| Soft rot is a bacterial disease that proning to occur on onions,and the main pathogen is Pectobacterium carotovorum subsp.carotovorum（P.carotovorum）.P.carotovorum can infect onion tissue owing to its strong pathogenicity,leading to softening and lodging of leaf sheaths and bulb decay.According to statistics,the field incidence of onion soft rot caused by P.carotovorum reach 40～50%in Gansu Province,and the production of onion is reducing by more than 30%.As the common plant essential oils,oregano and clove oil have strong antibacterial activity,but there are few reports on the control of onion soft rot caused by P.carotovorum.In this study,P.carotovorum was used as the test strain.By observing the changes of cell morphology before and after treatment,the effects of treatment on cell membrane,conductivity and cell membrane potential were analyzed.The changes of pathogenic factors treated with essential oils were detected,and the action mechanism of pathogenic factors was analyzed based on the molecular regulation mechanism.Finally,onion seedling experiment was used to evaluate the control effect of essential oil on onion soft rot.The main contents are as follows:（1）The results of GC/MS showed that the main component of oregano oil was carvacrol(C10H14),accounting for 77.65%.Eugenol(C10H12O2)was the main component of clove oil,accounting for 65.66%.The minimum inhibitory concentration（MIC）and minimum bactericidal concentration（MBC）of oregano oil against P.carotovorum were 200μL/L,respectively,the MIC and MBC of clove oil were 400μL/L and 500μL/L,respectively.Both essential oils showed an excellent antibacterial effect on P.carotovorum.Scanning electron microscope（SEM）observation showed that bacteria treated with essential oils exhibited shrinkage,deformation and fracture.At the same time,the integrity of cell membrane was destroyed by propidium iodide（PI）staining.Then the results from the relative conductivity and cell membrane potential showed that oregano and clove oil destroyed the integrity of cell membrane of P.carotovorum,changed the permeability of cell membrane and decreased the cell membrane potential,and led to the inhibition or death of cell growth.（2）Oregano and clove oil could inhibit the movement ability of P.carotovorum.In the swimming test,1×MIC essential oil treatment could completely inhibit the movement of flagella,while 1/2×MIC oregano oil and 1×MIC clove oil 100%inhibited colony diffusion in swarming treatment.The initial adhesion inhibition test of biofilm showed that oregano oil and clove oil were able to inhibit the formation of biofilm.In 1×and 2×MIC treatments,the inhibition rates of biofilm formation by oregano oil were 40.49%and 17.56%,respectively.The number of colonies were 4.85 log（CFU/plate）and 3.77 log（CFU/plate）.Clove oil biofilm formation inhibition rates were 56.49%and 31.38%,and the number of colonies were 5.38 log（CFU/plate）and 4.02 log（CFU/plate）,respectively.Moreover,essential oils inhibited the secretion of extracellular hydrolases.Compared with the control group,the secretion of protease,cellulase,pectinase,xylanase and lipase in 1/2×MIC oregano oil were decreased by83.18%,70.79%,5.66%,48.28%and 48.68%respectively.The inhibition rates of clove oil were decreased by 44.66%,70.05%,30.04%,47.37%and 73.42%respectively.The results of Quantitative Real-time PCR showed that the essential oils inhibited the gene expression of pathogenic factors of pathogenic bacteria,so the pathogenicity of P.carotovorum was reduced.（3）It was found that oregano and clove oil could effectively inhibit the spot size of onion soft rot.In the treatment group,the lesion diameter of 1×MIC oregano oil was 2.33 mm,and the inhibition rate was 92.29%.The diameter of clove oil lesion was 3.83 mm,and the inhibition rate was 87.08%.In the prevention group,the lesion diameter of 1×MIC oregano oil was 0.22mm,the inhibition rate was 99.28%,and the lesion diameter of clove oil was 2.94 mm,the inhibition rate was 89.94%.The control effect of essential oil with 2×MIC concentration on soft rot was more than 98%.Pot experiment showed that the control effect of 1×MIC oregano oil prevention group was 74.07%,and the control effects of treatment group was 67.23%,the control effects of 1×MIC clove oil in the prevention group and treatment group were 63.80%and 63.39%respectively.The control effect of 2×MIC concentration essential oil was more than80%.Moreover,the treatment of essential oil increased the activity of peroxidase（POD）in onion seedlings,so as to improve the disease resistance of onion.Therefore,oregano oil and clove oil may reduce the spot size of soft rot.It maybe the synergistic effect of direct bacteriostasis,inhibition of pathogenic factors and improvement of POD enzyme activity in onion. |
