Potato Review

www.potatoreview.com POTATO REVIEW JANUARY/FEBRUARY 2020 11 BP2019 “That said, there is a suspicion that we may be at the start of another step-change in P. infestans aggressiveness in the UK. There are strong similarities to 2007, a bad year for blight when genotype 13_A2 started to have a considerable impact on the effectiveness of blight control measures.” Will IPM raise risk of tuber blight? Dr Bain emphasised the importance of cultivar resistance to Integrated Pest Management (IPM) of late blight. However, there are industry concerns that reduced fungicide inputs may increase the risk of tuber blight for a number of varieties. This year’s work builds on the two previous years in which Dr Bain has been evaluating which varieties show the best resistance in an IPM strategy, ie give acceptable control of tuber blight in field trial conditions. “Funded by the Scottish Government, SRUC has been looking to understand whether cultivars with a combination of moderate ratings for both foliar and tuber blight offered sufficiently effective control of tuber blight through a reduction in inoculum density because of more resistant foliage, coupled with greater tuber resistance.” He found very good control of tuber blight was achieved in 2017 for Setanta, Harmony, and Gatsby and in 2018 for Carolus, Harmony, Orla, Setanta and Gatsby. However, results were more complicated than expected. The blight resistance ratings were presumably for blight strain 13_A2, and this was the inoculated strain in 2017 and 2018. However, in 2017, at the end of the season tests discovered that only 68% of the infections were 13_A2, with 27% 6_A1 and 5% 8_A1. Changes in 2018 also occurred, and the corresponding genotype percentages in September were even more extreme. Just 8% of the infection remained 13_A2, while a huge 79% was 6_A1. The new and aggressive 37_A2 was also found to have entered the crop, and formed 13% of infections. “Results should be regarded as preliminary at this stage but it should be noted that six top-50 cultivars (by GB planted area) have a combination of foliar and tuber resistance ratings the same, or superior, to those that gave very good control of tuber blight in the field experiments,” said Dr Bain. “This challenges a widely-held view that cultivar resistance-based IPM is not possible with current popular cultivars. One firm conclusion is that, with some further work, there is scope for cultivar resistance to contribute substantially to IPM.” Talking aphids Concerns about higher aphid numbers led to a number of discussions around the threat to the seed potato industry from the viruses that they can vector, including PVYN and potyviruses. Not all aphids are equal at transmitting virus, said AHDB Potatoes Senior Scientist (pests) Sue Cowgill, who was referring to previous AHDB-funded work that also included studies of what has been driving the competition so that PVYN dominates over PVYO. While the peach potato aphid (Myzus persicae) is the best at transmitting PVYN, the grain aphid (Sitobion avenae) and the willow-carrot aphid (Cavariella aegopodii) were shown to be better at vectoring the virus than previously thought. Genetics may also play a role. ➜ Sue Cowgill