Potato Review

24 POTATO REVIEW MAY/JUNE 2023 BLIGHT UPDATES Be ready for late severity We look at the considerations and trials ndings to take into account while planning the most e ective blight programme for 2023 crops. A combination of wet and cold weather acrossmuch of the UK saw many delays inmaincrop planting, but withmachines now rolling, attentionwill soon be turning to blight sprays. Late blight was a not a huge factor in 2022. Corteva Agriscience’s Field Technical Manager for potatoes Craig Chisholmsaid pressure in the middle of the seasonwas very low, and non- existent in some, but conditions so far this season point to amore severe late blight year in 2023. Despite the warm, dry climate in 2022, Corteva’s trials at SRUC in Scotland experienced signi cant blight pressure. It was at this site where the agriscience company continued to explore how best to deploy oxathiapiprolin and benthiavalicarb, the actives found in Zorvec Endavia. Craig said: “Applications of Zorvec in the rapid growth phase provided a delay in the development of blight later in the season by between one and three weeks over the standard programme (propamocarb + uopicolide). “Zorvec continues to provide unrivalled activity against phytophthora infestans, with no shift in sensitivity from the testing that’s been done. “Making use of that systemic and curative activity during rapid growth, plus its persistence into the stable canopy phase has, even in the modest disease pressure of 2022, againproven its worth once the blight epidemic started to take o .” Resistance monitoring - status update It was only late in the 2022 season that the blight started to be seen, probably more in volunteers than in crops, Craig said. As a result, fewer samples thanusualmade their way to the JamesHutton Institute for analysis. though enough to paint a picture of which late blight strains growersmight face in 2023. “36_A2, which is an aggressive strain that’s developed over the past four to ve years, has continued to grow in dominance, and 6_A1 continues to have a presence, showing its relative tness to maintain its position within the population,” said Craig. Strains which appear to have declined in prevalence are 13_A2, which is resistant to the phenylamides chemical group, and 37_A2, which has a reduced sensitivity to uazinam. “We can’t say that they have disappeared,” Craig said. “It’s just a re ection of their relative decline in the use of uazinamandmetalaxyl respectively, and the relative tness of the 36_A2 strain.” In terms of where blight strains are being found, 36_A2 and 6_A1 remain ubiquitous in all regions. Many potato growers have been concerned to learn of the challenges faced by growers, principally inDenmark, who have experienced resistance to the CAAs, giving rise to the 43_A1 strain (as detailed in theMarch issue of Potato Review). Craig said: “No samples submitted under Fight Against Blight were found to exhibit any level of CAA resistance in the UK, so UK growers can make use of up to six in a programme.” But Craig advises that any product containing a single active should be mixed with an alternative mode of action – mancozeb or straight cymoxanil being two good approaches. Chemistry available to growers appears to be diminishing as the limited number of active ingredients permitted to control late blight continues to dwindle. Craig said growers should make use of mancozeb while still permitted because it provides protectant activity against late blight and alternaria. He advises mixing it with the curative activity o ered by cymoxanil. “Cymoxanil is increasingly attractive as a mix partner, not just when blight pressure is high, by adding curative activity to protectant fungicides in the face of an ever more challenging pathogen,” he said. “ ere’s a good argument to say that we should be A clean crop after receiving two Zorvec Endavia applications at the rapid growth phase From above, the di erence in performance of blight fungicides in Corteva’s trials becomes apparent An aerial shot of SRUC’s Ayr trials site which saw quite heavy blight pressure in 2022 “36_A2, which is an aggressive strain that’s developed over the past four to five years, has continued to grow in dominance, and 6_A1 continues to have a presence.” Craig Chisholm, Field Technical Manager for Potatoes, Corteva Agriscience