Prevalence and severity of chestnut blight in South Tyrol (northern Italy) three decades after inoculation of hypovirulent strains of Cryphonectria parasitica

Cryphonectria parasitica causing chestnut blight is one of the most important pathogens of the chestnut tree (Castanea sativa). In the Autonomous Province of Bozen-Bolzano (South Tyrol) in northern Italy, chestnut represents the third common permanent crop by area and is an important landscape element of the slopy zones of the Etsch-Adige and Eisack-Isarco River valleys. Chestnut blight has been present in this area since 1958 and in the1990s, hypovirulent strains of C. parasitica, carrying a hypovirus, were widely inoculated to biologically control the disease. The present research aimed to survey the phytosanitary situation in 17 chestnut groves, where hypovirulent strains of C. parasitica were inoculated in 1991/1992, and to assess the long-term success of these measures. The visual inspection revealed the presence of symptoms of chestnut blight on each of the 339 single-stem or multiple-stem trees of C. sativa throughout the investigation area. However, on every inspected chestnut tree or tree group, both, virulent and healed cankers were present. The mean vitality score and mortality rate of the stems in coppices was 3.0 and 30% compared to 4.3 and 8% in fruit groves and 4.1 and 8% in mixed stands, respectively. The phytosanitary situation in fruit groves and mixed stands with larger and older trees can be considered far better compared to coppice stands, which are characterized by stems or sprouts with a smaller diameter. While hypovirulent strains of C. parasitica show a widespread presence, their prevalence is still rather low, and they seem to be less effective on younger trees.