| Nanfeng tangerine(Citrus reticulata Blanco var. kinokuni (Tanaka) H.H.Hu), belonging to Citrus of Rutaceae, is an important cultivated plant with high nutritional and medicinal value. In recent years, a serious disease of citrus black spot has been occurring on Nanfeng tangerine in Nanfeng county of Jiangxi Province, bringing heavy losses to local enconomy. This dissertation dealt with studies of symptom, pathogenic identification, biological characteristics, disease cycle and the control of the disease, and results are as follows.1. Symptom Fruits, twigs and leaves are infected, of which fruits especially mature fruits are mainly infected organs. After infection, circular and depressed necrotic spots with 1-3 mm size appear on diseased parts, the spots are red-brown at the beginning, but at later stage the colour turns dark brown and at this time some small black pycnidia appear at the centre of the spots.2. Pathogenic identification The fungus was cultured on potato dextrose agar. Mycelia of the fungus grew slowly, and with gray colour initially and black-brown colour at later stage. The mycelia is submerged, forming a plectenchymatous crust. Stromata developed in the form of hard and black masses in 15 days with more than one conidial and spermatial cavities in the upper region. At the same time a oozing conidial slime could be seen. Pycnidia are globose and dark brown, at the size of 120μm～350μm×85～190μm, with ostioles at the size of 10～15μm. Conidia were ovoid, elliptical or short rod, single cell, and colorless. The size of oval spore is 8.4～11.7×5.4～6.8μm, while the size of short rod spore is 6.o～7.5×1.9～2.5μm. The genomic DNA was extracted, and the sequences of rDNA-ITS were obtained. Through Nucleotide BLAST in NCBI website, the homology was invested, and the result revealed that it shared a similarity rate of 99% with Phoma citricarpa McAlpine. According to the morphological characteristies and the identification in molecular biology, this pathogen was identified as Phoma citricarpa McAlpine.3. Biological characteristics of pathogen The result of biological characteristics showed that the range of temperature for mycelium growth was 5～35℃and 25℃was the optimal temperature for the mycelial growth. The favorable pH values for mycelia growth were pH 5～7, with pH 6 as the optimal. In different media, mycelial growth was fastest on PDA. On OA, The fungus produced amounts of yellow pigment, and the pigment diffuses into the medium around the colony.4. Toxicity test of fungicides to the fungus in laboratory The results showed that 50% Kresoxim-methyl had the best inhibitive effect on Phoma citricarpa McAlpine, with 0.0199μg/ml as EC50 value.40% Flusilazole had the better inhibitive effect, with 0.0209 μg/ml as EC50 value. The EC50 of 25% Prochloraz,10% Difenoconazole,25% Pyraclostrobin,70% Streptomycin,30% Aureonuclemycin,70% Thiophanate-methyl and 80%Mancozeb were 0.0323,0.139,0.2475,0.3171,0.5041,1.8723 and 2.0760μg/ml, respectively.50% Carbendazim had the worst inhibitive effect with 4.3306μg/ml as EC50 value.5. Disease cycle The pathogen survives in the winter mainly on the fallen leaf and in the soil in the form of pycnidia or hypha. Conidia spread among trees through wind and rain and mainly invade into plant from the natural orifice or wound at the stages of new leaves, flowering and young fruits. Lesions on fruit were usually seen in the last ten days of May. The peak of disease generally appears in mid-October and at the end of October, and the disease grade reduced gradually in November. The emergence and development of the disease is closely related to both the number of pathogen and the conditions of local climatic. To a great extent, the temperature, rainfall and relative humidty have determined the development degree of this disease.6. Disease control The results revealed that removing diseased branches, leaves and managing tree crown properly had the best inhibitive effect on Phoma cilricarpa McAlpine, whose control efficacies were 48.25%. Deeply plowing soil had control efficacies of 27.28%. However, fertilizer and water had almost no effect. Field trials of fungicides showed that the control efficacies of 50% Kresoxim-methyl at 1:4000 dilution had the best inhibitive effect on Phoma citricarpa McAlpine, whose control efficacies were 99.35%. The control efficacies of 10% difenoconazole at 1:1000,1:1 mixture of 70% thiophanate-methyl with 80% mancozeb at 1:500,80% mancozeb at 1:400,50% Kresoxim-methyl at 1:5000,40% Flusilazole at 1:7000,70% thiophanate-methyl at 1:400,50% Kresoxim-methyl at 1:6000, and 10% difenoconazole at 1:1500 dilution were all over 83.56%.50% Carbendazim had the worst control efficacy effect of 30.7%. |
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