Establishment of Evaluation System and Screening of Disease- Resistant Accessions for Phomopsis Seed Decay in Soybean Germination Stage

An accurate and rapid indoor evaluation system was established by using soybeans with different resistance levels to Phomopsis seed decay as test materials. And then 170 soybean germplasm accessions were employed to screened out disease-resistant varieties, so as to provide methods and material basis for high-throughput assessment of Phomopsis seed decay in soybean and cultivation of resistant varieties.

In terms of establishing a reliable evaluation method for Phomopsis seed decay, Qihuang 34, Williams, Zhongzuo 09-560, z13-631-2, ZDD26268, Chenxiqingpidou 1 and Tongxianhuangdou were selected as experimental materials. For each soybean accession, the seeds with uniform size and undamaged seed coat were germinated in the dark after disinfection. At different germination stages, the pathogen of Phomopsis seed decay was inoculated for 24 h, 48 h, 72 h and 96 h. The mycelium coverage rate and seed decay rate of seed surface under different infection time were counted to determine the optimal identification period for evaluating Phomopsis seed decay in soybean. Then, the resistance of 170 different soybean germplasms in natural population was identified by using the coverage rate of mycelium on the surface of seeds and the decay rate of seeds as evaluation indexes. The high disease resistance varieties were screened based on 5 disease resistance levels.

The soybean accessions showed the most significant differences in disease resistance levels after 96 h of germination when mycelium coverage rate and seed decay rate of soybean surface were used as evaluation indexes. Further comparison of the incidence of 24 h, 48 h, 72 h and 96 h after infection showed that the difference in disease resistance between different varieties after infection for 72 h was the most obvious. Therefore, it was the most suitable period, 72 h of infection at the bud stage after 96 h of germination, for evaluating the resistance level of different soybean varieties to Phomopsis seed decay. The resistance of 170 soybean varieties to Phomopsis seed decay was identified and classified into five disease resistance grades, namely, high resistance, medium resistance, medium susceptibility, susceptibility and high susceptibility. Among them, there were 30 varieties of grade I (high resistance to disease), 51 varieties of grade Ⅱ (medium resistance to disease), 71 varieties of grade Ⅲ (medium disease susceptibility), 4 varieties of grade Ⅳ (disease susceptibility) and 14 varieties of grade V (high disease susceptibility), idicating that there are extensive variations in the resistance to Phomopsis seed decay of soybean germplasm resources in China.

In this study, the most optimum stage of disease identification was considered as soybean seeds after 96 h germination to infect the Phomopsis longicolla for 72 h. After that, the mycelium coverage rate and seed decay rate of soybean surface were counted as evaluation parameters. The evaluation system has high accuracy and reliability, which can provide an effective method for high-throughput identification of different varieties in the laboratory. And 30 highly resistant varieties were further screened to provide a material basis for the breeding of resistant varieties.