| Watermelon is an important cash crop and occupies a significant position in agricultural production system of China. In recent years, watermelon Fusarium wilt has become an increasing threat to watermelon production around the world. Fusarium wilt of watermelon is caused by the fungus Fusarium oxyporum f.sp. niveum, which commonly spreads from one field to another by infested soil and can survive more than 10 years in soil. Highly resistant cultivars are the most effective and economical measure to combat Fusarium wilt. The country's advances in breeding of wilt-resistant varieties are much less than the watermelon production demands as result of lack of basic research in pathogenicity differentiation, identification and selection of resistant varieties, etc. besides old research methods.From 2001 to 2003, the differentiation of disease fungus, mutation for pathogenicity, and varietal resistance were studied and discussed by using 46 isolates from watermelon-growing areas of Hebei and 4 isolates from other place, as well as 50 watermelon cultivars from Hebei Modern biotechnological methods were used in combination with traditional approach for the study.1. According to observed characteristics of watermelon wilt fungus and based on response to host inoculated by isolates it was obvious that the watermelon Fusarium wilt fungus was Fusarium oxyporum f.sp.niveum.2. There was significant difference among isolates from Hebei by identifying the pathogenecity.The strongest isolates were FON31 and FON40, the weakest isolates was FON12 on the pathogenecity.3. The differentiation of physiological races of Fusarium oxyporum f.sp.niveum was thoroughly identified by the response of commonly used hosts. The resulsts indicated that 46 isolates from Hebei were fall into 3 races like race 0, race 1, race2, which were 8(17.4), 30(65.2%), and 8(17.4), respectively. Race 0 mainly existed in Handan, Shijiazhuang, Baoding, Xingtai, Langfang areas, etc.; the race 1 was the predominant race and distributed around Hebei province; race 2 covered Baoding, Shijiazhuang, Xingtai areas, etc.;4. AFLP patterns of ten isolates, which were of four cucurbit-infecting formaespeciales, were amplified by 10 primer pairs. When all polymorphic bands were used for cluster analysis using the SPSS program, isolates were classified into 4 groups, which were the same formae speciales isolates were grouped into the same group, so their genetic backgroud were similar, at the same time, different groups include different formae speciales isolates, which indicated that AFLP could reveal the difference between different formae speciales. In addition, there were genetic differentiation in the same formae speciales.5. The genetic differentiation of 50 isolates of Fusarium oxysporum f.sp niveum from Hebei watermelon growing areas and 4 identified isolates from other regions was analyzed by RAMS (random amplified microsatellites) molecular marker technique. 134 polymorphic markers were obtained from the total 188 amplified markers by 21 primers. Among the isolates the genetic distance was from 2.45 to 8.60 and the average was 5.20, which proved that the genetic diversity of tested isolates were abound. Based on the hierarchical analysis of the molecular markers, 50 isolates could be classified into three genetic groups: RGI, containing 39 isolates, which were 32 Race 1 from Hebei, Xinjiang and Beijing, 3 Race 2 and 6 Race 2; RGII, including 2 isolates from Hebei province and one of Race 0 isolates from Xinjiang region; and RGIII, containing 6 Race 2, among them 5 isolates from Hebei province and one of Race 2 isolates from USA. The results of this study showed that RAMS groups were generally associated with the physiological races which were identified on the response of host to isolates, but there was no relation between RAMS groups and geographic regions of isolates.6. The genetic differentiation of 50 isolates of Fusarium oxysporum f.sp niveum from watermelon growing areas of Hebei and 4 identified races from other regions were analyzed by AF...
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