| Cucumber Fusarium wilt,caused by Fusarium oxysporum f.sp.Cucumerinum,is a destructive soil-borne vascular bundle disease.As one of the three major diseases in cucumbers,Fusarium wilt is hard to be controlled.The rhizosphere microbiome,which is considered as the first line of defense against soil-borne pathogens,could create a bridge between plants and soil,and is intricately involved in plant health.In this study,the pot experiments were carried out to study the bacteria community composition from Fusarium wilt-resistant and-susceptible cucumber cultivars based on Illumina Mi Seq high throughput sequencing technology.Furthermore,how the rhizosphere bacteria assist in the host to defend against pathogens was explored.In addition,the effects of two commonly used fungicides(thiophanate-methyl and carbendazim)on the cucumber rhizosphere microbiota were assessed from a microbial perspective.The aim of this study is to clarify the relationship between pesticides,rhizosphere bacteria and the Fusarium wilt.The main results of this study are as follows:(1)In this study,the embryo root inoculation method was used to evaluate the resistance of 19different cucumber germplasms to Fusarium wilt,and the disease indices ranged from 27.9%to 100%with the pathogen inoculation concentration of 1×10~7spores/m L.Among all cucumber germplasms,Longyuanxiuchun and Shuyan 2 were identified as the disease-resistant cultivars,the disease indices of which were 27.9%and 47.9%,respectively;moreover,Zhongnong 6 and Zhongnong 38 were disease-susceptible cultivars,the disease index of which were 93.5%and 100%,respectively.Finally,these four cultivars were selected for the subsequent experiments.The results showed that the disease severity for the four cultivars increased with an increase in initial inoculation concentration of pathogens.The disease indices ranged from 5.0%to 76.7%at inoculation concentration of 1×10~5spores/m L,from 17.5%to 87.9%at inoculation concentration of 1×10~6spores/m L,and from 32.5%to100%at inoculation concentration of 1×10~7spores/m L.When the inoculation concentration was 1×10~6spores/m L,the disease indices differed significantly between resistant and susceptible cultivars.The disease indices for Longyuanxiuchun and Shuyan 2 were 17.5%and 18.3%,while those for Zhongnong6 and Zhongnong 38 were 85.8%and 87.9%,respectively.In addition,the fresh weight and stem length of cucumber seedlings for susceptible cultivars were less than those for the resistant cultivars after inoculation with pathogens,but root length had no significant difference.(2)The soil types were the dominant factor to affect the rhizosphere bacteria composition,followed by cucumber cultivars.In the black soil,the results of PCo A suggested that bacterial community structure differed significantly between resistant and susceptible cucumber cultivars(ANOSIM,R=0.6618 and P=0.001).Moreover,the Shannon indices for the resistant cultivars,i.e.,Longyuanxiuchun and Shuyan 2,were higher than that for the susceptible cultivar Zhongnong 38.The LEf Se analysis showed that the abundance for Bacillus,Arthrobacter,Rubrobacter,Skermanella and Gaiella were relatively higher in the rhizosphere soil of Longyuanxiuchun and Shuyan 2.The bacterial co-occurrence network analysis at the genus level showed that Bacillus was the key genus with the degrees of 27,which was important to maintain the stability of bacterial community;and more importantly,the relative abundance of Bacillus for the resistant cultivars was significantly higher than that of susceptible cultivars.In the fluvo-aquic soil,the results of PCo A suggested that bacterial community structure differed significantly between resistant and susceptible cucumber cultivars(ANOSIM,R=0.3279 and P=0.001).The?diversity had no significant difference among different cucumber cultivars.The LEf Se analysis showed that the abundance for Arthrobacter was relatively higher in the rhizosphere soil of Longyuanxiuchun and Shuyan 2.The bacterial co-occurrence network analysis showed that Leptolyngbya?FYG had the highest degrees of 8 in the fluvo-aquic soil.Overall,it was speculated that resistant cultivars were able to selectively recruit Bacillus in their rhizosphere soil to defend against the pathogens in the black soil.(3)Thiophanate-methyl significantly changed the bacterial community composition in the rhizosphere soil of cucumbers,but the influence level was related to the pesticide application dosages and cucumber cultivars.The Shannon indices for Longyuanxiuchun and Shuyan 2 were significantly decreased with the treatment of thiophanate-methyl at the recommended dose(93.8 mg a.i./each seedling),and the Shannon index for Longyuanxiuchun was also significantly decreased at the dose of9.38 mg a.i./each seedling.The LEf Se analysis showed that thiophanate-methyl decreased the abundance of Bacillus in the rhizosphere soil for Longyuanxiuchunk,which would possibly weaken the ability of the resistant cultivar to defend against pathogens.(4)As the major metabolite of thiophanate-methyl,carbendazim serves as one of the most commonly used fungicides in agricultural production.In this study,the use of carbendazim significantly altered the bacterial community composition in the rhizosphere soil of different cucumbers,but the influence level was related to the pesticide application dosages and cucumber cultivars.Carbendazim significantly decreased the Shannon indices for Longyuanxiuchun and Shuyan 2 at the high dose treatment(93.8 mg a.i./each seedling),and for Longyuanxiuchun at the low dose treatment(9.38 mg a.i./each seedling).The LEf Se analysis showed that carbendazim decreased the abundance of Bacillus in the rhizosphere soil for Longyuanxiuchun.From a microbial perspective,long-term application of carbendazim may not be conducive to the control of cucumber Fusarium wilt. |
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