Blight strain shifts point to new strategies
Potato growers and agronomists should look to historic shifts in blight strain when creating effective strategies for the current season, advocates to Syngenta Technical Manager, Andy Cunningham.
“New genotypes emerging each pose unique new challenges in devising blight programmes,” he warned. “That’s been shown by the rise of EU36 to around 65% dominance of the blight population in England in just three seasons. While current fungicides don’t appear to have any issues controlling the strain, the fact that it is fitter and very aggressive does indicate that it can get into crops sooner.
“That means blight programmes need to start earlier and go on for longer, which has implications for costs in the current climate and agronomic decision making for alternating actives and spreading risk of resistance.”
Speaking at the latest Syngenta Potato Science blight briefing, he urged growers to take extra steps to better utilise available chemistry to counter the threat from blight genotypes. That included alternating chemical groups through the programme, using the best-in-class products for each group - such as Revus among the CAA group - and mixing modes of action in each application.
The season-long challenge been further exacerbated by the loss of diquat for desiccation, which means blight risk can be extended longer into the autumn, along with making control of blight sources from dumps and waste piles more difficult.
Mr Cunningham (below) cited previous experience of the EU37 genotype, which was less sensitive to fluazinam. That demonstrated how one strain could increase from less than 5% to over 25% of blight incidences in a single season - and result in serious concerns for controlling tuber blight.
“Conversely, EU13 that showed reduced sensitivity to phenylamides, reached a peak of 80% of blight samples in 2008 and still at 55% in 2013, but last year accounted for only 1% of monitored cases,” he pointed out.
“That may give a useful opportunity to utilise this highly systemic active group, including metalaxyl-M in Fubol Gold, as a limited part of the overall blight programme.”
He added that while A1 blight strains are present in the population there will always be the risk of sexual reproduction and the creation of new genotypes. Most will fade away, but occasionally it will throw up strains of increasing concern. Monitoring revealed that the A1 strain, EU6, was still present in a quarter of all GB blight samples tested.
Growers should also be aware of prevailing blight strains in their area, highlighted Mr Cunningham. “AHDB monitoring by James Hutton institute showed EU36 accounted for 64% of cases in England, but only 6% in Scotland, for example. However, EU6 was responsible for 31% of cases in Scotland - where ‘other’ unclassified strains were also far higher, at 41% of samples, compared to 7% in England.”
Even if multi-site activity availability were limited, the modes of action that are available, along with the IPM measures growers can put in place, does put less selection pressure on the development of resistant strains. Independent trials at Eurofins last season (above) showed a mancozeb free Syngenta blight programme could prove just as effective as one containing the active.
“Creating robust strategies and following good practice for preventing fungicide resistance remains crucial,” he advocated.
“Ongoing resistance research and field trials last season highlighted how effective the active mandipromaid – as in Revus and Amphore Plus – remains in preventing all existing blight strains.
“Coupled with good application and effective interval timing, growers and agronomists have the chance to keep new strains in check.”