Effects of thermal neutron irradiation on the frequency of crown gall and bacterial canker resistance in seedlings of Prunus rootstocks

Abstract

Seeds of Prunus persica (peach), P. amygdalus (bitter almond), P. mahaleb (cherry), and P. cerasifera (myrobalan plum) used for rootstocks were irradiated with thermal neutrons or X-radiation. Comparisons of seed germination, seedling growth, and leaf abnormalities caused by different dosages and kinds of irradiation were made in a pilot experiment to determine the radiosensitivity of the seeds. Thermal neutron dosages of approximately 2·0 × 10 13 for peach and almond and 1·5 × 10 13 neutrons/cm 2/sec for plum and cherry were used in subsequent experiments on disease resistance. The combined results of a 4-year glasshouse study involving 3000 seeds of each Prunus species showed that the frequency of crown gall ( Agrobacterium tumefaciens) resistant seedlings was 2·0–8·3 per cent higher in irradiated populations of almond and peach but 6·9 per cent lower in cherry compared with non-irradiated controls. In contrast, bacterial canker ( Pseudomonas syringae) resistant seedlings were 2·1–22·4 per cent less frequent in irradiated populations of almond and plum but 14·6 per cent more frequent in cherry. Seedlings with combined resistance to canker and crown gall were retested under orchard conditions. No change was found for the canker resistance of plum and cherry; however, some seedlings of peach and almond gave positive reactions to canker, and some seedlings of all stone fruits gave weak crown gall reactions. After the orchard evaluations there were still seedlings of plum, almond, and cherry that had combined resistance to both diseases but their frequency in control and irradiated populations was not significantly different.