Short and long term efficacy and prevalence of Cryphonectria parasitica hypovirulent strains released as biocontrol agents of chestnut blight

Abstract

Chestnut blight, Cryphonectria parasitica (Murrill) Barr, was reported in Portugal in 1989. The disease rapidly spread within the chestnut region of the Trás-os-Montes (the Northeast of Portugal). Eradication and mechanical/burning exclusion failed to control the disease and natural hypovirulence is still absent or of very low incidence. The introduction of human-assisted therapeutic hypovirulence is therefore required to control the disease. We presente here the efficacy of field applications (short and long term) of a locally characterized hypovirulent strain (CHV1-RBB111), by the punch method in two chestnut orchards (Valpaços and Vinhais). The Valpaços disease incidence was 68% (65 out of 96 trees) and in Vinhais it was 46% (78 out of 169 trees). In both locations, diversity of vegetative incompatibility (vc type) was low and largely dominated (> 85%) by a single vc type (EU11). In Valpaços, all cankers of the orchard were treated (63 cankers) and in Vinhais 31 trees (cankers) were treated. One year after application, field efficacy was very high with 93.8% of healed cankers in Valpaços and 90.3% in Vinhais, these values increased to 100% and 93.5% (respectively) after four years. The persistence of the introduced CHV1-RBB 111 strain was high and present in almost all treated cankers and it was dominant both inwards and on the application site revealing an endophytic behavior. The non-C. parasitica microfungi were abundant with 14 different species. Biscogniauxia mediterranea (De Not.) Kuntze was the most abundant in both orchards. Treatment with the introduced hypovirulent strain (CHV1-RBB111) was found to be effective in controlling chestnut blight with regeneration of cortical tissues and recovery of treated trees.