| It is well known that China is the biggest potato producer in the world. Up to 2001, the total planting area had come to 4.72 million hectares, about 25% of the total planting area in the world, and about 60% of that in Asia. Potatoes are planted in 22 provinces and autonomous regions in China. Among them, Inner Mongolia is one of the major seed potato production areas and the largest areas to plant potatoes in China. The potato production has been becoming a very important supporting industry in Inner Mongolia agriculture. However, potato late blight caused by Phytophthora infestans is the most devastating disease for potato production in Inner Mongolia. In this paper, phenotypic and genotypic diversity analyses were performed on P. infestans isolates from Inner Mongolia. The aim of the investigation is to set out genotype research on P. infestans in China and to provide a solid academic base for controlling potato late blight in Inner Mongolia.During past years, more and more pathologists focused on studying P. infestans, especially on virulence and avirulence factors of pathogen, and expected to unravel the mechanism of pathogen pathogenesis and host resistance. Among avirulence factors of P. infestans research, identification of avirulence genes (Avr) is a hotspot. In potato - P. infestans pathosystem P. infestans exhibits a gene-for-gene interaction with potato, in which Avr gene products from the pathogen are recognized by the host plant expressing the cognate resistance gene. Based on latest understanding of race-specific Avr genes in P. infestans, in this paper we aimed at genetic mapping and transcriptional profiling of Avr genes further. The main results in the thesis as follows:1. A total of 51 isolates of P. infestnas were collected from China (40 isolates from Inner Mongolia) from 1997 to 2003, and compared with 9 isolates from other countries. The structure of the populations was analysed phenotypically and genotypically. Phenotype characteristics of the isolates included mating type and virulence on potato differentials. Genotypes were assessed for mitochondrial DNA haplotype with restriction fragment length polymorphism-polymerase chain reaction (RFLP-PCR) as well as amplified fragment length polymorphism (AFLP) and simple sequence repeats (SSR). In addition to isolate V12 from Yunnan was A2 mating type, all other isolates from China were the A1 mating type. In the 38 isolates from Inner Mongolia tested there were 18 different races. Race 3,4,7,10 showed the highest frequency of 21.1%, followed by 1,2,3,4,5,6,7,8,9,10,11 and 3,4,6,7,10 with frequencies 13.2% and 10.5%, respectively. All 48 isolates collected from North China presented mitochondrial DNA haplotype IIa, whereas, three isolates from Yunnan have Ia and IIb haplotype. Our results showed that genotypes with the IIa halotype were predominant in Inner Mongolia, and maybe also ;n North China. SSR analysis with primers GCA-00012, revealed that 40 isolates from Inner Mongolia have the same SSR genotype SG1 indicating that those isolates belong to one clonal lineage. The other 8 isolates from North China also have the same SSR genotype SG1. However, The 3 isolates from Yunnan, have three different SSR genotypes SG2, SG3 and SG4. AFLP analysis of all 60 isolates...
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