| Phytophthora infestans?Mont.?de Bary,which causes potato late blight,belongs to the Algae,Oomycota,Oomycetes,Peronosporales,Pythiaceae,and Phytophthora.The A1 mating type of the pathogen was only observed in origianl and many other potato production areas in early stage.But due to the allocation and transportation of potato varieties and germplasm resource as well as with differentiation and variation or other reasons,A2 mating type,self-breeding type and A1A2 type appeared and their incidences were increased year by year.This situation result in greatly enhancement of producing-oospores potential of P.infestand.It leads P.infestans more adaptable to the external environment,and makes late blight more difficult to prevent.At present,the prevention methods for late blight mainly rely on chemical pesticides,and among many chemical pesticides,metalaxyl is the most widely used and the effect is the most ideal.However,due to the abuse of pesticides in recent years,especially excessive application of metalaxyl,the resistance of P.infestans to metalaxyl is becoming stronger and stronger.The search for alternatives to metalaxyl has become a necessary condition for ensuring agricultural production.Although chemical pesticides can effectively and quickly control potato late blight,which brings huge benefits to farmers,the pollution it brings to the environment should not be underestimated.Therefore,utilizing the beneficial microorganisms and developing biological agents instead of chemical pesticides has long been an important direction of biological research.The SR13-2 strain studied in this experiment was isolated by our lab from vegetable soil of Shangrao district,Jiangxi province in China,belongs to Enterobacteriaceae,Serratia,and Serratia rubidaea.The strain had very strong inhibitory effect on mycelial growth of P.infestans and the characteristics of high stability and stress resistance.In addition,it could effectively protect potato in vitro tissues from infection of P.infestans.However,subsequent studies on this strain are still not known,such as the disease prevention effects on seed potatoes and living plants.Besides,it is also not known whether SR13-2 strain could promote growth of potato,since the mixed bacterial solution of several antagonistic bacteria containing SR13-2 displayed significant growth promotion in seed tuber germination and seedlings.The main research content of this experiment are as follows:?1?To monitor the frequency and distribution of metalaxyl resistant strains of P.infestans in five northern provinces in China in 2017.?2?To investigate whether P.infestans has the cross-resistance to metalaxyl and azoxystrobin,and whether azoxystrobin can replace metalaxyl to prevent late blight.?3?To further clarify the effect of promoting growth for potato and disease prevention of SR13-2 during potato seed tuber germination stage,and to find the optimum concentration of HT-6 bacterial liquid for soaking seed tubers.?4?To evaluate the effect of SR13-2 bacterial solution on potato growth and disease prevention of potted plants,and to test the best way to treat potato potted plants.?5?To initially explore the some mechanisms of HT-6 bacterial liquid in promoting potato growth and disease prevention.The research results obtained in this experiment are as follows:1.In 2017,137 isolates of P.infestans were collected and isolated by our lab from 5northern provinces in China,and 82 strains were purified and tested for their metalaxyl resistance.Among them,sensitive,medium and high resistant strains were 7,59 and 16,respectively,and they were 8.5%,72.0%and 19.5%,respectively.The high-resistance strains were distributed in all 5 provinces.Of which,the highest frequency was found in Liaoning Province?28.6%?and the lowest in Hebei Province?16.7%?.2.The toxicity of some high-metalaxyl strains was detected by azoxystrobin.The results showed that the EC50 value was between 0.0322-0.1122?g/mL,and the average value was0.0841?g/mL,which was much lower than that of the sensitivity evaluation baseline of1.0?g/mL.This indicates that there was no cross-resistance in P.infestans strains for metalaxyl and azoxystrobin.In other words,Azoxystrobin could be used instead of metalaxyl to control potato late blight.3.After soaking potato seed tubers with different concentrations of SR13-2 bacterial liquids,it was found that the excessive concentrations of SR13-2 liquid,such original liquid and diluted to 10-1 concentrations,had evident corrosion for seed tuber,while that of relative low concentrations had very strong growth promoting effect on seed tubers germination process.Among them,that diluted to 5×10-1 concentrations had the best promotion effect after 9th day for soaking treatment potato seed tubers,the bud length was 1.8 cm,the bud thickness was 1.1 cm,and the fresh weight was 0.31 g,which was the best in all treatments.4.The seed potato block soaked by SR13-2 liquids showed strong inhibition for mycelial growth of P.infestans on the blocks and infection to block in comparison with control.Among them,the effects treated with the 5×10-1,10-2 and 2×10-2 different concentrations were better than that of original liquid and 10-11 concentration.Under these three concentrations,P.infestans could not grow on the seed potato blocks,in which the effect treated with 5×10-1concentration was the best.5.After soaking potato seed tubers with different concentrations of SR13-2 bacterial liquids,the content of both soluble sugar and protein reached the highest level on the 9th day.The highest content of soluble sugar treated with 5×10-11 concentration reached 106.37mg/g,which was 27.9%higher than that of control,and the highest content of soluble protein was 14.29 mg/g,which was 98.7%higher than that of control.The activity of the PPO,respiration related enzyme,reached the strongest on the 6th days,with the treatment of 5×10-1concentration.And its activity was 69.61%higher than that of control.The strongest activity of AAO,another respiration-related enzyme,was 93.76%higher than that of control after treated with 5×10-1 concentration on the 9th day.6.The comparison of resistance-related enzymes revealed that all of the activities of POD,SOD,and PAL reached their peaks on the 4th day,but there was no significant difference in the activity of SOD between the experimental group and the control group.Both POD and PAL showed their strongest enzyme activities in the seed potato after treated with5×10-1 concentration.Compared with the control group,the enzyme activity of POD was124.02%higher,while the enzyme activity of PAL was 65.19%higher than that of control.7.After soaking potato seed tubers with 5×10-1 concentrations of SR13-2 bacterial liquids,the emergence time was 6 days earlier than that of control,and the young leaves growing showed a healthier dark green color,and the transpiration rate was doubled compared with the control.And in vitro leaves also showed resistance to late blight.8.The growth promotion effects of different treatment ways for potato were compared in this experiment.It was found that the promotion effect of whole treatment including soaking seed tuber blocks,root irrigation and spray leaves was greatly better than that of any one of the single treatment.At the 60th day after whole treatment,the plant height was 42.7 cm,the stem diameter was 1.9 cm,and the number of leaves was 54 and the maximum leaf area was44.58 cm2.In addition,The disease prevention effects of different treatment ways for potato were evaluated in this experiment.It was found that whole treatment had the best effect compared with other single treatment while spray leaf treatment had little effect.9.After whole treatment,the contents of soluble sugar,protein,total chlorophyll and the activities of PPO,AAO,POD and PAL in potato leaves all reached the highest levels.The content of soluble sugar was 60.32 mg/g,41.80%higher than that of control;and the content of soluble protein was 32.54 mg/g,86.05%higher than that of control;and the total chlorophyll content was 3.97 g/mg,41.28%higher than that of control.The activities of PPO,POD and PAL were 105.54%,303.75%and 280.11%higher than those of the control,respectively.But the activity of AAO was not much different from the control.In summary,high resistant strains of P.infestans were distributed in all 5 provinces in2017,and their incidences were relative high.Aoxystrobin could replace metalaxyl to prevent late blight.The content of soluble sugar and protein,respiration-related enzymes PPO and AAO activities,as well as resistant-related enzymes POD and PAL activities were increased greatly after SR13-2 liquid treatment,it might promote the seed tuber germination and seedlings growth along with disease control.The treatment including soaking seed tuber blocks,root irrigation and spray leaves could significantly increase the content of soluble sugar,protein,and chlorophyll,as well as respiration-related enzymes POD,and PAL activities,and promote seedlings enmergence,seddlings growth,and prevention of late blight. |
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