Potato Review

www.potatoreview.com POTATO REVIEW MARCH/APRIL 2023 29 BLIGHT In 2022, 43_A1 was found to be equally as aggressive as 36_A2, having a similar latent period and comparable sporulation capacity. Testing down at 7C showed that Phytophthora was still able to complete its lifecycle, but about 6-12 days slower than at 18C. “Interestingly, lower temperature didn’t in uence the sporulation capacity to the same degree, so it slows the infection, but not necessarily the spread. e risk of late blight will be there, even if it feels too cold for late blight, and it will be elevated where the dominant or new genotypes are present,” said Ed. David believes that such studies undertaken by industry, which link clones to their traits, are important and help improve understanding of disease risk. is is particularly important in GB, since AHDB Potatoes was wound down. “ is informs blight management practices and highlights the value of monitoring the pathogen population through initiatives like FAB here and Euroblight, which tracks genotypes and fungicide performance across continental Europe,” he said. Tackling a complex situation Norfolk-based independent potato specialist Martyn Cox says the late blight situation is now so complex that growers should leave little to chance. In a perfect world, he says that would mean following the example of the organic sector with more emphasis on resistance rather than fungicides. is would not only cut use, but also reduce the risk of resistance in the fungicides applied, as less blight means less selection pressure. However, owing to reluctance of retailers to change varieties, most grown in the UK are relatively susceptible to blight. e focus therefore has to be on eliminating inoculum sources while using preventative fungicide programmes to limit epidemics. One area Martyn says needs more attention is the proliferation of volunteers, which can be poorly controlled in other crops, stubbles and at potato eld edges beyond the reach of the sprayer boom, providing major sources of late blight outbreaks. Along with other members of the Association of Independent Potato Consultants (AIPC) group, he is also increasingly nding late blight on hairy nightshade (Solanum sarrachoides). en there is the usual suspect of cull piles, which despite annual warnings are still often left unmanaged. Proactive on inoculum Spore trapping work in Belgium has shown that Phytophthora spores move around in the atmosphere throughout winter, which raises many questions about alternative hosts, but may simply mean they survive longer in cool conditions. “Combined with more aggressive genotypes like 36_A2, it means attention to detail in these areas is so important. Identify the sources that might cause a problem now and do something about them before they do,” Martyn said. He adds that problems are often seen later in the season, when crops grow into eld edge vegetation or under trees and become shielded from the fungicide. If there is a miss with the pre-emergence herbicide, that potato area needs knocking out. ere were a lot of problems in the high- risk year of 2021 where sprayers were not switching on and o fast enough and backing into corners and drawing away can fool GPS systems and result in underdosing. Irrigation pipes with a tiny leak can also cause a ne spray that washes fungicide o surrounding plants and infection is almost guaranteed. is is another ne detail to consider. ➜ “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 Cooke, leader of Fight Against Blight Cyazofamid, the active in Ranman Top, is extremely important to the potato industry because of its excellent tuber blight activity and needs to be protected with sensible anti-resistance fungicide strategies. 2022 GB P. infestans population change