| Rice sheath blight caused by Rhizoctonia solani is an important disease on rice.Following the popularization of high-yielding and dense cultivation techniques and the increase of nitrogen fertilizer application,rice sheath blight is becoming more and more serious.At present,the control of the disease in field mainly relies on fungicides including azoxystrobin,thifluzamide and some triazole fungicides.However,long-term and excessive use of single fungicide has caused strong resistance of the fungus to some fungicides.In order to reduce the resistance risk of R.solani against the fungicides and improve their efficacy on rice sheath blight,toxicity and the action mechanism of the fungicides were determined.In this paper,we had clarified the effect of bixafen on R.solani,including the virulence,fresh and dry weight,DNA and soluble proteins contents,sclertium formation and germination,etc,and the resistance induction effect of it on rice.In addition,the combined virulence of bixafen compounded with some commonly used fungicides against R.solani were definited and the optimum formula of 20%bixafenpyraclostrobin suspension concentrate was developed.The main results are as follows:1.In order to clarify the effect of bixafen on the growth and development of R.solani and the induction of resistance to rice,the fungicide was used to treat the pathogen and rice plants.Results showed that bixafen had strong inhibitory activity against R.solani.After being treated with different concentrations of bixafen,the fresh and dry weight of the pathogen decreased with the increase of the fungicide concentration.When the concentration of bixafen was 10 μg/mL,the fresh and dry weight of the mycelium were only 2.54%and 1.79%of the control,Bixafen could inhibit the formation of the sclertium of R.solani.When the concentration of the fungicide was 1.0 μg/mL,no sclertium produced.However,there was no effect on the germination of sclerotium of R.solani within the concentration range of the fungicide tested.After bixafen treatment,the DNA content from the equal amount of mycelium decreased with the increase of the concentration of the fungicide,while the soluble protein content increased and malondialdehyde content decreased first and then increased.When the concentration of the fungicide reached 10μg/mL,the malondialdehyde contents between the treatments and control had no difference.The enzyme activities of CAT,POD and PPO in rice plants significantly increased after fungicide treatment,and reached the highest values on the 1st,3rd and 3rd day,respectively.Compared with control,the contents of malondialdehyde in rice plants increased first and then decreased during the treatment period,and increased with the increase of the concentration of bixafen.The β-1,3 glucanase activity in rice plants was not affected by bixafen.2.The mycelium growth rate method was used to determine the combined virulence of bixafen combined with different fungicides.Results showed that when bixafen combined with epoxiconazole at 3:1,1:1 and 1:3,the synergistic ratio(SR)values were 1.33,0.81 and 0.84,ranging from 0.5 to 1.5,which meant the additive effect.In the combination of 2:1 and 1:2,SR values were both greater than 1.5,which meant the synergistic effect.The SR values of bixafen and tebuconazole compounds were all between 0.5 and 1.5 when they were combined with the above mass ratio.When bixafen and prochloraz were combined according to the mass ratio above,the SR values of other mass ratios were all greater than 1.5,2.58,5.32,1.93 and 2.08,respectively,except for the SR value of 1:2 was between 0.5 and 1.5.When bixafen was combined with difenoconazole,the SR values of 4:1 and 1:4 mass ratio were 1.77 and 2.04,and the SR values of 2:1 and 1:1 mass ratio were 0.91 and 0.56,and 1:2 mass ratio was 0.35,which showed synergistic,additive and antagonistic effect,respectively.The SR values of all the mass ratios were greater than 1.5,showing synergistic effect when bixafen and pyraclostrobin were combined according to 3:1,2:1,1:1,1:2 and 1:3,respectively.Therefore,bixafen and pyraclostrobin were selected to furtherly research and develop into preparation.3.Through the screening of wetting dispersant,antifreeze,thickening agent and defoamer,the best formulation combination was obtained as follows:10%bixafen,10%pyraclostrobin,5%A755,3%Morwet EFW,0.2%xanthan gum,1%magnesium aluminium silicate,4%ethylene glycol,0.1%organosilicon defoamer,and replenished with water to 100%(mass ratio).After being stored at 54±2℃ for 14 d,the suspension rate was 96.7%;the pH value was 6.5;the average particle size of D50 wall.21 μm;there was no water separating out,and the lasting foaming ability was less than 25 mL.After being stored at 0±2℃ for 7 d,the suspension rate,pH value,average particle size D50,lasting foaming ability and viscosity were 97.3%,6.5,0.97 μm,less than 25 mL and in the range of 200～600 mPaS,respectively.4.60 gai/ha,90 gai/ha and 120 gai/ha 20%bixafen+pyraclostrobin SC had good control effects on rice sheath blight,and the control effects could reach 69.23%,76.59%and 87.23%,respectively.The control effects on rice sheath blight of 90 gai/ha and 120 gai/ha 20%bixafen+pyraclostrobin SC were higher than that of 150 gai/ha pyraclostrobin,and the control effect of 120 gai/ha 20%bixafen+pyraclostrobin SC was significantly higher than that of 90 gai/ha thifluzamide.All above results showed that bixafen has strong toxicity to R.solani.The compounded of bixafen with commonly used pesticides in production could effectively enhance the toxicity of the fungicides,and the compound has better control effect on rice sheath blight in the field. |
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