Potato Review

16 POTATO REVIEW MAY/JUNE 2022 BLACKLEG products that might be able to help limit its impact during the growing season, as independent potato consultant Mark Stalham has found out over recent seasons. No disease In his previous role at NIAB CUF, he tested Harpin αβ protein biostimulant product ProAct (see box) for three seasons between 2018 and 2020 and direct influence on disease was difficult to detect. In the first year, variety Jelly was planted with no detectable seed-borne infection, and no blackleg or rots were seen in the plots, so little was learnt about ProAct. The following year, the susceptible variety Markies was chosen, with seed brought in from a rogued Scottish crop with known infection. Again, no disease or response to Harpin αβ was observed. In the final year of the investigation at NIAB CUF in 2020, blackleg symptoms were seen in the trials and where irrigation was applied throughout the season disease was worse than where irrigation was ceased following scab control. This suggests infection was linked to either the water or the environment in the crop canopy created by irrigation during July. Again, very little effect was seen from ProAct applications. However, one common observation over the first two years of trials was a significant increase in tuber calcium concentration, something that Mark says is notoriously difficult to achieve with calcium nutrition. “In work we’ve done on internal rust spot and hollow heart – both symptoms of calcium deficiency – it’s been very easy to get additional calcium into the foliage, but not down into the tubers,” he explains. Mark was asked to continue the work by ProAct manufacturer Plant Health Care in 2021 and in many cases, he would have said no, with the inconclusive results enough to suggest that it was not worth more time. However, from other research it is known that the Harpins contained in ProAct are proven activators of plant defence mechanisms, with the product widely used in the US. Scottish work In addition to evidence overseas, work carried out by SAC Consulting in Scotland from 2014 for several seasons suggested ProAct can influence blackleg incidence but results over several years of experiments were inconsistent. Stuart Wale, potato agronomist and researcher with SAC, says the experiments were part of a larger project looking for elicitors for use in potato crops. After little response to the product in the first two years, in 2016 the product achieved a 50% reduction in blackleg in high-risk variety Amora. However, for the next three years the pattern of little response returned. On reflection, the key finding Stuart takes from the six years of work is that the product was somehow influencing environmental spread, which is not always a significant source of inoculum. “We know from AHDB and Scottish Government work that blackleg inoculum mainly spreads through infected mother tubers, but also environmentally. “If you are stimulating resistance with ProAct before a plant is exposed to blackleg, then in theory environmental spread will be reduced, but proving it on a consistent basis is very difficult,” he adds. Although SAC results were inconsistent, a positive effect was seen in a bad blackleg year.