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New detections of chestnut blight in Great Britain during 2019–2020 reveal high Cryphonectria parasitica diversity and limited spread of the disease

Cryphonectria parasitica was detected for the first time in UK in 2011. A 2017-2018 survey identified the disease at different sites in Berkshire, Derbyshire, Devon, Dorset, and London, while the present study comprises the results of the 2019-2020 survey with new findings and additional sites in Berkshire, Buckinghamshire, Cornwall, Derbyshire, Devon, London, West Sussex, and the island of Jersey, reflecting the progressive diagnosis of more infected trees. A total of 189 samples were collected from 52 surveyed sites, and 123 samples tested positive both by real-time PCR and/or isolation from 43 sites. A total of 115 isolates were tested for mating type, vegetative compatibility group (VCG) and Cryphonectria hypovirus 1 (CHV-1). Twelve VCGs were identified, with four of them being first records in UK. The highest diversity of VCGs was found in Devon followed by West Sussex while London and Derbyshire presented the lowest. Both mating types were present (41% MAT-1 and 59% MAT- 2), and no heterokaryons were observed. Only one mating type was present per site and VCG, with the exceptions of two sites in West Sussex, two in London, one in Derbyshire, and one in Devon, where both mating types for EU2 and EU33, EU10 and EU37, EU2, and EU9 were detected respectively in different trees. Perithecia of C. parasitica were not observed at any site during this survey. Cryphonectria hypovirus 1 (CHV-1) was infecting three isolates in very low concentration (1.9 to 48.1 ng/µl) from three different locations in London and was always non-mutated subtype-I haplotype E-5. A greater diversity of VCGs at outbreak sites compared with previous surveys combined with their scattered distribution and the slow spread of the pathogen, supported the hypothesis that this disease has been introduced through imports over time from Europe.

chestnut blight, detection, hypovirus, mating types, trade, vegetative compatibility groups

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