Abstract:Phytophthora blight symptomatic cucumber and wax gourd plants were collected from 9 sampling areas in Guangxi Zhuang Autonomous Region and Guangdong Province during 2007-2010, from which 193 isolates of Phytophthora melonis were isolated and identified. Virulence of the isolates and their host preference were tested and compared using inoculating mycelial disks on wax gourd and cucumber plants using non-wounding method. The results showed that marked difference of virulence existed in the P.melonis populations. The isolates were divided into highly virulent, moderately virulent and weakly virulent groups. The highly virulent isolates were dominant in most of the area sampled on both host plants. At least three types of host preference were found among the P.melonis isolates. The isolates of A-type host preference were highly virulent on both wax gourd and cucumber plants regardless of their original isolation hosts. The isolates of B-type host preference were highly virulent only on the wax gourd plants from which they were isolated, and weakly or moderately virulent on the cucumber plants. The isolates of C-type host preference were highly virulent only on the cucumber plants from which they were isolated, and weakly or moderately virulent on the wax gourd plants.