Establishment of an identification method of citrus resistance to target spot

Abstract:【Objective】Citrus target spot is becoming increasingly severe in China, damaging the young leaves, shoots and fruit in more and more citrus varieties. This study was carried out in order to explore the most suitable inoculation conditions for establishing a rapid, simple and reliable method to identify resistance to the disease.【Methods】In vitro leaf inoculation method was used to inoculate the pathogen to leaves under 5, 10, 15, or 20 ℃ in a moisture box and the diameters of the speckles were recorded at different time after inoculation. Then, 27 different citrus varieties were inoculated with a typical strains Pc-W ZBY1 and kept in a humidified box using the established inoculation method and the severity of the symptom was recorded. The resistance was evaluated based on the average diameter of the speckle. 【Results】At 5, 10, 15, and 20 ℃ with a humidity of 90%, no lesions were found on adaxial or abaxial leaf surface without artificial wound, while lesions occurred in all inoculated leaves with wounds. The wounded leaves were more likely to be infected by Ps. citricarpa at different tested temperatures, and the most suitable inoculation site was the abaxial side of leaves. At 28 days after inoculation in all tem-peratures, the lesion hardly expanded and the size of the lesions differed significantly among treat-ments. Therefore, 28 days after inoculation was the best time for identification of disease resistance. Le-sions developed much faster on the abaxial side than the adaxial side of the leaves. Different tempera-tures affected the disease development greatly. At 10 ℃, the diameter of lesion spots was the largest, significantly larger (p ＜ 0.05) than under the other temperatures. Therefore, 10 ℃ was the optimal for  inoculation and incubation for resistance identification in laboratory. Evaluation of resistance to Ps. cit-ricarpa among the citrus varieties showed that most varieties were susceptible. Pc- WZBY1 strain showed different pathogenicity in different cultivars with significant differences (p＜0.05) among some varieties. The results showed that Shatianyou had an immune phenotype. The other varieties of Citrus grandis could be classified into highly resistant type. In Citrus limon, the average diameter of the speckle of Eureka Lemon was 5.9 mm, and the diameter of lesion spot in some leaves was as large as 12 mm; seedless Orah 091 produced the largest lesion spot of 14 mm and was the most susceptible variety. Jinqiu Shatangju showed the strongest resistance in hybrids. In other varieties, Fukumoto navel or-ange was the most susceptible variety in Citrus sinensis, and C. sinensis ‘Karakara’, Yubei licheng and Tarocco had a similarly higher resistance to Ps. citricarpa. Compared with the aforementioned two pomelo varieties, disease spots in most pomelo varieties were hard to see, suggesting excellent performance in disease resistance. In all tested varieties, Shatianyou was immune, and Meyer lemon was of high re-sistance to Ps. citricarpa. The high incidence of citrus target spot in the other tested varieties showed the strong virulence of the pathogen which is casting high risk in citrus production.【Conclusion】A method leaf inoculation to identify resistance to citrus target spot disease was optimized with inoculat- ing on the abaxial side of leaves and incubating at 10 ℃ for 28 days in moisture box. Shatianyou is immune, and Meyer lemon has high resistance to Ps. citricarpa. Orah 091 was the most susceptible variety and Jinqiu Shatangju most resistant in hybrids, which are mostly susceptible. C. grandis is more resistant compared with C. sinensis and C. reticulata.