Study On Identification,Biological Characters And Infection Of Ganhuangcao Brown Leaf Spot Pathogen

Penthorum chinense Pursh is a kind of medicinal plant which is the material of Gansuning pharmacon. Penthorum chinense Pursh brown leaf spot is a new and destructive disease of Penthorum chinense Pursh in Sichuan in recent years. The dissertation reported on the symptoms of the disease, the isolation, identification of the pathogen, biological characteristics, infection and indoor fungicides screening. It was studied systematically first. The main results are as follows.1. Symptoms of Penthorum chinense Pursh brown leaf spot and the pathogen identificationThis is a new kind of disease on Penthorum chinense Pursh. It damaged the leaves. At first, the spots occurred on the margin or blade of leaf, which were approximate rounds or half round taupe spots. The colour of spots on the upper epidermis was brown and light brown on the lower epidermis. As the development of the diseases, the lesion gradually expanded in size of 3～10 mm. In the center of the spots, colour was light brown, and the outer colour of spots was darker. At the late development period of the disease, the lesions were joined and numerous small black dot (stroma) scattered on the surface of lesions were observed especially in the rainy day. The disease led to leaf wilting or leaf- cast.Based on the morphological classification and molecular biology technology, the pathogen belongs to Cercospora genus. Penthorum chinense Pursh brown leaf spot pathogen was a new record of Cercospora genus in China. It was named Cercospora sedoides. The pathogen's stroma in diameter of 7.5～45μm with brown cells scattered or gathered under the host's epidermis. Conidiophore in length of 47.5～150μm with 1-4 diaphragm and 0-3 geniculation scattered or gathered. The conidia were colourless, aciculiform in size of 12.5～452.5μm×1.25～3.75μm.2. The biological characteristics of Cercospora SedoidesThe influence of temperature, light, carbon source, nitrogen source, culture media and techniques on radial growth of mycelium and sporulation was tested. Mycelial growth of Cercospora Sedoides occurred on all media and with all inoculation procedures. In the utilization of carbon source, it was better with glucose and disaccharides than polysaccharide. And the most suitable nitrogen sources were the nitrate nitrogen. Abundant sporulation occurred on V8 media with both the flooding technique and with the transfer of mycelial fragments. But mycelial disk transfer failed to produce sporulation on any of the media tested. The temperature range for mycelium growth was 10～30℃and for sporulation 10～30℃. The optimum temperature for mycelium growth was 25℃and for sporulation 25℃. And the most sporulation was at 4 days after inoculation. Sporulation was remarkably more in the continuous dark treatment than in the alternation light/dark treatment.Conidia germinated at 10 to 35℃.The optimal temperature range for spore germination was between 20 and 25℃. After 2h of incubation, the spores started to germinate. And the ways of germination were various. Glucose stimulated conidial germination of the pathogen. The germination ratio increased with the raising RH and the maximum rate of conidia germination was in water. Light was neither inhibitory nor stimulatory to germination. The lethal temperature of conidia was 45℃in 10min.3. The infection conditions of Cercospora Sedoides and the influence of environmental factors on the infectionSpores germinated after 3hr at room temperature. Germ tubes may arise from any of the conidial cells, but the apical cell usually germinated first, followed next by the basal cell, and then intercalary cells. The appressorium was formed before the penetration of the pathogen. Then the appressorium came into mycelium to penetrate through stomata or epidermis. Some conidia germinated and formed mycelium that penetrated from the interval of the cells. The symptom of chlorotic dots occurred at 7 days after inoculation, at room temperature and keeping 72-hr moistureLeaf spots developed over a range of temperatures, with the most rapid development and most severe symptoms at 25℃. No symptoms developed on plants receiving Ohr of high humidity. Symptoms could develop when plants were exposed to 48hr and more time. The disease severity increased with length of keeping moisture time increasing after inoculation.4. Virulence determination of fungicides to the pathogenThe four fungicides were tested in laboratory to determine the effect on the hypha growth and germination of conidia. The results showed that carbendazim was the most effective one. Imazalil and diniconazole were inferior to carbendazim. Then triazolone had little effect.