According to Stef Dierickyx, Zonal Expert in potatoes for global agrochemical manufacturer Belchim Crop Protection, whilst the 36_A2 blight strain still represents the main threat, three new strains have now been identified in Northern Europe bringing potential new challenges to UK growers.
“Every year Phytophthora infestans continues to surprise us. Looking back to 2020 the dominant blight strain across most of Northern Europe was 36_A2 with 37_A2 beginning to stabilise and a decline in outbreaks of the older blue 13_A2 strain. Pink 6_A1, still an issue for some UK growers, no longer seems to be a threat in France, Belgium or the Netherlands.
“In terms of advice for UK growers, flexibility in adapting blight protection programs in response to unusual weather patterns and sudden temperature fluctuations is now a priority. There are many local factors that can influence leaf wetness on potato crops so monitoring localised weather conditions is essential given that fields are often spread over a significant geographic area.
“Keep programs strong from the outset with full dose rates to stop blight spore build up. After the crop has flowered, if pressure is high, all following sprays should include products offering effective tuber protection – particularly if the variety being grown has an early tuber set. Ranman Top (cyazofamid) is rainfast and delivers excellent tuber protection with the ability to kill zoospores from all known blight strains” confirms Stef.
Of future concern to all potato growers are the new blight strains; EU_41, 42 and 43. With outbreaks in both Denmark and Northern Germany last year it’s clear that all are capable of cycling at extremely low temperatures (10o C). This confirms currently thinking that Phytophthora infestans is even more dynamic than originally assumed says Stef.
“The EU_43 outbreak in Denmark at the beginning of last summer was devastating. Reinforcing the message that, even at moderate temperature levels, if leaves are wet then there is a potential risk. Based on the way blight pathogens are constantly evolving I think it’s dangerous to dismiss any actives. Therefore I would still advocate use of fluazinam as part of a tank-mix because of its excellent track record on Sclerotinia, Alternaria and Botrytis” concludes Stef.
Frontier Agriculture agronomist Sam Cooke, who advises on a significant area of potatoes between South Lincolnshire and North Cambridgeshire supports Stef’s view on the importance of localised forecasting and agrees that the UK’s trend towards more unpredictable weather patterns has made effective blight control even more challenging.
“I still use Blightwatch but much of my forecasting is also based on localised information, sourced and shared with other locally based Frontier agronomists. Knowing where allotments are in relation to fields of commercial crops and being able to respond quickly to sudden flash showers is critical to successful protection programs.
“Mancozeb is an excellent multi-site offering good resistance management and, along with cymoxanil, it’s been the foundation of my early rosette sprays. Although we will have to wait to see the outcome of Mancozeb from a regulatory stance, it will be interesting to see how seed breeders are developing more blight resistant varieties suitable for the UK potato market during the next 2 years.
“Whilst a strong start to blight programs is important, rapid canopy growth in early summer combining warm and often wet weather with catchy showers is when products with rainfastness are required. With rainfastness of 15 minutes onto a dry leaf, Ranman Top is an effective protectant. If temperatures drop and there’s an increased risk of zoospore activity then it’s also a key weapon in killing zoospores. I would generally advise saving 2-3 sprays of Ranman Top for late seasonal control and holding back 1 spray to kill any remaining zoospores in or around the bottom of newly flailed crops.
“If kickback is required, key mid-season options should include oxathiapiprolin or propamocarb. Oxathiapiprolin also offers high anti-sporulant activity. In late season, fluopicolide plus propamocarb is a useful alternative if you have not been able to hold back Ranman Top for late season use.
“Through desiccation I would always advise minimising risk by using all products at full rate to ensure you have done everything possible to protect the crop as it goes into store. With a number of new and aggressive blight strains on the horizon growers cannot afford to take any unnecessary risks” concludes Sam.