Potato Review

24 POTATO REVIEW MARCH/APRIL 2020 BLIGHT FOCUS Tuber Blight authorisation to be reviewed in 2020. “If we were to lose mancozeb it would be a big blow for sustainable blight control in northern Europe. It is the only multi-site available for mixing and has probably done more to protect resistance build-up than any other active ingredient,” said Denis. FUTURE CONTROL Protecting product efficacy for the future is a concern shared by Scottish Agronomy agronomist Greg Dawson. Ensuring these messages are adopted will not be without its difficulties, he concedes, but there are options for growers to consider. “We now have 36_A2 confirmed in the main potato producing counties of Scotland. This is significant as it has shown greater aggressiveness in response to the main fungicides and is further evidence that we must find new options if crops are to be protected without letting costs run out of control,” he said. The concept of integrated crop management (ICM) has been accepted and incorporated into regulation, notes Greg. Some of the components could be more widely adopted through combining responsible fungicide use with better decision support systems and the use of in-field sensors, but he believes the tools required for the holistic nature of ICM to be fully embraced are still unavailable to growers. A lack of some fundamental knowledge, market preferences and the controversy around novel breeding methods limit options, he believes. “There is a clear need to formulate general principles for synergistically combining traditional and novel ICM actions to improve efforts to optimise late blight control solutions. It is not sustainable to be dependent solely on either genetics or chemistry. Integrated blight control requires a suite of effective fungicides each performing as expected. It is worth noting that of the 49 new active substances submitted for approval since June 2011, just six had been approved by February 2017. Of these, only two have activity on late blight. As an industry, we must improve stewardship of existing products to retain efficacy and choice,” he says. Part of the difficulty in adopting a fully ICM approach, suggests Mr Buckley, rests in the limited accuracy of longer-term weather forecasts. Research shows that a five-day forecast is now as accurate as a three-day forecast was in the mid-1980s, but at about 85% he questions whether this is enough to encourage a change in behaviour when what is needed is an accurate seven-day forecast. “ICM is useful and can work in ideal conditions, but so much of what happens in crop production is driven by the weather and yet the limited accuracy of weather forecasts, especially at seven- and 10-day intervals, means grower and adviser reluctance to fully embrace other aspects of ICM is unlikely to change,” said Denis. Unreliable weather forecasts may serve as a valid reason not to adopt more aspects of ICM, but they are not an excuse to repeat the mistakes that led to the emergence of resistance to fluazinam in 37_A2. “The industry in northern Europe had a fluazinam scare with Green 33 around eight years ago but got away with it. It didn’t heed the warning though, and as a result we have now got fluazinam-resistant 37_A2. And we are at significant risk of repeating the same mistake with other crucial active ingredient groups,” Denis said. Varietal resistance For tuber blight control, other than fluazinam, only fluopicolide and the Qil- products cyazofamid and amisulbrom are recognised as having significant activity. “Clearly, as growers and advisers we need to manage our use of these products carefully if we are to maintain their effectiveness for the future. For our part we now recommend that single actives such as cyazofamid and amisulbrom are always mixed with a suitable partner belonging to another mode of action,” says Greg. “But to make real progress we need to look further than adopting best practice. It’s time that we accepted the contribution science can make to crop protection. Until attitudes change, the use of fungicides will increase as will the disease pressure on crops and the cost of production,” added Greg. An issue that frustrates Scottish Agronomy’s desire to develop more considered ICM policies is the absence of up-to-date varietal resistance profiles. “The published resistance ratings are likely to be significantly overstating a variety’s ability to resist the late blight pathogen. It is not an area of research that has been considered since 2010 when 13_A2 and 6_A1 were dominant and tests revealed that the published resistance ratings were overstated by at least one point for most varieties.” The precise reasons for rapid population invasions by the likes of 36_A2 are yet to be fully elucidated, but some phenotypic characteristics of the newcomers are probably part of the equation. His point is significant because work published by INRA in France showed that 36_A2 was among the most aggressive of current populations and that genotype 41_A2 had the ability to overcome many race-specific resistance genes available to breeders or present in current potato cultivars. Scottish Agronomy is also critical of the lack of research that considers the behaviour demonstrated by the current strains in the field environment (phenotype). Mr Buckley agrees too that attitudes need to change and new breeding techniques such as CRISPR-CAS, which has the potential to improve disease resistance without affecting the qualities that make varieties so desirable, should be approved for commercial application. “By using the gene editing approach made possible with CRISPR-CAS scientists can improve specific aspects of a variety. This means retailers and consumers can continue to exhibit a preference for established varieties such as Maris Piper, yet it could have a level of resistance to late blight that would reduce the need for fungicides,” he said. “It’s time that the wider public embraced what science can contribute to plant breeding, and not just dismiss it,” he added.