Identification of Black Spot Resistance in Broccoli (Brassica oleracea L. var. italica) Germplasm Resources

Black spot disease, caused by Alternaria alternata, results in enormous losses in broccoli production. The current measures to prevent black spot disease mainly rely on seed disinfection and chemical control, but excellent disease-resistance resources are relatively scarce. In this study, we screened primers for black spot disease identification and conducted black spot disease resistance identification of 173 lines, including 70 hybrid lines and 103 inbred lines. Based on the phenotype, we have set five grades to present different symptoms of illness: high disease resistance, disease resistance, disease tolerance, susceptibility, and high susceptibility (the disease resistance gradually weakens). According to our phenotypic evaluations, 3, 55, 65, 45, and 5 lines were classified into high disease resistance, disease resistance, disease tolerance, susceptible, and high susceptibility, respectively. By comparing the proportion of resistant lines between hybrid and inbred lines, we noticed that the frequency of hybrid varieties with high disease resistance and disease resistance (28.57%) was lower than that in inbred lines (36.89%), indicating that the resistance resources have not yet been effectively utilized in hybrid broccoli breeding. Therefore, our results identified the resistance resources to black spot disease in broccoli, which lays the foundation for the exploration of disease resistance genes as well as the analysis of disease resistance mechanisms in the future.