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The devastating effects of the chestnut blight fungus Cryphonectria parasitica have been controlled with relative success in Europe due to the natural spread of hypovirulence. This phenomenon is caused by dsRNA viruses named CHV, and in particular the Italian subtype of type CHV1 has proved to be with best results. In our study, we investigated host dependency at clonal level to canker size and morphology/fructification, as well as if hypovirulent isolates of differing virulence will cause significantly different sizes of cankers, and possibly variability in conidiation (hypovirus-isolate dependent). For this purpose, we converted 1 virulent isolate (Smo062B) with 2 hypovirus clones (J12 and Sk28, labeled HV1 and HV2 respectively, for this study) previously described as being of different virulence towards the host fungus. The obtained hypovirulent isolates were used for inoculations of thin chestnut (1,5 – 5cm diam.) stems within chestnut clusters. The trials were performed on 2 separate sites, on a total of 78 inoculation (39 pairs) in Brezno (25 clusters), and 68 (34 pairs) on 10 clusters in Kalishte. For comparison of the reaction of the host plants to hypovirulent isolates, both hypovirulent isolates were inoculated in stems of exact width on the same chestnut clusters (i.e. stems), multiple times per cluster, up to 18 (i.e. 9 pairs) in Kalishte and 6 (i.e. 3 pairs) in Brezno. Control inoculations were with virulent isolate Smo062B. 140 days after inoculation cankers were assessed for conidiation and their width and length were measured. Stromata were collected from all fruiting cankers, and used for reisolation. Obtained cultures were determined as hypovirulent or virulent depending on culture morphology. Canker surface area was highly dependent on the host clone. In Kalishte, surface area of cankers caused by HV2 ranged on average per cluster from 408mm2 , up to 3020mm2 . In Brezno, the range was even more pronounced with HV2 cankers ranging from 131mm2 up to 3159mm2 on average per cluster of chestnut stems. Cankers caused by HV1 were on average with a significantly smaller surface area when compared to cankers caused by HV2. The extreme case was in cluster 9 in Kalishta, were the average surface of cankers caused by HV1 were 560mm2 , while HV2 caused cankers with nearly 4 times bigger area (3020mm2 ). Similarly, in Brezno, canker area in clonal stems for HV1 on average was 245mm2 , and for HV2 was 1677mm2 . Counterintuitively, the smaller cankers, caused by HV1 containing the more virulent hypovirus (J12) produced as often or more spore baring stromata as isolates containing the less virulent hypovirus. This partially explains the spread and sustainability of even very virulent hypoviruses within populations of C. parasitica.

Hans Em Faculty of Forest Sciences, Landscape Architecture and Environmental Engineering

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