| Green Mold caused by Penicillium digitatum is one of the principal postharvest diseases of Citrus. The pathogen can produce a lot of conidia which makes the disease spread very rapidly and has caused serious loss in production. To find potential biocontrol agents for citrus green mold, a total of 414 strains of bacteria and 58 strains of actinobacteria were isolated from the rhizospheric soil of citrus, among them, strain X33 exhibited the strongest antibiosis capacity. Here are the main research methods and results:(1) Based on the results of morphological, cultural characteristics and 16SrDNA analysis, strain X33 was identified as Streptomyces lavendulae. This is the first report of Streptomyces lavendulae for controlling citrus green mold.(2) The fermentation medium which is appropriate to Streptomyces lavendulae X33 antibacterial active material synthesis and bacteria growth was obtained by screening step by step and optimized through single factor experimentin. The optimized shaking flask cultivation medium were as follows:soluble starch 20.0g/L, beef extract 3.0g/L, peptone 5.0g/L, KNO3 lg/L, K2HPO40.5g/L, NaCl 5.0g/L, initial pH 7.0. The optimized cultivation conditions were as follows:age of inoculum 48h, inoculum level 9%, working volume 30ml/250 mL, during 0-48h, the cultivation was performed at 32℃,200r/min;and after 48h, the cultivation was performed at 28℃,180r/min.(3)The bacteriostatic mechanism of the fermentation broth of Streptomyces lavendulae X33 against Penicillium digitatum were initially researched.The results showed that the antibacterial substances could deformity and dissolve the mycelium of Penicillium digitatum,they could significantly inhibit the growth of conidiophores and conidia production.In addition, the antibacterial substances could significantly inhibited conidia germination.(4) The antimicrobial activity of the fermentation broth of Streptomyces lavendulae X33 on bacteria, yeast and plant pathogenic fungi were tested by using the der-plate assay and mycelium growth rate. The results showed that the fermentation liquid had better inhibitory effects against 6 kinds of bacterial,4 kinds of yeasts and 7 kinds of plant pathogenic fungi, the best inhibitory effect was against Geotrichum candidum, and the inhibition rate was 100%.(5) The stability and solubility of the antibacterial substances produced by Streptomyces lavendulae X33 were initially researched. The results showed that the kind of antibiotic is stabilization in alkalescence circumstance, and also is stabilization in acid or neutral condition, but it is not stabilization in higher temperature. The activity of repressing Penicillium digitatum is zero after in 115℃ heated 30min; the antibacterial substances easily dissolve in strong polar solvents such as water and methanol,insoluble in weak polar solvent such as N-hexane and petroleum ether.(6) In order to assess the edible-safety of the fermented product of Streptomyces lavendulae, the toxicological safety properties were examined by acute oral toxicity test and subacute oral toxicity test.The results of the acute oral toxicity test showed that the median lethal dose (LD50) of the fermented product of Streptomyces lavendulae was more than 10g/kg·bw, which fell into the scope of non-toxicity. The results of the subacute oral toxicity test showed that significant differences were not observed in blood routine examination and chemical indexes, hepatic and renal functions indexes,organ coefficients among the 3 different dosage groups of the fermented product compared with normal control group through out the 28-day test period of mice. In addition, there was no abnormal condition for the pathologic examination of main organs. |
