Characterisation of isolates of Cryphonectria parasitica from western Hungary for their suitability to establish a biological control program for chestnut blight

Chestnut blight disease caused by the fungus Cryphonectria parasitica (Murr.) Barr is a devastating tree disease in forests and other areas with chestnut cultivation in central Europe. The virulent strain of the pathogen is responsible for the deadly necrotic lesions or cankers on infected chestnut trees. The discovery of Cryphonectria hypovirus 1 (CHV1) in Europe has opened wide possibilities for biological control of C. parasitica in sweet chestnut populations (Castanea sativa) once considered lost. Inoculation of virulent chestnut cankers with hypovirulent isolates of the fungus carrying CHV1 leads to non-lethal and inactive cankers. Since the hypovirus can only be transmitted by hyphal anastomoses among fungal individuals sharing the same vegetative compatibility type, it is imperative to analyse the diversity of vegetative compatibility types of C. parasitica to develop improved, site-specific biological control applications and disease management strategies. Infected bark samples of 12 chestnut trees were collected (in 2022) from the site Farkasgyepű (Veszprém county, Hungary). Isolates were cultured in the laboratory and vegetative compatibility between isolates was evaluated on PDA medium by a co-culturing assay. An important task of the laboratory work was also to identify local hypovirulent fungal strains and to determine their usability (conversion capacity) in biological control applications. The main aim of our research was to base of chestnut blight disease control with special emphasis on hypovirulent strains of C. parasitica. During our surveys, we visited a production site in west-Hungary (Farkasgyepű/Veszprém county) where infected bark samples had not yet been collected or hypovirulent fungal strains had not been used earlier. With our tests, we wanted to establish the successful field treatments. Based on the results, at least 2 vegetative compatibility groups of C. parasitica isolates were found on a relatively small plot. The first group (EU12) contains FG3, FG8, FG10 and FG12. The second group (EU13) contains FG5, FG6 and FG7 whereas FG2 and FG4 belongs to both groups (“bridge” isolates). At the same time, among the hypovirulent strains collected from the area, only 1 (FG2) was found to be suitable for in vitro conversion of all local, virulent isolates. It was also proven with a wound inoculation test on apple fruits that FG2, FG5, FG6 and FG7 hypovirulent isolates have the strongest hypovirulent effect on virulent isolates in in vitro conversions.