The Study Of Platycodon Anthracnose Caused By Colletotrichum Gloeosporioides

Platycodon grandiflouorum, a kind of Perennial herbs that is used for treatment, eating and enjoyment belong to Aster ales, Campanulaceae, Platycodon grandiflorus. It has a long cultivation history in China. The dry roots of platycodon is used as a medicine, It tastes bitter and spicy, and it is mild. It makes expectoration easy, relieves a cough, releases lungs, discharges pus. The tender stem and root are available for foods. Platycodon anthracnose occurs frequently in planting area in recent years, It has a certain impact on the yield and quality of platycodon, and caused serious economic losses to the local farmers. However, the report about anthracnose of platycodon has not be seen, what’s more, no one has done a systematic study. The disease symptom and pathogen identification, biological characteristics of the pathogen, invasive approach and condition, screening of fungicides of platycodon anthracnose were studied preliminarilly in this paper. It provides a certain theory basis for the diagnosis of the disease to understand the disease occurrence regularity, and application in agricultural production on the prevention and control.1.Disease symptomPlatycodon anthracnose mainly appears on the base of stalks and leaves. The disease spot on leaves is oval or irregular shape, the disease spot on stalks is elongated and slightly concave. In early time, the color of disease spot on leaves is yellow-brown on the front, light brown on the back. Then the disease spot expends gradually, the central color of the disease spot is reddish brown, and the color outward deepen as dark brown. It has apparent ring of dark brown halo between them. At last, disease department generates a lot of small black spots, and more on the back which is conidia disc. After the occurrence of the disease, the disease spreads quickly and gradually when the temperature is appropriate with much rain, leading platycodon to piece of death.2.Pathogen identification Separation and tie-back:Pathogen was isolated from the diseased plant leaves, inoculate the health plant and reseparation. With the onset of natural field show similar symptoms, the typical symptoms of leaf tissue separation again. It has been proved that the isolate was the pathogen of anthracnose by the method of Koch’s Rule. Based on morphological classification and with molecular biology identification.Morphological identification:The colony is white, fuzzy, dry, round or nearly round in the early, and then the colony gradually becomes light brown. Acervulus and conidium appear after4days past, the wet orange sticky spore ball appears after about7days. Acervulus is round or nearly round, a beige dot in the early, and then a black dot, with no bristles; Conidium is cylindrical, single spore, with1oil ball, both ends obtuse, The spore is about12.7μm～23.6μm×3.1μm～4.2μm, the color of attached spore is shallow brown, suborbicular or irregular shape, its edge is smooth, size is6.3μm～7.8μm×4.6μm～6.1μm, conidium generated in the conidium pan. Under the culture condition, the generation of appressorium after conidium germination is a identification feature of colletotrichum. On the basis of traditional morphological classification and morphological identification, the pathogen of platycodon anthracnose isolated from the nearly circular lesion was identified as Colletotrichum gloeosporioides.Molecular biology identification:we get the the same result by using molecular biology techniques and rDNA-ITS sequence analysis on platycodon anthracnose pathogens.According to the result of morphological identification and molecular biology techniques, we know that:platycodon anthracnose is caused by Colletotrichum gloeosporio ides.3.Biological characteristicsThe range of temperature which supports mycelial growth is5℃to35℃, and the optimum temperature is25℃for growing. Conidial produced at15℃to35℃and germinates at20℃to30℃, while optimum temperature is30℃to produce conidial and germinate. The pathogen grows best in a neutral environment, the influence of light condition on mycelial growth rate and the spore germination is not obviously. Sporulation quantity is different under different light conditions. Dark condition is more conducive to produce conidial and spore germination. Light condition has a little effect on spore germination, while dark condition is good for producing conidial. Lethal temperature of conidium is53℃. And conidium survives on the sick body of plants to go over the whole winter.4.1nfection process of the pathogen and infection conditionsUnder the optical microscopic observation, conidia of the pathogen has begun to germination after inoculation at different time. After inoculation of pathogenic bacteria to the platycodon for8hours, conidium begins to germinate on the surface of leaves of the platycodon, blade at one end or both ends of germ tube. After some conidium germ tube’s elongation, hyphae is formed, and infect the host from epidermis, intercellular spaces and pores. Some conidium produce appressorium on germ tube, invasion of silk grow on the germ tube, directly through the skin into cells.Moisturizing treatment or not and the length of different time after inoculation have obvious influences on the development of the disease and infection of pathogenic bacteria. After inoculation of pathogen of platycodon grandiflorum about48hours, the plant will be infected, at the time about96hours is most conducive to pathogen invasion. It will be infected during20℃～30℃. The best temperature to infect is28℃, and20℃is harder. Long hairy wet processing and high temperature conditions are conducive to invasion of platycodon grandiflorum anthracnose pathogen.5.The fungicides screening in laboratoryThe five fungicides (carbendazim, mildothane, mancozeb, chlorothalonil and imazalil) were tested in laboratory to determine the effect on the germination of conidia and hypha growth. The result shows that, five kinds of fungicides have different degree of inhibition for pathogen hyphal growth and spore germination. Carbendazim has the best effect of inhibition on pathogen in the hypha growth and the germination of conidia, the inhibition effect of chlorothalonil is second only to carbendazim for pathogen. But imazalil has the worst inhibition effect on pathogen.