Potato Review

28 POTATO REVIEW MARCH/APRIL 2023 BLIGHT ‘It’s when, not if’ DAVID Cooke, leader of the Fight Against Blight monitoring programme at e James Hutton Institute (JHI) in Dundee has said that while Great Britain hasn’t yet seen the fungicide-insensitive blight strain that is taking hold in Europe, it is a case of ‘when not if ’ it arrives, and growers need to be vigilant. “Growers and agronomists need to be aware of the developing European situation and composition of the British population and ensure a pragmatic approach is taken to late blight control in 2023 and beyond,” he said. Compared to 2021, when there were around 210 outbreaks of late blight reported in GB, 2022 was a low-pressure year, with just 84 outbreaks reported thanks to the hot and dry weather during July and August. From samples sent to JHI by FAB blight scouts during 2022, 435 genotypes were isolated and the GB- wide data shows 13_A2 and the uazinam- insensitive 37_A2 all but disappearing. e GB population was dominated by 36_A2 at 53% and 6_A1 at 35%. By country, England had the highest proportion of 36_A2 at 67% and 37_A2 disappeared in Wales from 41% in 2021, while 6_A1 continued to dominate in Scotland. “ e 41_A2 genotype, which was found in Fife the previous year, was not found. ere was a concern it might establish, but it’s good news we haven’t seen it again. However, the caveat is the dry, low pressure year – it might still be lurking,” said David. ere are also samples categorised as “Other”, made up of genetically diverse late blight isolates from oospores created by sexual reproduction between two mating types. David says the proportion of “Other” declined in 2022 largely down to the dry conditions, with less blight pressuremeaning a reduced likelihood of picking up these diverse clones in sampling. Any newproblemstrainwill come from this source he says. “ ey don’t come from nowhere and oospores in the soil are what drives population change. In shorter rotations andwhere volunteers and other hosts aren’t controlled, this can give you early infection, extending the risk period and putting pressure on fungicides. It is the usual message of managing primary inoculumsources as closely as possible.” He says this also serves as a reminder that GB needs to keep one eye on the European situation where a few new strains have emerged in recent seasons. ese represented around 49% of the 2021 population sampled as part of Euroblight monitoring and include EU_43_A1, which is resistant to mandipropamid, the active ingredient in fungicide Revus. is clone jumped fromDenmark to the Netherlands and Belgium over recent seasons, and it is felt that it will be on British shores soon. “It is a major concern that we have crossed this resistance to CAAs in Europe and we need to pre-empt problems here with careful management,” David said. Phenotyping work Manufacturer Certis Belchim has been undertaking work to learn more about the tness of recently-established and emerging strains since 2020. At its Londerzeel trials station in Belgium, Phytophthora infestans strains are multiplied on an agar plate. Leaves of the variety Bintje, a popular and relatively susceptible variety popular amongst Belgian growers, are inoculated. Leaves are then incubated for ve to seven days at 18C and spores from the developing lesions harvested and kept at 4C for two hours. ese are then used to inoculate another clean leaf, before assessments are made on latent period, lesion development and sporulation capacity of the lesion. Suitable humidity for blight development is maintained throughout the experiments. Ed Bingham, Certis Belchim’s Potato Crop manager for Europe, says in 2020 the Londerzeel team investigated these phenotypic traits at di erent temperatures, including 10C, 18C and 24C. Results showed that the latent period of EU_36_A2 is almost one day shorter than EU_6_A1 at 18C and 24C. However, perhaps more concerning to growers from a practical perspective is the growth (cm2/day) and the sporulation capacity (spores/cm2) of those lesions at the lower temperature of 10C. Sporulation capacity e sporulation capacity of lesions caused by 36_A2 – now dominant in GB – is more than ve times that of 6_A1. Although 37_A2 is not quite as aggressive, it still has an impressive ability to sporulate at lower temperatures. e work was repeated in 2021 and similar results were observed, but this time some samples of unknown genotypes were added in, which would make up part of the “Other” category. eir average sporulation at 18C was almost three times higher than 36_A2 showing the signi cant threat of a new, aggressive strain rising to prominence from this diverse group. “If we are unable to continue in 2023, it will be di cult to react if 43_A1 does arrive.” Martyn Cox, Independent potato specialist Work by Certis Belchim at its trials station in Londerzeel, Belgium suggests that genotype 36_A2 has a significantly shorter latent period, and its sporulation capacity is more than five times higher at 10C, than 6_A1.