The Study On Anti-Saprolegnia Drug

With the continuous development of aquaculture industry,the deteriorating aquaculture environments resulted in fish disease more and more frequently.Saprolegniasis is one of main fungal disease in aquaculture,spreading worldwidely.To date,no wholly effective alternative treatment to malachite green was identified,since the use of malachite green has been prohibited.Therefore,the exploitation of the anti-saprolegniasis drug is very important in raising the eggs hatching rate,treating saprolegniasis and decreasing economic loss.Based on this,using Saprolegnia parasitica as the test strain,this thesis is screening on 56 kinds of chemical drugs and 142 kinds of Chinese herbal medicine,according to rapeseed liquid-media method and solid-media method.The next the combined effect assay and safety assessment assay were tested,obtaining the following results:1.In order to find effective drugs to control saprolegniasis,56 fungicidal chemicals used in agriculture were screened,in which kresoxim-methyl and azoxystrobin,with minimum inhibitory concentration(MIC)values of 1.0 mg/L and 0.5 mg/L respectively,showed good in vitro antifungal activities against S.parasitica.Through the acute toxicity assay using goldfish(Carassius auratus),azoxystrobin exhibited wider margin of safety with a safe concentration(SC)value of 0.553 mg/L than kresoxim-methyl with a SC value of 0.131 mg/L.Based on the acute toxicity,azoxystrobin has the potential for controlling saprolegniasis.Kresoxim-methyl may need chemical modifications to enhance its inhibitive effect or could be used in combination with other drugs.2.In order to find natural agents to control and treat saprolegniasis,methanol extracts of 142 traditional Chinese medicinal plants were investigated.Cnidium monnieri,Magnolia officinalis and Aucklandia lappa exhibited antifungal activity on S.parasitica mycelium at concentrations=62.5 mg/mL and were selected for further evaluation.The plants that were active were extracted with four solvents i.e.petroleum ether(PE),ethyl acetate(EA),methanol(ME),water(WA),and the extracts were evaluated with an in vitro bioassay using a Rapeseed MicroPlate Method.Among the extracts tested,all PE extracts of three plants exhibited the highest efficacy.These results could provide reliable information for the following test for isolating active substances.3.According to the results of the combined inhibitory effects on kresoxim-methyl and azoxystrobin with C.monnieri,M.officinalis and A.lappa respectively,the combined effect of azoxystrobin and M.officinalis was found that is a synergistic inhibitory effect on S.parasitica,containing 0.25 mg/L azoxystrobin and 15.6 mg/L M.officinalis.4.After studying Praeparatum H for the application effect and the toxic effects on aquatic beneficial organisms,the results were obtained:Praeparatum H exhibited antibacterial activity with MIC value of 6.25 mg/L against Bacillus subtilis,and with MBC value is higher than the maximum test concentration 50 mg/L;Praeparatum H could significantly inhibite the growth of Chlorella vulgaris when the concentration is 1.0 mg/L;Praeparatum H was found to be having toxic effects on Artemia salina for a long time,but there were no significantly difference between 0.250 mg/L test group with the control group within 48 h;Praeparatum H exhibited safe concentration of 0.543 mg/L against Carassius auratus;Praeparatum H is not toxicity to Gobiocypris rarus eggs while eggs mortality was 0 at the concentration of 1.0 mg/L.Based on the above results,Praeparatum H was found that can effectively inhibit S.parasitica mycelium and spores.According to the results of the safety evaluation tests,Praeparatum H was considered to be a safety drug,and could be used to prevent and treat saprolegniasis caused by S.parasitica,while providing a viable way for the prevention and control work of saprolegniasis.