Apple burrknots involved in trunk canker initiation and dying of young trees

Conditions associated with an unusually high occurrence of burrknots in two young commercial apple orchards at two locations, Těšetice and Stošíkovice, South Moravia, the Czech Republic, were analysed. In the first orchard, planted in spring 2003, the occurrence of burrknot on trees of cvs. Early Smith, Jonagold and Gala on M.9 rootstock was evaluated. In the second orchard, planted in autumn 2002, cv. Gala on M.9 rootstock was assessed. Planting material used at each location was obtained from the same commercial propagator and had been established from the same propagation stock materials. Of 60 trees per cultivar surveyed in the first orchard, incidence of burrknots in cvs. Early Smith, Jonagold and Gala trees was 98, 97 and 92%, respectively. The burrknot severity (mean number of burrknots on above portion of rootstock) was significantly higher on Jonagold trees, i.e. 3.65, than on the other two cultivars. Of 60 Gala trees in the second orchard, symptoms of burrknot appeared on 73.3% of trees planted on a slight slope and 70.0% of trees planted on a plane. The burrknot severity was significantly higher on the Gala trees planted in the Těšetice orchard than in the Stošíkovice location. Burrknot incidence and incidence of root-suckers were the highest on Jonagold trees at Těšetice. However, correlations between burrknot number and number of root-suckers were not statistically significant. Five years after the tree planting, increased dying of Jonagold trees was recorded at Těšetice. Of 290 trees examined, 5.5% had died. On the rootstock portion of trunk, each dead tree exhibited burrknots associated with bark cankers that more or less girdled the trunks. Only sporadic occurrence of canker symptoms and no premature dying of young trees were observed at Stošíkovice. Attempts at isolation of the fire blight bacterium, Erwinia amylovora, and oomycete Phytophthora spp. from necrotic tissue surrounding burrknots on rootstocks were not successful. The stem associated apple tree viruses Apple stem pitting virus (ASPV) and Apple stem grooving virus (ASGV) were detected frequently in the rootstock and scion parts of cvs. Jonagold and Early Smith and less frequently in Gala cultivar. The virus positive trees included individuals both with various burrknot severity and without symptoms of burrknots. There were no correlations between the incidence of burrknots and the presence of ASPV and ASGV.