The protection of ash trees against ash dieback by tree injections

Ash dieback caused by a non-native pathogenic fungus Hymenoscyphus fraxineus has been decimating populations of European Fraxinus species for over 30 years. Nevertheless, there is still a considerable amount of valuable ashes of this species in European cities and landscapes. Although it has been shown in many studies that the effect of H. fraxineus on adult and senescent trees tends to be rather chronic, the infection by this fungus also causes a substantial decrease in the lifespan of these trees. This research is dealing with the control of ashes using the tree injection technology. It is focused on the development of bark necroses, as this symptom results in the highest damage to a tree. Trees were first inoculated with two different strains of H. fraxineus. Trees with proven positive inoculation were injected with six different fungicides. Subsequently, over the course of nine months, the area of developing bark necroses was measured using image analysis methods. After the field part of the experiment was finished, the extent of the infection by H. fraxineus in tissues was determined and confirmed by PCR. Three out of six tested variants (Tr. 1: 7.1% propiconazole, Tr. 3: 7% propamocarb and 2.7% fosetyl, and Tr. 6: 0.015% sodium selenite) displayed significant deceleration of the growth of bark necroses compared to control. Tr. 3 demonstrated the highest effectiveness with only a very small increment of the necrosis area during the first three months after the injection treatment and a follow-up complete cessation of growth. Reisolations confirmed high mortality of H. fraxineus after this treatment. Hence the presented study can contribute to substantial prolongation of the lifespan of ashes in Europe.