| Clubroot is caused by Plasmodiophora brassicae Woronin(Plasmodiophora brassicae)infesting the roots of cruciferous plants,which seriously endangers the safe production of cruciferous plants such as rapeseed.The Plasmodiophora brassicae is extremely resistant and infectious,making it more difficult to prevent and control.Chemical control is one of the main ways of clubroot control,and it is a challenge for current researchers to select and find the efficient,economical and safe pesticides to prevent and control the clubroot.In this study,several pesticides were selected for field trials.The effective pesticides were used for indoor trials so as to investigate their effects on the growth of rapeseed and the inhibition of the biological activity of Plasmodiophora brassicae.The microstructure of the Plasmodiophora brassicae before and after pesticide treatment was observed to investigate the characteristics of the changes in each stage of its life cycle,and to reveal the mechanism of action of the pesticides.According to the results,the proper pesticides and their rational concentrations were obtained to conduct pot trials after compounding with fulvic acid,so as to investigate the inhibitory effect of pesticides on Plasmodiophora brassicae infestation and the effect on resistance of rapeseed,and to improve chemical control effect on the clubroot.The research results are as follows:1.Screening of control pesticides for rapeseed clubroot:In this paper,several pesticides were selected to prepare“seed mates”for filed trails.The trial results indicated that cyazofamid and fluazinam had a good effect on clubroot control.The combinations of carbendazim and other pesticides were 100%effective in the control of Huayouza 62 in the field.Based on this,indoor trials were conducted to select the species and concentrations of pesticides.The five single pesticides,namely cyazofamid,fluazinam,carbendazim,uniconazole,thiophanate-methyl with different concentrations,and the combinations of cyazofamid/fluazinam,cyazofamid/carbendazim,fluazinam/carbendazim,and uniconazole/thiophanate-methyl,were considered to be effective for the control of rapeseed clubroot.The mentioned pesticides were not toxic to the indoor rapeseed cultivation and could be used for subsequent experimental investigations.2.Toxicity of pesticides to Plasmodiophora brassicae:In this paper,spore suspensions at a concentration of about 2×10~9 CFU/m L were obtained.The spore suspensions were incubated in vitro in pesticides and spore germination rates were calculated by spore staining tests.The results of the experiment were as follows:the inhibition rates of the spore germination of Plasmodiophora brassicae were 22.73%,13.99%,15.46%,6.37%and15.47%for cyazofamid,fluazinam,carbendazim,uniconazole and thiophanate-methyl respectively.While the inhibition rates of spore germination of the four combination agents,cyazofamid/fluazinam,cyazofamid/carbendazim,fluazinam/carbendazim and uniconazole/thiophanate-methyl,were 24.17%,32.97%,38.53%and 17.73%.It is important to note that the fluazinam/carbendazim had the highest inhibition rate on spore germination of Plasmodiophora brassicae,being 1.7 times more effective than that of cyazofamid alone.Histological observations of the infestation process of Plasmodiophora brassicae before and after pesticide treatments were carried out by observations of root hair infestation and cortical infestation.It was found that treatments with cyazofamid and fluazinam/carbendazim could reduce the rate of root hair infestation by more than 50%.Cortical infestation was observed in healthy rapeseed roots with neat and orderly cell arrangement,while the cells of diseased rapeseed roots were disordered,with abnormal cell swelling and serious deformation.Pesticide treatments significantly reduced the development and severity of the cortical infestation of Plasmodiophora brassicae.3.Changes in physiological indicators of rapeseed before and after inoculation and pesticide treatment:The spore suspension of Plasmodiophora brassicae had a significant inhibitory effect on the root length of rapeseed,and the higher the concentration of spore suspension,the shorter the main root length of rapeseed.When the concentration of spore suspension was 5×10~7 CFU/m L,the main root length of rapeseed was 5.63 cm,which was24.63%shorter than that of the water control group.Pesticide treatment on non-inoculated rapeseed showed both a promotion of rape root elongation and an inhibition of rape root growth.Pesticide treatment inhibited the activity of Plasmodiophora brassicae to a certain extent.After pesticide treatment,the root length of rapeseed was longer in the inoculated group than in the non-inoculated group,probably because the pesticides inhibited the activity of Plasmodiophora brassicae and induced a stress response in rapeseed under the combined effect of both Plasmodiophora brassicae and the pesticides,resulting in rapid root growth.The chlorophyll content,MDA content and SOD activity of rapeseed were measured before and after inoculation and pesticide treatment.The inoculation treatment was detrimental to the growth of rapeseed for inoculation,resulting in a 13.65%decrease in chlorophyll content and an increase in MDA content and SOD activity.The treatments with cyazofamid-fulvic acid and fluazinam/carbendazim-fulvic acid could improve the health condition and resistance of rapeseed.Under inoculated conditions,the two chemical agent treatments increased chlorophyll content by 43.54%and 51.20%,decreased MDA content by 18.85%and 67.43%,and increased SOD activity by 44.98%and 98.86%,respectively,compared to the inoculated group only.Chemical agent treatments reduced the pathogenic effect of Plasmodiophora brassicae on rapeseed and improved the resistance of rapeseed. |
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