| The rice sheath blight is a kind of rice disease caused by the fungus from Deuteromycotina. It occurrs on every main rice planting area and also the rice-growing areas of China every year. It is becoming an outstanding problems hindered the production development of rice. As the use of hybrid rice varieties, the changes of the agricultural system and field conditions, the rice sheath blight had become increasingly serious in recent years. The rice sheath blight always become to come on the base of rice leaf sheath, then mainly formed the clouded lesions and scaled up gradually. In case of high temperature and humidity weather, it could lead a rice yield loss rate to 20-50%, or total loss at all. Validamycin is commonly used to control the Rhizoctonia solani and prevent the damage caused by the disease. However, with the long-term and large-scale use of validamycin, most pathogenic fungus have had resistance to the chemical agent and the control effect may reduce. To overcome this drawback, search new botanical active materials to prevent and control of rice sheath blight has become a hotspot of research. The study determined the antifungal activities of active materials from usual perilla frutescens, medlar, celery, pepper, motherwort, angelica sinensis, lotus plumule and radix isatidis, selected the target active materials and measured the inhibition effect on Rhizoctonia solani. This paper also did preliminary study on the inhibition mechanism based on those with strong activities. The main studies are as follows:1) The antifungal activities of botanical active materials on Rhizoctonia solani were investigated by Haloes methods. The results showed that the active materials from roots, stems and leaves of radix isatidis, motherwort and angelica sinensis had little inhibition on Rhizoctonia solani. But the active materials from perilla frutescens, medlar, celery, pepper and lotus plumule had some inhibition effect on the fungus. The five plant with the greatest inhibition effect were, in order, celery leaf, medlar fruit, pepper fruit, perilla frutescens leaf, lotus plumule.2) The inhibition rate of the five plant on Rhizoctonia solani were determined with the method of growth rate. The inhibition rate of active material from celery leaf was 74.73%, 26.88 times of the lowest inhibition rate. The inhibition rate of perilla frutescens leaf was 25.95%, the medlar fruit and pepper fruit was between 2.78-16.55%. The inhibition rate of all the four botanical materials were lower than validamycin.3) The green house tests revealed the five botanical active materials were effected on controlling Rhizoctonia solani, but the pesticide effectiveness gradually declined as the time went on. At the 7th day after sprayed, active material from celery leaves crude extracts had a highest inhibitory rate of 61.01%, then was medlar fruit, perilla frutescens leaf was 38.88%, pepper fruit was 27.05%. At the 14th day, celery leaves crude extracts got the highest inhibitory rate of 61.53%, then was medlar fruit, perilla frutescens leaf was 43.21%, pepper fruit was 28.97%. At the 21th day, celery leaves crude extracts maintained the highest inhibitory rate of 62.75%, followed by perilla frutescens leaf, medlar fruit was 41.78%, and pepper fruit was 30.38%.4) Action mechanism① Antifungal ways. It was found that the active material from celery leaf cannot entirely inhibit the growth of Rhizoctonia solani when the concentration was 50 μ g/mL. This may demonstrat that the inhibitory mechanism of the active material from celery leaf against Rhizoctonia solani was mainly via the growth inhibition but not to kill the fungus directly.② Mycelial morphology. The mycelium of Rhizoctonia solani became thick, bending and darker, with uneven edge and slowly growth after cultured on the medium with active material from celery leaf. The mycelium growth rate gradually slow down but the color lighted up with the increased agent concentration. Controlled by the well growth and neatly edge of Rhizoctonia solani mycelium on the medium without any agents.③ The sclerotium. The different concentration of active material from celery leaf had different effect on the germination rate of Rhizoctonia solani sclerotium for 40.17-95.62%. The germination rate gradually reduced with the increase of agent concentration, which suggested the active material from celery leaf could not entirely inhibit the sclerotium germination but to delay it.④ Mycelial growth. The mycelial dry matter reduced with the increasing concentration of active material from celery leaf. The mycelial dry matter were among 5.31 and 5.35 mg when the concentration of active material ranged between 5 and 15 μ g/mL, but it had not significant difference. The mycelial dry matter were among 4.76 and 4.95 mg when the concentration of active material ranged between 30 and 60μg/mL, and had significant difference. When the concentration of active material reached 120μg/mL, the mycelial dry matter was 3.63 mg which was only 0.64 time the weight for the untreated. The study showed that active material from celery leaf could inhibit the growth of mycelial under liquid culture condition.⑤ Cell membrane permeability. After treated by the active material from celery leaf, the cell membrane conductivity of Rhizoctonia solani was higher than that of control though the difference was not significant. When the concentration ranged from 5 to 15 μg/mL, there existed no significant difference on variation amplitude of electrolytic leakage between the active material and the control. When the concentration was 30μg/mL and the treating time was 60 min, the variation amplitude of electrolytic leakage of the active material was obviously higher than that of control, though the differences were also not significantly. These results suggested that if the concentration of active material was less than 30μg/mL, the cell membrane penetrability of Rhizoctonia solani could not be damaged and the cytoplasm could not be leaked. |
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