Evaluation of resistance of Xinjiang wild walnuts to walnut canker

Abstract:【Objective】In recent years, walnut canker disease (Cytospora chrysosperma) has become one of the important diseases affecting the development of walnut trees in Xinjiang. The investigation on the natural incidence of wild walnut trees in Xinjiang wild walnut populations suggested that Xinjiang has a rich wild walnut germplasm resource, and there are obvious differences in disease resistance among different varieties, but there is still a lack of systematic research. Through the disease resistance evaluation with different methods, the disease- resistant germplasm resources can be screened and can lay a solid foundation for resistance breeding of cultivated walnut, and also provide a scientific basis for the protection and utilization of wild walnut in Xinjiang.【Methods】The annual branches of 28 Xinjiang wild walnut accessions of 7 types were used as experimental materials. Three wounds up to the xylem were made on each branch with a punch, about 5 mm in diameter. Then, agar plates with pathogen purified for 6 days were inoculated through the wound, and the lesion length was recorded on the 12th day. The phloem tissue around the branch lesion was taken and stored in the refrigerator at -40 ℃ after quick freezing with liquid nitrogen. The activities of POD, SOD, CAT, and MDA in the phloem were measured. The resistance of Xinjiang wild walnuts to walnut canker was evaluated by principal component analysis and cluster analysis.【Results】After inoculation with walnut canker pathogen for 12 days,the disease degree of the 28 isolated branch materials showed a significant difference. Among them, the lowest length of disease spot in the branch was 5 mm and the highest was 100 mm. The disease spots on the surface of branches were obvious, and the color of the diseased part of branches turned black and lose water. According to the multiple comparison results, the difference between L-1 with the largest disease spot on branches and the nine single plants without disease was the most significant. The tested materials were divided into 8 groups according to the differences. Among the 28 wild walnut materials, the highest SOD enzyme activities were found in X-4, XT-2, XY-5, and T-1, and all showed susceptible individual plants. The SOD activity in L-1, XT-1, and X-1 with the largest lesion was the lowest, which might have reached the physiological threshold with the infection of pathogenic bacteria. The highest POD enzyme activity was in T-1, XT-1, and X-3, and the three accessions were susceptible materials. The lowest POD activity was found in X-5, XT-4, and J-1, and none of the three materials displayed disease symptom. The highest CAT enzyme activities were in XT-2, L-2, and X-1, of which L-2 and X-1 were severely diseased. The lowest CAT enzyme activity was in J-3, J-1, and XT-5, among them, J-1 and XT-5 were not infected, and the disease of J-3 was mild. The highest MDA content was detected in XY-3, XT-1, L-1, and X-1, which were all susceptible materials, and the lesion length of XT-1, L-1, and X-1 were longest. The lowest content of MDA was in XT-4, T-2, XY-1, XY-4, and XY-2, which were all non-diseased materials. Compared with the non-diseased branches, the average POD enzyme activity, SOD enzyme activity, CAT enzyme activity, and MDA content in diseased branches increased by 160.6%, 18.9%, 183.6%, and 92.6%, respectively. The POD enzyme activity, CAT enzyme activity, and MDA content were significantly positively correlated with the size of the disease spots (p＜0.01) . Although the SOD activity of diseased plants was higher than that of non-diseased plants, there was no significant linear relationship between SOD and branch lesion length. The SOD activity of diseased plants of wild walnut was higher than that of the non-diseased plants, but there was no significant linear relationship between SOD and lesion length. The results of the principal component analysis showed that two principal component matrices with eigenvalues greater than 1 were extracted, and the variance contribution rate of the two principal components reached 73.42%. According to the matrix formula, the disease resistance of Xinjiang wild walnut was -1.45-1.32, among which XT-4 had the highest disease resistance and XT-1 had the lowest disease resistance. Xinjiang wild walnut germplasm with a score of 0.93-1.32 had good disease resistance, including seven individual plants XT-4, J-1, J-2, XY-4, XY-1, XY-2, and T-2. The results of cluster analysis showed that the disease resistance of 28 wild walnut accessions could be divided into three categories at 4 Euclidean distances, among which class B walnut had the best disease resistance, class C walnut had the worst disease resistance, and class A belonged to the intermediate type. Most of the 7 high resistance germplasms belonged to class B.【Conclusion】The annual branches of the 28 Xinjiang wild walnuts showed different disease resistance after inoculation with walnut canker. The activities of POD, CAT, and content in MDA can be used as physiological indexes for the preliminary identification of walnut canker resistance. Finally, 7 high disease resistant materials were selected from the 28 wild walnut materials in Xinjiang. The results provide reference for disease resistance breeding of walnut.