First report of <i>Calonectria henricotiae</i> causing boxwood blight in Switzerland and Italy

Calonectria henricotiae is one of two pathogens which cause boxwood blight (Gehesquière et al., 2016). Unlike C. pseudonaviculata, which is widespread globally (Aiello et al., 2022), C. henricotiae has only been found in nine countries, mostly in central Europe (Hong, 2023). Because of its high temperature tolerance, C. henricotiae is likely to be present in southern Europe. Following the first discovery of C. henricotiae in Spain in 2023 (Kong et al., 2024), we expanded the survey of boxwood gardens and street plantings to include Switzerland, Italy and Greece in the summer of 2024. Thirty-five boxwood (Buxus spp.) twig samples showing either black streaks and/or leaf blight symptoms (Figure 1a) were collected from gardens or lining streets in 12 cities in Greece, Italy and Switzerland (Table 1), and were transported to and processed in the lab under a USDA permit (P526P-21-05960). Samples were surface sterilised with 10% bleach for two minutes and were rinsed three times in deionised water. They were then induced for sporulation in closed plastic containers at 23°C as previously described (Kong et al., 2024). Samples producing white mould were examined under a dissecting microscope. Fruiting bodies from suspected samples were suspended in 15 µL sterile deionised water and transferred to a glass slide for confirmation under a compound microscope. Only three samples from Italy and one from Switzerland produced cylindrical conidia. Fifteen 1-µL drops of the conidia suspensions from each sample were placed on an acidified potato dextrose agar (PDA) plate. Three colonies from each plate were subcultured onto regular PDA. Species identification was done using PCR with purified culture DNA and C. henricotiae- and C. pseudonaviculata-specific primers (Guo & Pooler, 2021). All isolates from the four samples which produced cylindrical conidia were positive for C. henricotiae and negative for C. pseudonaviculata. Their identities were confirmed by sequencing the Hb3 gene (GenBank Accession Nos. PQ464598 - PQ464601) and by conducting thermal tolerance and fungicide sensitivity tests to discriminate between C. henricotiae and C. pseudonaviculata (Gehesquière et al., 2016). The mean growth (mm) of these isolates after 10 days was: 10.3 (± 2.7) at 28°C and 2.1 (± 0.7) at 30°C, and 25.4 (± 7.1), 22.4 (± 8.4) and 24.5 (± 9.6) at 23°C at 1, 10 and 100 ppm of kresoxim-methyl (Sovran®, BASF), respectively. The growth of these isolates was substantially greater than that of a C. pseudonaviculata isolate under the same conditions (Figure 2a). A pathogenicity test with Buxus sempervirens twigs dipped in the conidial suspension verified these isolates as the causal agent of boxwood blight (Figure 1b). To our knowledge, this is the first report of C. henricotiae causing boxwood blight in Italy and Switzerland. This pathogen has the potential to be more destructive with fewer control options due to its tolerance to certain fungicides. Knowing the distribution of C. henricotiae is critical to preventing its spread to other currently unaffected areas. The work was funded by the USDA National Institute of Food and Agriculture - Specialty Crop Research Initiative (2020-51181-32135). None of the authors have a conflict of interest.