Analysis of genetic diversity and evaluation of disease resistance to pear fire blight of Pyrus betulifolia Bunge seedling

Abstract: 【Objective】Pyrus betulifolia Bunge is widely used as a rootstock for pears in Xinjiang due to its deep root system, drought resistance, waterlogging resistance, cold resistance, salinity resistance and other characteristics. In recent years, pear fire blight has been spreading rapidly in Xinjiang and become a major threat to the development of characteristic forestry and fruit industry in Xinjiang, especially to the Kuerlexiang Li pear industry in the south of Xinjiang. P. betulifolia and Kuerlexiang Li pear are of poor resistance to pear fire blight. Germplasm resources are the important material basis for pear breeding, production, as well as scientific research. Studying the genetic diversity and disease resistance of the seedling offsprings of P. betulifolia is important for the subsequent research in the genetic diversity of P. betulifolia at molecular level and the excavation of disease- resistant genes.【Methods】The morphological genetic diversity analysis of 19 qualitative traits and 6 quantitative traits of 196 cultivated P. betulifolia seedlings were conducted. Seedling resistance was identified by artificial inoculation with pear fire blight fungus, and the comprehensive evaluation was combined with the field disease investigation.【Results】Leaves of P. betulifolia seedlings were more diversified in morphological characters. Leaf shape ranged from ovate, elliptic, to lanceolate. The shape of leaf base was narrowly to broadly wedge-shaped, round, or truncate. The leaf apex was broadly to narrowly acuate. The leaf margin wasmostly acutely serrate. All the 196 samples investigated showed no lobe, no seta on leaf margin, no pubescence on the back of mature leaf, no stipules, and no thorn. The leaf shape was mainly ovate (84.18%); the shape of leaf base was mainly wide wedge-shaped and wedge-shaped (55.10% and 30.61%, respectively); the shape of leaf apex was mainly broadly or narrowly acuate (45.92% and 47.96%, respectively); the leaf margin was mainly serrate (serration) (99.49% ). The status of leaf surface was mainly enclasped (60.71% ), and the latitude of leaf in relative to the shoot was mainly downwards (70.92%). The bark was generally longitudinally cracked, accounting for 93.88%. The tree posture was mainly semi- spreading or spreading, accounting for 50.51% and 36.22%, respectively. The density of lenticels on one-year-old branches was mainly medium or many, accounting for 53.06% and 45.41%, respectively. The leaf bud posture was mainly oblique, accounting for 70.41%. The size of bud receptacle was mainly small, accounting for 92.34%. The tree vigor was generally strong. The color of one-yearold shoot was mainly red brown and yellow brown, accounting for 38.27% and 35.71%, respectively. The genetic diversity index of the 14 qualitative traits ranged from 0.03 to 1.20. The genetic diversity indexes of leaf base shape (1.04), tree habit (1.08), tree vigor (1.01), and annual branch color (1.20) were relatively high. Six quantitative traits had a coefficient of variation ranging from 11.14% to 22.27% . The of leaf area had the largest coefficient of variation of 22.27% , followed by leaf petiole length (21.73%), and the coefficients of variation of the 6 quantitative traits were relatively low ranging from 11.14% to 11.73%. The genetic diversity index of the 6 quantitative traits ranged from 1.95 to 2.07. The genetic diversity indices of the quantitative traits were much greater than those of the qualitative traits, indicating that the phenotypic diversity of the quantitative traits was relatively high. No high resistance materials were found from the 196 P. betulifolia seedlings. There were four disease resistant (R) seedlings and ten moderately resistant (MR) seedling. There were 36 seedlings (18.37%) exhibiting moderate susceptibility (MS), 58 (29.59%) exhibiting susceptibility (S), and 88 (44.90%) exhibiting high susceptibility (HS). Most of the seedlings (92.86%) showed different degrees of susceptibility to pear fire blight fungus, indicating a lack of resistant materials in the tested P. betulifolia seedlings. The disease pattern in the field showed that the degree of disease was light in the juvenile stage and increased after entering the adult stage, and the stronger the tree, the higher the disease incidence.【Conclusion】This result provides a material basis for further study of the germplasm diversity of P. betulifolia, as well as for breeding of rootstock resistant to pear fire blight and the excavation of disease resistance genes.