flupicolide in Infinito will remain important here too,” he adds.
