| The watermelon,known as the king of summer fruits for its cool,sweet,juicy qualities,has long been plagued by fusarium wilt.Watermelon fusarium wilt is a soil-borne fungal disease caused by Fusarium oxysporum f.sp.niveum(FON).FON usually infects the vascular bundles of watermelons,and the disease develops rapidly after successful infection and can occur throughout the growing season of watermelon.At present,there is no effective methods to control the watermelons that have suffered from fusarium wilt,so the prevention and control of fusarium wilt is more focused on prevention.At present,the common methods of watermelon fusarium wilt control mainly focus on three aspects,disease resistance breeding,stock grafting and chemical pesticide.According to the requirements of developing green agriculture,biological control methods have gradually become the mainstream of plant disease control.Biological control is usually environment-friendly and safe to humans and domesticated animals and supposed to be a promising plant protection method.At present,the application of biological control in watermelon fusarium wilt is still in its initial stage,and the development prospect is attractive.The application technique and research of entomopathogenic nematodes(EPNs)to control agricultural insects have been increasingly mature around the world.However,application of the symbiotic bacteria carried in EPNs intestines to control plant pathogens has not received enough attention.Entomopathogenic nematode symbiotic bacteria produce metabolites which enable EPNs the capability to kill insects and antibacterial activity to control fungi including many plant soil-borne diseases.Therefore,the application of entomopathogenic nematode symbiotic bacteria in biological control,especially in the control of watermelon fusarium wilt,deserves more attention.In the present study 11 strains entomopathogenic nematodes symbiotic bacteria were selected as materials to evaluate their biocontrol potent against two physiological races of Fusarium oxysporum f.sp.niveum(Fon-1and Fon-2)by using methods of conidium germination and mycelial growth inhibition.Of these eleven bacterial strains,three were found with good antibacterial ability,so that were selected for further research of preparation of compound bacterial agents.The main research results are as follows:1.Five symbiotic bacterial strains of entomopathogenic nematodes were found to have strong capability to inhibit completely the spore germination of both Fon-1 and Fon-2In this study,the inhabitation efficacy of all strains of species of Xenorhabdus on spore germination of Fon-1 and Fon-2 was measured by treating spores with different components of fermentation broth of 11 entomopathogenic nematode symbiotic bacterial strains.The results show that spores of Fon-1 and Fon-2 did not germinate and grow on PDA plate when treated with fermentation broth and its aseptic filtrate of X.bovienii NN5,X.bovienii NN6,X.cabanillasii NN7,X.bovienii NN8 and X.bovienii NN20 for 12 h,indicating that the above components of the fermentation broth of these 5 symbiotic bacterial strains had a complete inhibition effect on the germination of the conidia of the two physiological species of FON.However,the spores treated by those of the other 6symbiotic bacterial strains as the above-mentioned methods as those 5 strains can germinate and grow on PDA plate,indicating that they did not completely inhibit the germination of Fon-1 and Fon-2.2.Five strains of entomopathogenic nematode symbiotic bacteria were found to have significant inhibitory effect on mycelial growth of Fon-1 and Fon-2.The oxford cup method was used to determine the inhibitory ability of each component of fermentation broth of 11 strains of entomopathogenic nematode symbiotic bacteria on the mycelial growth of each physiological species of FON.The results showed that 5strains,X.bovienii NN5,X.bovienii NN6,X.cabanillasii NN7,X.bovienii NN8 and X.bovienii NN20,were found to have significant inhibitory effect on mycelial growth of Fon-1 and Fon-2.The other 6 strains of symbiotic bacteria had poor inhibitory ability to the growth of mycelia of Fon-1 and Fon-2.Among these five strains,X.cabanillasii NN7 was the most effective on inhibiting the growth of mycelia of Fon-1 and Fon-2,with the inhibition rate of 50.74% and 64.75% respectively,which indicated that the strain NN7 has the greatest biocontrol potential.In addition,our results also showed that the anti-fungus activities of the fermentation broth of all those strains were the best,and those of the bacterial cells of them were the worst.3.A variety of compound ratios of the fermentation broth of three symbiotic bacterial strains with good inhibitory effect on the growth of Fon-1 hypha were screened out.X.bovienii NN5,X.bovienii NN6 and X.cabanillasii NN7,three strains of symbiotic bacteria with the best comprehensive control effect,were selected for determination the best combination proportions to control FON.The inhibition ability of each bacterial strain combination on the growth of Fon-1 mycelia was determined.Antagonistic tests showed that there was no obvious antagonism between the three strains,which indicated that they are suitable to be used as the compound bacteria test materials.The inhibition rate of compound ratio of X.bovienii NN5 to X.bovienii NN6 with 1:1 and 1:2,and that of X.bovienii NN5 to X.cabanillasii NN7 with 2:1,and those of X.bovienii NN6 to X.cabanillasii NN7 with 1:2,2:1 and 4:1 on the growth of Fon-1 mycelia were more than50%,and the highest inhibition rate was the 2:1 combination of X.bovienii NN6 to X.cabanillasii NN7,which was 57.90%.Compared with the inhibition rate of single bacteria,the combined bacterial control effect of X.cabanillasii NN7 did not significantly improve,and the combined bacterial control ability of X.bovienii NN5 and X.bovienii NN6 was significantly improved. |
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