| Clubroot disease, caused by obligate biotrophic parasite Plasmodiophora brassicae Woronin, is one of the most serious soil-borne recurrent diseases in cruciferous crops worldwide. First discovered in the western Mediterranean coast and southern Europe, now it is widely distributed in Europe, Asia and North America. Especially in recent years, clubroot is spreading rapidly in China. Under favorable environmental conditions, germination of a large number of P. brassicae resting spores results in the formation of tumors on roots which affect the transfusion of moisture content and nutrient. Thus such disease leads to a significant drop in yield and quality losses of cruciferous crops, leading even to total crop failure. In addition, if the soil is infested by P. brassicae, it will carry resting spores for a long time. Resting spores of P. brassicae can survive in soil for many years and soil inoculum loads decrease very slowly. This disease seriously restricts the production of cruciferous plants and oilseed rape.Currently, although resistant varieties are the most safe and effective methods to control clubroot, the pathogenicity of P. brassicae is easy to change so that resistant varieties soon lose the ability of disease resistance. Traditionally, the use of liming to improve the soil pH is somewhat effective; however, long-term usage would have a detrimental impact on the soil structure. Chemical prevention is still one of the important measures used to control clubroot in cruciferous plants. Therefore, it is necessary to study on the methods of clubroot management. Select for appropriate chemicals to curb the disease development in order to reduce the economic losses of incidence area and improve productivity of cruciferous crops.In this experiment, the susceptible host Chinese cabbage (Brassica rapa) cultivar ’Zaoshu-5’was tested. Four kinds of fungicides which have the ability of controlling clubroot were screened. Meanwhile, we have determined the duration of plant defense inducers methiadinil. A pot experiment was conducted to test the effect of the combination of methiadinil and cyazofamid, and then in Dayi County, Sichuan Province and Jiulong County, Sichuan Province field trials, we evaluated their chemical control of clubroot disease including incidence, severity index, control efficacy and the physical growth parameters. Moreover the physiological and biochemical indexes of treated cabbage were measured in order to further explore the effect of compound agent. We hope that our research can provide a reference for clubroot management with ensuring control efficiency and lowering costs, improving the economic benefits of cruciferous crops.The main results were as follow:1. Indoor pot experiment of fungicide and compound agentThrough pot experiment, four kinds of fungicides that have been commonly used in the management of clubroot were tested by the method of artificial inoculation. The results indicated that 10% cyazofamid SC got the best effect, up to 90.85% and 50% carbendazim was the worst, only 22.01%. Thus, we select cyazofamid as one of the compound agent. The duration of six kinds of plant defense inducers for inducing resistance were measured. 25% methiadinil SC performed steadily, while its control efficacy was lower than methiadinil SZG. Through the test results, we decided tentatively the spraying interval for 7d. In pot experiment of compound agent, the compound containing an effective ingredient dosage of 30 mg/L cyazofamid+150 mg/L methiadinil was found to have the greatest effect,88.15%.2. Field efficacy trials of the compound agent of methiadinil and cyazofamidThe difference in control efficiency was studied for different compounds in field trails. Only the effect of the combination of methiadinil SZG1# or methiadinil SZG4# and cyazofamid were higher than methiadinil SC compound. Considering reagent costs and practical production conditions, we recommend the compound of methiadinil SC and cyazofamid SC for further proportion optimization.The proportion optimization results indicated that all fungicides which mixed the protective fungicide, cyazofamid with the plant-induced resistance agent, methiadinil tested in field trails significantly decreased clubroot disease severity relative to the untreated control. Moreover, the combination performed better than a single dosage of 10% cyazofamid SC. The compound containing an effective ingredient dosage of 30 mg/L cyazofamid+150 mg/L methiadinil had the greatest effect,83.16% in Dayi and 99.19% in Jiulong, which were better than the standard agent, cyazofamid. Such growth indexes as root length, plant height, and fresh weight performed well in plants treated with the compound agents, with the highest rate of growth at 56.7%, which was superior to that of plants treated with the single agent cyazofamid. Because of environmental and other impacts factors in field trails, more researches on proportion optimization and mechanism are needed to deeply conduct. This trial is merely a preliminary investigation of the effects of compound agents for field evaluation.3. Physiological and biochemical indexes analysis of field treated Chinese cabbageThe treated cabbage samples were determined, including POD, SOD, soluble sugar content, chlorophyll content. The results suggest that SOD, soluble sugar content and chlorophyll content of treated cabbage had increased tendency to some extent compared to the control group. And only POD of treated cabbage with 10% cyazofamid SC was slightly higher than control agent, no significant difference. The others treatments were lower than CK. There was no significant relationship between the POD activity and the effective concentration of the compounds. There is a certain correlation between changes of physiological and biochemical indexes of different treatments and their control efficiency on cabbage clubroot. The better the control effect is, the higher the physiological and biochemical indexes are. However, there are many external factors influencing enzyme activity assay. Whether the physiological and biochemical indexes are related with the mechanism of compounds and the specific mechanism needs to be studied. |
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