Potato Review

www.potatoreview.com POTATO REVIEW MAY/JUNE 2022 17 BLACKLEG Figure 7. Time course of development of plants with blackleg symptoms. a) WO & PO Jolly, Challenger; b) EG Harrison, Heraclea; c) G Harrison & Son, Agria. Control, ●, ProAct, ●. Error bars based on 7 D.F. (Source: MSPC Ltd) a) b) c) (Source: MSPC Ltd) 0 5 10 15 20 25 20-May 10-Jun 01-Jul 22-Jul 12-Aug 02-Sep 23-Sep Blackleg incidence (%) 0 5 10 15 20 25 20-May 10-Jun 01-Jul 22-Jul 12-Aug 02-Sep 23-Sep Blackleg incidence (%) 0 5 10 15 20 25 20-May 10-Jun 01-Jul 22-Jul 12-Aug 02-Sep 23-Sep Blackleg incidence (%) Combined with the unusual ability to increase tuber calcium levels significantly and consistently in the NIAB CUF work, leading to Mark hypothesising about how it could be suppressing blackleg. “Essentially, we think that there is a physical mechanism whereby the increased calcium uptake is strengthening cell walls and other protective membranes that prevent a plant succumbing to a pest or pathogen,” says Mark. New trials This encouraged Mark to set up another series of trials on behalf of manufacturer Plant Health Care in 2021 across three commercial sites, two in Norfolk and one in Cambridgeshire. Typical fertiliser, herbicides and fungicide programmes were applied when required to keep the plots healthy and clean, and irrigation was scheduled using an irrigation model based on local weather data from tuber initiation. ProAct was applied three times across eight replicates – the first spray at end of tuber set (initial tuber initiation + 7 days, T1), the second 21 days post T1 and the final application 42 days after T1. Unlike the previous years at Cambridge, 2021 offered conditions conducive to blackleg development, with emergence delayed by cool conditions. Then in June, temperatures increased, and recently emerged crops were subjected bright conditions, leading to rapid growth. As the spring had been relatively dry, early irrigation was required from tuber initiation, providing the damp conditions ideal for bacterial development at a time when plants were already stressed. “We started with very low levels of infection, but when combined with the favourable conditions, there was just enough inoculum to really get the blackleg going, particularly on one site,” explains Mark. Significant blackleg This led to symptom development at all three sites, with up to 22% of plants infected in control plots at one site and a lower, but still significant incidence of between 3 and 9% across the other two sites. The figure below shows disease progression across sites in untreated (black) and ProAct treated plots (grey) and Mark says when carrying out assessments, there was a noticeable reduction of infected plants where the product was used. “The statistical error was very low between plots, so the data agreed with the observations and results were very consistent across the replicates.” ProAct will not have an impact on seed-born infection, underlining the importance of sourcing clean seed, but the data from this trial suggest it is effective at reducing environmental spread of Pba. Any reduction in air-borne spread did not translate into a significant reduction in tuber rotting. However, at the two sites where significant rotting occurred, there was a numerically lower rot count where ProAct had been applied.