| Root rot disease of Isatis indigotica Fort was found to be more and more seriousin many areas of Northern China in recent years. In this study, the pathogens werefirst isolated and tested for the pathogenicity, then the pathogens were identified, thebiological characteristics of the pathogens were examined afterward, and finallysoilborne antagonistic microorganisms were screened against the pathogens.One hundred and eight samples of root rot diseased I. indigotica were collectedfrom Yutian county of Tangshan city in 2005 and 2006. 172 fungi isolates wereisolated from the symptomatic plant materials, all the isolates were classified to 3species of 2 fungal genera: Fusarium solani, Fusarium oxysporum and Rhizoctoniacerealis. F. solani was isolated at the highest frequency (44.8％); from the root rotdiseased I. indigotica. F. oxysporum had lower frequency (44.2％); R. cerealis had thelowest frequency (7％). One or two isolates from every species were chosen toexamine the following experimentations, including F. solani isolates P4 and P5, F.oxysporum isolate P1 and R. cerealis isolate P2. The pathogenicity tests showed thatisolates P4 and P5 were the most important pathogen with 100％disease incidenceand disease index 55, isolate P1 was the less important pathogen with 89％diseaseincidence and disease index 36, while isolate P2 was the weak pathogen with 67％disease incidence and disease index 31.A polymerase chain reaction (PCR) protocol was developed to amplify internaltranscribed spacers ITS1 and ITS2 of the nuclear ribosomal DNA in isolates P1, P2,P4 and P5. The PCR products were sequenced and analyzed. Results indicated thatthe identity between P1 and F. oxysporum (DQ459007) was 100％by comparing theirITS sequences, the identity between P2 and R. cerealis (AJ242887) was 100％, theidentity between P4 and F. solani (AY310442) was 99％, and the identity between P5and F. solani (AJ608989) was 100％. The results proved that ITS sequences analysiscould be a method of fungi identification in species level, but not in lower specieslevel.The colony growth, conidial germination and sporulation of the four isolates P1,P2, P4 and P5, were tested under different temperature, pH, carbon sources, nitrogensources and relative humidity. The lethal temperature for conidia was examinedafterward. Results showed that the optimum temperature for colony growth, conidialgermination and sporulation of isolate P1 was 25℃on potato dextrose agar, theoptimum pH value for colony growth and conidial germination was pH6, the optimumpH value for sporulation was pH5, the optimum carbon source was sucrose, the optimum nitrogen source was sodium nitrate, the conidia couldn't germinate underrelative humidity 90％, and the lethal temperature for conidia was 55℃at duration of10 minutes. The suitable temperature for colony growth of P2 was 30℃on potatodextrose agar, the optimum pH value for colony growth was pH6, the optimumcarbon source was soluble starch, and the optimum nitrogen source was sodiumnitrate. The optimum temperature for colony growth and conidial germination of P4and P5 was 30℃on potato dextrose agar, the optimum temperature for sporulationwas 25℃, the optimum pH value for colony growth, conidial germination andsporulation was pH7, the optimum carbon source was lactose, the optimum nitrogensource was sodium nitrate, the conidia couldn't germinate under relative humidity90％, and the lethal temperature for conidia was 50℃at duration of 10 minutes.One hundred and eleven antagonistic microorganisms, including 73 isolates ofactinomyces and 38 isolates of bacteria were obtained from 34 soil samples and testedfor antagonistic effect against Fusarium solani (isolate P4). 41 strains of them wereinvestigated by inhibition test in vitro. The results showed that isolate B4228 andA4075 were the best two. Further studies on their inhibition effect on the other plantpathogens were conducted. Significant inhibitory effects against Alternaria solani,Botrytis cinerea, Veticillium dahliae, Rhizoctonia solani Kühn, Colletotrichumlagenarium and Rhizoctonia spp. were found. Antagonistic isolate B4228 wasidentified as Bacillus subtilis B4228 finally.
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