Abstract: 【Objective】 To identify the pathogen causing Pythium root rot of tomato in Inner Mongolia,providing theoretical reference for accurate identification and scientific control of soil-borne diseases in greenhouse tomatoes in the region. 【Methods】 A total of 47 samples of greenhouse tomato seedlings with wilting of aboveground parts,black lesions at the stem base,and plant collapse and wilting were collected from three sites in Hohhot and Baotou,Inner Mongolia. Tissue isolation methods were used to isolate and purify the pathogen. Morphological and molecular biological techniques were employed for pathogen identification. Pathogenicity tests and re-isolation of the pathogen were conducted. 【Results】 A total of 21 isolates were purified and obtained from the 47 samples. On PDA medium,these isolates produced well-developed white aerial mycelium. Microscopic examination revealed the presence of oospores,oogonia,and antheridia,which were consistent with morphological characteristics of the Pythium. Further sequencing of the ITS and β-tubulin,followed by phylogenetic tree construction showed more than 99% homology when compared with the sequence of Pythium aphanidermatum. P. aphanidermatum was re-isolated and identified from artificially inoculated diseased plants,validating Koch′s postulates and confirming that the isolated P. aphanidermatum was the causal pathogen Pythium root rot of tomato. 【Conclusion】 P. aphanidermatum is the pathogen responsible for Pythium root rot of tomato in Inner Mongolia.

Key words: Inner Mongolia, Pythium root rot of tomato, Pythium aphanidermatum, Isolation, Identification