| Fusarium wilts of bitter gourd and cucumber belong to the typical soil-borne disease category.Both diseases are often devastating to the agricultural production of bitter gourd and cucumber,which occupy an important proportion of vegetable crops in China and other countries as well in the world.Biocontrol has been used as a safe and efficient measure of crop disease management and partially depends on distinct constituents of bacterial species represented by bacteria in the Streptomyces genus.Biocontrol bacteria that have been classified as various Streptomyces species fulfil the biocontrol function either by affecting interactions between microbes(including plant-pathogenic pathogens)distributed in the soils or on plant rhizospheres,or by activating immune responses in plants.Under both circumstances,biocontrol bacterial strains possess common characteristics in genetical,physiological and biochemical relevance to the geographical and biological(plant species,for example)sources and to environmental conditions.These factors not only determine the biologically basic property of a biocontrol bacterial strain,but also impact the effectiveness of the bacterial use in biocontrol against local crop diseases.Considering these biodiversity constituents,the present study was devised to characterize two biocontrol Streptomyces stains isolated from the local(Taian,Shandong Province)plantings,with respect to the physiological basis of the biocontrol function and the potential of agricultural application in the future.A total of 97 Streptomyces strains were isolated from soils of a local field that had been used to grow bitter gourd plants and incur Fusarium wilt for 5 years.From the same soils,68 Streptomyces strains were isolated in total.By plate confrontation(biocontrol bacteria and the target pathogen incubated on medium at the opposite sides)tests and fermentation filtrate inhibition test,it was determined that strain GX-117 had the strongest antibacterial activity against the pathogen of bitter gourd Fusarium wilt,and strain GX-8 had the strongest antibacterial activity against the pathogen of cucumber Fusarium wilt;and the plate confrontation test was carried out with another 13 pathogens stored in the laboratory,the antibacterial effect of strain GX-117 on 10 pathogens was more than 50%,and strain GX-8 on8 pathogens.The antibacterial effect of the original strain was more than 50%,which indicated that the antibacterial effect of strains GX-117 and GX-8 were extensive;strains GX-117 and GX-8 grew well on Gaoshi No.1 liquid medium,entered logarithmic growth period on the third day,and entered stable period on the fourth day.In addition,GX-117 and GX-8 bacteria were identified to be members of the Streptomyces genus based on their morphylogical,physiological,biochemical,and molecular characters.Then,GX-117 and GX-8 was subjected to bioactivity assays applied to bitter gourd and cucumber plants in enviromentally controlled plant growth chambers.The results showed that strain GX-117 and GX-8 could promote the growth of bitter gourd and cucumber seedlings in different degrees.On one hand,compared with control(CK)treatment in which plants were inoculated with the corresponding pathogen but not treated with GX-117 or GX-8,the plant height,stem diameter,fresh weight and dry weight of bitter gourd seedlings treated with strain GX-117 increased by 20.76%,16.88% and 82.93%,respectively.Compared with CK treatment,the plant height,stem diameter,fresh weight and dry weight of cucumber seedlings treated with strain GX-8 increased by 60%,12%,54.05% and 20.59%,respectively.On the other hand,compared with CK treatment,the incidence rate of bitter gourd wilt disease treated by strain GX-117 decreased by 54.04%,and the control effect was 57.92%.The incidence rate of cucumber wilt disease treated by strain GX-8 decreased by 42.83% and the control effect was 43.01%.The results showed that the antagonistic strains GX-117 and GX-8had the ability to resist the disease of bitter gourd Fusarium wilt and cucumber Fusarium wilt respectively.Next,defense responses in bitter gourd and cucmber plants were analyzed following treatment with GX-117 or GX-8 and the subsequent inoculation with the pathogen in contrast to uninoculation CK.The activities of defense enzymes of bitter gourd seedlings treated with strain GX-117 and cucumber seedlings treated with GX-8 were significantly higher than those of CK treatment in the whole sampling period except in the early stage of control;the accumulated MDA content was significantly lower than that of CK treatment;the chlorophyll content was also significantly higher than that of CK treatment;it indicated that antagonistic strains GX-117 and GX-8 could improve the control ability of bitter gourd and cucumber seedlings against Fusarium wilt respectively.Furthermore,the single factor fermentation conditions of strains GX-117 and GX-8 were screened from six aspects: culture medium,initial pH value,temperature,rotation speed,inoculation amount and fermentation time,the results showed that strain GX-117 had the best inhibitory effect on Fusarium oxysporum f.sp.cucumebrium when it was fermented with soybean powder liquid medium,initial pH value of 8.0,temperature of 28℃,rotational speed of 180 rpm,inoculum volume of 2 ml and fermentation time of 4 days;strain GX-8 had the best inhibitory effect on FOM when it was fermented with potato glucose liquid medium,initial pH value of 7.0,temperature of 28℃,rotational speed of 200 rpm,inoculum volume of1 ml and fermentation time of 5 days had the best antibacterial effect on Fusarium oxysporum f.sp.momordicae.After single factor fermentation,the antibacterial effect of strains GX-117 and GX-8 on pathogens increased by 29.42% and 37.96%,respectively.Taken together,these results open an experimental aveneu to the agricultural application of GX-117 and GX-8 to control bit gourd and cumcumber wilts and provide a technical basis for further studies of different biolcontrol strains from the Streptomyces popolations. |
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