Controlling late blight (Phytophthora infestans) in potatoes has become much more complicated. Growers and agronomists have to think about many factors, including disease pressure, blight strains, growth stage of the crop, weather conditions, fungicide mode of action, movement of the fungicide within the plant, resistance management, and many more.
Growers need to protect their crops from emergence right up to harvest. They must also protect tubers from tuber blight, starting from tuber initiation onwards, more of an issue as fewer fungicides are able to do this.
Blight strains in the UK have changed radically recently and Dr. David Cooke of the James Hutton Institute in Dundee has identified newer strains which are more aggressive, produce more spores from larger lesions and have a faster life cycle.
Consequently effective blight control has become more challenging. His analysis of late blight samples showed that the most widespread genotype in 2019 was the Strain 6_A1, accounting for 36% of all samples. This declined from 47% in 2018. In 2011 it dominated the UK population with nearly 80% of all samples.
The next most widespread strain last year was 36_A2 which increased from 17% in 2018 to 27% in 2019. The genotype 37_A2 which entered the UK in 2016 is of concern as it is insensitive to fluazinam, which was one of the most widely used fungicides, particularly for tuber blight at the end of the programme.
The phenotype 37_A2 is found mostly in the East of the country, less so in the North or West but has declined from 16% in 2018 to 6% in 2019, perhaps mirroring the drastic reduction in the use of fluazinam and reduction in selection pressure.
The Pesticide Usage Survey in 2014 showed over 360,000 hectares of fluazinam were sprayed. In 2018 this has gone down to 78,000 hectares, leaving a significant gap to fill in the programme, says Paul Goddard, potato expert from BASF.
Paul explains that dimethomorph in both Percos and Invader has systemic activity which protects new growth and its translaminar activity which protects the upper and lower surfaces of the leaves.