As many as 35 per cent of late blight samples from 2018 were recently-discovered strains 37_A2 (Dark Green) and 36_A2, revealed James Hutton Institute research leader. David Cooke, speaking at the Agronomists’ Conference. Heather Briggs writes.
Genotype 36_A2 appeared in a cluster in Kent, and outbreaks of 37_A2 were discovered in Shropshire, Cheshire, Lancashire and Kent, plus a single finding in Norfolk but it was not found in south west England, Wales or Scotland. However, aggressive blight genotype 6_A1 remained predominant, found in 46 per cent of reported blight outbreaks.
The number of outbreaks caused by 13_A2 continued to decline across Britain, and now comprises just 6 per cent of late blight samples, the lowest level since it first appeared in 2005, he said. Blight pressure had been much lower than the previous year, with just 40 outbreaks reported compared with 158 in 2017, and Dr Cooke (right) warned that this will have skewed the sampling.
“The start of the year was very cold, followed by a wet period in March and April which generated primary inoculum, but the hot, dry conditions between May and July resulted in very few outbreaks,” said Dr Cooke.
Localised rainfall in August and September led to disease activity picking up quickly in some areas, he added.
Blight strain 36_A2 remains a concern because of its aggressiveness, and Dr Cooke has been sending isolates to INRA, France, for aggressiveness testing. “Comparing isolates of 36_A2 with other lineages showed 36_A2 formed the largest mean lesions next to 6_A1, and produced abundant sporangia,” he said. “Industry is concerned that changes in fungicide sensitivity are driving increasing incidence of fit and aggressive blight genotypes 36_A2 and 37_A2 across Europe. “Increased insensitivity to the fungicide fluazinam has been demonstrated in 37_A2 leading many growers to adjust their fungicide strategies.”
Commissioned by the AHDB, research conducted by James Hutton Institute potato pathologist Alison Lees is monitoring the efficacy of fluazinam, cyazofamid, mandipropamid, propamocarb and fluopicolide across the blight strains dominating the populations in GB crops.
“None of the actives in the experiment were chosen because of any issues, but we cannot test all the available actives, and these were the logical ones to test,” he explained. Further evidence of insensitivity to fluazinam in genotype 37_A2 was discovered, with lesions still forming in the assays at the field rate of 1,000ppm.
So far the team has found no evidence of resistance in 36_A2 to any of the five actives. However, slightly larger lesions were caused by 36_A2 compared to other genotypes at the lowest doses of all actives. “This shows a generic effect which may explain its dominance in field epidemics. The mechanisms are unknown but we assume it is something related to aggressiveness. “It remains important to monitor the emergence of 36_A2 and other genotypes in context of their aggressiveness and combine our findings with those of other researchers.”
As a member of Euroblight, Dr Cooke works closely with Wageningen University and Research plant pathologist Geert Kessel (right), who also spoke at the conference. Dr Kessel said that 10 years of genotyping the pathogen in the Netherlands have shown similarities to the UK populations, with clonal strains such as 13_A2 (Blue 13) being prominents.
However, less prevalent genotypes and a large number of genetically unique individual strains created by sexual reproduction are also present in the Netherlands, particularly in potato starch-growing areas, he explained. Strain 37_A2 was first found in the country in 2013 with EU_36 appearing one year later in the German/Dutch starch-growing area. During the next two years their area of population expanded a little, and then accelerated in 2016 and 2017 at the expense of 1_A1, 6_A1 and 13_A2. 2016 was the year 37_A2 was first found in the UK followed by 36_A2 in 2017.
“At Euroblight, we have been assessing control options and their efficacy on a pan-European scale. “Best practice methods to help control late blight include removal of primary inoculum, crop rotation, resistant varieties and effective spray application.” He encourages growers to opt for resistant varieties, but uptake is slow.
Timing and application
“Correct timing of the application and fungicide choice play a crucial role in efficacy. “We aim to spray shortly before a predicted infection event, and all the registered late blight fungicides can prevent infection when sprayed before the spores land in the crop. “However, if you spray after the infection event, the phytophthora will already be inside the plant so you will need a curative active ingredient.”
Decision support tools can help manage the late blight situation, with the more sophisticated ones taking into account fungicide degradation and unprotected new growth. In addition, they may issue specific warnings for tuber infection or oospore infections. Dr Kessel has also been identifying potential benefits and problems linked to the introduction of an increasing number of resistant varieties. “Trials have shown that by using integrated pest management (IPM) strategies, the number of sprays on susceptible Desiree could only be marginally reduced.
“However, just three sprays controls infection in the resistant variety Sarpo Mira.” Nevertheless, he warned, zero tolerance to blight is especially important in resistant varieties.
“A small infection in a resistant variety is the first sign of the pathogen breaking down resistance.”