| Root rot of Isatis indigotica was found to be more and more severity in many areas of Northern China in recent years. In this study, the pathogens were isolated and tested for the pathogenicity, then the pathogens were identified, the biological characteristics of the pathogens were examined afterward, and finally ethanol extract of 123 species Chinese herbs were tested for the antifungal activity against the isolate P4.One hundred and eight diseased samples of I. indigotica with root rot symptom were collected from Yutian county of Tangshan city in 2005 and 2006. 172 fungi isolates were isolated from the symptomatic plants. All the 172 isolates were classified into 3 species of 2 fungal genera according to Koch's postulates, they were Fusarium solani, F. oxysporum and Rhizoctonia cerealis. The frequency of F. solani, F. oxysporum and R. cerealis was 44.8％, 44.2％and 7％, respectively. A polymerase chain reaction (PCR) protocol was developed to amplify internal transcribed 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 that the identity between P1 and F. oxysporum (DQ459007) was 100％by comparing their ITS sequences, the identity between P2 and R. cerealis(AJ242887)was 100％, the identity between P4 and F. solani (AY310442) was 99％, and the identity between P5 and F. solani (AJ608989) was 100％.The colony growth, conidial germination and sporulation of the four isolates P1, P2, P4 and P5, were tested under different temperature, pH, carbon sources, nitrogen sources and relative humidity. The lethal temperature for conidia was examined afterward. Results showed that the optimum temperature for colony growth, conidial germination and sporulation of isolate P1 was 25℃on PDA media, the optimum pH value for colony growth and conidial germination was pH6, the optimum pH value for sporulation was pH5, the optimum carbon source was sucrose, the optimum nitrogen source was sodium nitrate, the conidia couldn't germinate under relative humidity 90％, and the lethal temperature for conidia was 55℃at duration of 10 minutes. The suitable temperature for colony growth of isolate P2 was 30℃on PDA media, the optimum pH value for colony growth was pH6, the optimum carbon source was soluble starch, the optimum nitrogen source was sodium nitrate. The optimum temperature for colony growth and conidial germination of isolates P4 and P5 was 30℃on PDA media, the optimum temperature for sporulation was 25℃, the optimum pH value for colony growth, conidial germination and sporulation was pH7, the optimum carbon source was lactose, the optimum nitrogen source was sodium nitrate, the conidia couldn't germinate under relative humidity 90％, and the lethal temperature for conidia was 50℃at duration of 10 minutes.The antifungal activity of ethanol extract of 123 species of Chinese herbs against isolate P2 was tested on Petri dishes. The results indicated that ninety nine species of Chinese herbs had significant inhibitory effect against the colony growth with the concentration of 0.01 g/mL, for example, the inhibition ratio of TS-42 and TS-84 were over 50％, and TS-10 reached 100％. One hundred and twelve species of Chinese herbs had significant inhibitory effect against conidial germination, ten of them were over 50％inhibition, and the inhibition rates of TS-10 and TS-11 were 100％. These extracts with inhibition rates over 50％were chosen for the secondary screen at the concentration of 0.005 g/mL. Opposite to TS-10 had the 100％inhibitory effect in the second screening tests, the inhibition ratio of TS-42 and TS-84 against colony growth reduced significantly. The inhibition rate to conidial germination of TS-10 was 74.50％, which was also the best one with the EC50 3.621 mg/mL.
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