Symptoms and occurrence regularity of citrus target spot in the field and the resistance determination of 47 citrus varieties

Abstract: 【Objective】Citrus target spot has been discovered on Citrus unshiu and Fortunella margarita in Shaanxi Province, C. unshiu in Hubei Province, C. reticulata var. ponkan in Hunan Province, as well as C. reticulata var. ponkan, C. limon var. eureka, C. meyerii var. meyer and C. sinensis var. newhall in Chongqing, which has seriously threatened the citrus production in China. Thus, the symptomatic change and occurrence regularity of the target spot in Chongqing were investigated, and the resistance difference of the 47 varieties to 3 strains from C. unshiu in Shanxi, Hubei and C. limon var. eureka in Chongqing were evaluated in this study.【Methods】The symptomatic changes of the target spot on C. limon var. eureka in Wanzhou, Chongqing were investigated by photography. The time of appearance of the typical target spot symptoms, and the incidence and disease index in various orchards under different conditions were also investigated when the disease condition was stable. The leaves of the 47varieties were inoculated in vitro with the 3 strains. And the virulence of the 3 strains and the resistance of the 47 varieties were analyzed and evaluated based on the incidence and average diameter of disease spots.【Results】After the Citrus trees were infected by Pseudofabraea citricarpar, the symptoms could appeared on the leaves, branches, shoots, fruits, petioles, fruit stalk, and calyx, etc. At the early stage of the disease, the reddish brown spots formed on the leaves, and then they gradually expanded to dark brown round lesions. The disease spots were scattered, and the multiple disease spots would connect to one large spot. The leaf spot was slightly concave, round, oval, fusiform or irregular, and the concave and water stain were more obvious on the young tissues. One side or multiple sides of the lesion could produce tail along the vein. The diameter of the disease spot was mostly 0.2-2.2 cm. The outer edge of the spot on the front of the leaf was oil stained, and then it became dark brown and reddish brown inward, and light white in the center. Most of the black dots (acervuli) were arranged in concentric circles, and there was no any conidiamata on some diseased leaves. Later, the black dots (acervuli) on the diseased leaves could falled off with the leaf abscission, which could be the main source of infection in the next year. The disease spots on some perennial leaves were relatively small, and they could remain on the leaf surface without perforation or expansion until conditions were suitable for infecting the other leaves. There were also symptom differences on the leaves of the different Citrus varieties and different growth stages. The incidence of the citrus target spot peaked from January to February, and the time of appearance of the typical target spot symptoms was as early as December 3 in the high-altitude areas, the orchards in canopy, the weak trees and the orchards of susceptible varieties, of which the disease index was higher than that of other orchards. From November to January, the lower of the average maximum temperature, the average low temperature, and the extreme low temperature were, the higher of the disease index was, while from February to April, the higher of the above three climate factors were, the higher of the disease index was. From December to April, with more rain days, high relative humidity and rainfall the disease index was higher. The different strains from the different sources had certain virulence to the 47 varieties with significant difference. Overall, the virulence of YLSDP80 was the strongest (the average diameter of the lesion was 6.40 mm), and that of CGDS2 was medium (the average diameter of the lesion was 5.03 mm), but the incidence was the highest (85.75%). WZSS1 was relatively weak (the average diameter of the lesion was 4.52 mm), and the incidence was the lowest (66.94%). Among the47 varieties, 24 varieties were resistant to the 3 strains, including Tarocco Blood Orange No. 9, Liduo Blood Orange, Fengwan Navel Orange, Cara cara Navel Orange, Jinxiang Orange, Fukumoto Navel Orange, Tarocco Blood Orange No. 8, 98-1 Long Leaf Orange, Sanhongyou Pomelo, Huangjinyou Pomelo, Dianjiang Laohongyou Pomelo, Liangpingyou Pomelo, Shatianyou Pomelo, Longduzaoxiangyou Pomelo, Chandler Pummelo, Daya mandarin, Orah, Aiyuan No. 30, Satsuma mandarin No. 1, Nankou, Shiranui, Kouzhumi, Hongjü and Poncirus trifoliata Eureka lemon was susceptible to the strains from three provinces at the same time, and the resistance of the other 22 varieties to the strains from three provinces was different.【Conclusion】Comparing with the black spot on citrus, the conidiomata on the diseased leaves of the citrus target spot were obviously arranged in concentric circles, and the citrus target spot usually developed in mid December to March of the next year. Comparing with the anthracnose, the diseased spots of the citrus target spot were smaller in size and more in number, and the conidiomata were still black rather than those of the anthracnose were orange when the relative humidity were very high, which was benificial for distinguishing the citrus black spot, the anthracnose and the target spot for the field worker of plant protection. The 24 varieties with resistance to the 3 strains could be inter-cropped with the other 25 varieties to retard the spread of the disease.