New, emerging clones of Phytophthora infestans in Europe

Over the last five years, EuroBlight has undertaken and coordinated an extensive survey of European populations of the late blight pathogen, P. infestans. The most recent data from the EuroBlight monitoring initiative highlighted the emergence of three new clonal lineages, named EU_36_A2, EU_37_A2 and EU_41_A2, in different parts of Europe. As ever, this raises the question of the epidemic potential of these newcomers, and of their impact on late blight management strategies. Are those strains more aggressive than other types? Are they less sensitive to fungicides? What cultivars are now under threat? Do I have to change my control strategy? These are the questions farmers and potato advisors are asking.  

Alongside the genotypic analyses carried out within the EuroBlight survey, several research teams have committed themselves, as part of the IPMBlight2.0 project, to also generate and analyse important phenotypic traits, in particular aggressiveness (disease severity on compatible hosts), virulence, (the ability to overcome identified host resistance genes) and fungicide sensitivity. Since the number of isolates analysed is still limited and because not all analyses are complete yet, the results from these studies are still preliminary. However, the findings suggest that the reasons behind the success of these lineages are different.

EU_37_A2: a fluazinam insensitive, fit lineage?

First detected in the Netherlands in 2013, where it remained local until 2015, this lineage has since spread south (it is now dominant in Belgium and northern France), to the west (expanding in England) and east (some locations in Germany and Poland). It has displaced other lineages.

One key trait in this lineage is its strongly reduced sensitivity to fluazinam compared to other P. infestans widespread lineages. Fluazinam has been, and remains, a major tool in the current IPM toolbox for controlling late blight. This low sensitivity was first reported by H. Schepers and his colleagues at Wageningen UR and is confirmed in tests carried out in Norway and Estonia, within the IPMBlight 2.0 project, on isolates collected in 2016. Further analyses are currently on the way regarding isolates collected in 2017.

EU_37_A2 isolates collected in 2016 proved remarkably aggressive to the susceptible potato cultivar Bintje in lab tests conducted at INRA, France. However, we also have indications that isolates from the same lineage collected in 2017 are not as aggressive as their 2016 counterparts. This might be due to the more diverse origins, and the larger sample size of the 2017 isolates compared to the 2016 ones, and warrants further investigation.  Our finding of EU_37_A2 as a cause of tuber blight is also a cause for concern which is focussing attention on late-season blight management.

Another genotype, EU_33_A2, was also found to be insensitive to fluazinam, but trials and evidence from monitoring indicate that it is not fit and it failed to become established in field populations. Conversely our evidence on EU_37_A2 points to good epidemic fitness and it seems likely it will continue to expand its range.

As visualised on the EuroBlight mapping tool, EU_37_A2 is spreading in the direction of Denmark. In 2018, intensive sampling in the south of Denmark will be conducted early in the season with the samples genotyped as rapidly as possible (less than 14 days). This data will warn farmers about potential problems with reduced sensitivity to fluazinam; a product used mainly later in the season for tuber blight protection.

EU_36_A2: going west?

The genotype EU_36_A2 was first detected in Germany and the Netherlands in June and September 2014, respectively. In 2015, a low disease pressure season it spread within these countries and was detected in Belgium for the first time. It then further expanded its range to France, and to some spots in Poland, in 2016. Its mainly westward expansion continued in 2017, when it increased from 4 to 9% of the collected samples and reached England, Denmark and Poland. Although its spread suggests it is fit, the phenotypic characteristics of this lineage are still very poorly explored, as it was absent from the 2016 samples collected within IPMBlight2.0. The first analyses are underway, but we have still no clear evidence to explain the current distribution of this lineage, nor to predict its actual fitness or persistence over time.

EU_41_A2: a Nordic invader of sexual populations ?

The last newcomer, EU_41_A2, is a very rare case of emergence of a clonal lineage within populations thought to reproduce mainly sexually. Its current distribution spans Denmark and the south of Norway, Sweden and parts of Poland. Because of its recent detection, few isolates of this lineage have been characterised to date, but the first indications from analyses made by INRA suggests that it has a high level of aggressiveness towards susceptible potato cultivars. More data are now being collected on its aggressiveness and fungicide sensitivity profiles at INRA and SLU, and this will be helpful for understanding possible reasons for its emergence, and for predicting its epidemic potential.

For Danish conditions, there are concerns about the spread of the EU_41. It seems to be very aggressive, and in 2017 severe attacks in some areas could not be avoided despite intensive use of fungicides. In Sweden, there are the same concerns about this lineage, and during 2018 sampling, we will map any further spread of EU_41. Aggressiveness and fungicide sensitivity in this clonal lineage will be determined and compared with samples from sexual populations of P. infestans. Since clonal lineages can only overwinter vegetatively, the incidence of EU_41 in infected tubers will be determined.

EuroBlight mapping tools

The simultaneous emergence and spread of these clones in European populations of P. infestans highlights the value of constant monitoring of these evolving populations and maintaining the competences and infrastructure for rapid genotyping and phenotyping of emerging clones.

New IPM initiatives taking pathogen information into accout

In the IPMBlight2.0 project, one activity is to make a proof of concept on how to include pathogen information in national Decision Support Systems (DSS). The IPM2.0 approach as defined by Geert Kessel, WUR, adds three extra components to the current control strategy for potato late blight: the use of resistant varieties, monitoring of naturally occurring genetic adaptations in the pathogen and a ‘do not spray unless’ strategy, which dictates that a grower only needs to apply fungicides when a resistant variety is at risk of infection due to pathogen adaptation.

In a collaboration between Denmark and Sweden, field trials will be conducted during 2018 to test strategies to control new and more resistant starch cultivars. The first part of the strategy is to start spraying with reduced dosages (Revus / Ranman TOP) only when late blight is found in the region. The weather-based infection pressure will be used to determine the fungicide type and dosage. The trial setup includes trap nurseries with 20 differential cultivars with known resistances and cultivars in the pipeline for commercial use. The second part of the strategy is to increase dosage and use stronger compounds when late blight appears in differential cultivars with the corresponding resistances. With this strategy, the goal is to reduce the fungicide use in starch potatoes by up to 50 % depending on the blight season. 

This news was written by Didier Andrivon, Jens G. Hansen, Björn Andersson and David Cooke - based on results from the EuroBlight Monitoring initiative, the IPMBlight2.0 project and national blight projects.