| Valsa canker, caused by fungal pathogen of Valsa Fr. species and their Cytospora Ehrenb. ananmorphs, is the most destructive disease of apple tree that causes serious economic losses. It is called"cancer"of apple trees in some places for its difficulty to control, and composes one of the major limiting factors of apple production. Species identification of the genus Valsa and Cytospora have long been recognized as a difficulty for the prevalence of intraspecific morphological variations and plasticity of some cultural characteristics. In China, there is no general agreement on the identity of the causal pathogen of apple canker. Molecular markers, for their richness in information, stability through life cycles and immune to environmental impact, have been widely used in phytopathologyical studies, becoming a reliable tool for pathogen detection and identification.In this study, the pathogens of apple canker were investigated through multiple approaches, combining morphological observation, culture characterizing study, rDNA-ITS sequences analysis and pathogenic comparison. Basing on the study of 150 isolates from 10 provinces, the following conclusions have been obtained.1. There are three Valsa species occurred on the apple tree in China, viz. Valsa ceratersperma sensu Kobayashi, V. malicola Z. Urb. and V. persoonii (Nitschke) H?hn.2. The species concept of the former V. eratersperma(Tode:Fr.)was very inspecific, and in the phylogenetic analysis its members scattered in two highly divergent clades. In this study we narrowed the species concept down to V. eratersperma sensu Kobayashi, which composes 96% of the isolates studied and was obviously the dominant species on apple tree in China.3. Low genetic divergences were detected among isolates of V. eratersperma, but with no relation to the geographical origin of the isolates. Two distinct clades (Ca and Cp) formed in the phylogenetic trees, with a mean genetic distance (p-distance) of 1.32% revealed by rDNA-ITS sequences. There was a dominant haplotype of ITS sequence among the isolates studied, which widely distributed across all the apple growing regions.4. Some phenotypic variations were observed among the isolates of V. ceratersperma. Generally, there were three colony coloration types, yellowish-brown, milky-white and dark-gray. The milky-white isolates could develop on PDA medium at 37℃, but the isolates of the other two types could not. On the 20% ABA medium, the yellowish-brown and the milky-white isolates formed macro-conidiomata, while the dark-gray isolates formed mini-conidiomata.5. The pathogen of pear canker is also V. eratersperma sensu Kobayashi. The ITS sequences of pear canker isolates was identical to or differing only at one nucleotide site from that of the isolates from apple tree belonging to clade Cp (Ha13). The colony was milky-white, and it formed macro-conidiomata.6. V. persoonii, the major causal agent of peach canker, was discovered to be able to occasionally (1 out of 150 isolates) infect apple tree. The ITS sequences of isolates from apple tree was identical to those of the isolates from peach tree. However, there were great phenotypic differences between the V. persoonii from the two kind of host. The isolates of peach tree have multi-locule conidiomata, possessing black conceptacle, with the colony olivaceous, compact and homogeneous, but without growth zones and aerial mycelia。On the contrary, the isolates from apple trees form conidiomata with one locule, lacking black conceptacle, with the colony yellowish-brown, loose and radiate, producing growth zones and white aerial mycelia.7. V. malicola was firstly reported from China. Its colony remained milky-white throughout the cultivation, and on the 20% ABA medium it formed mini spherical conidiomata.8. Three valsa species differ in pathogenicity. V. malicola has weak virulence and V. persoonii has medium virulence. However, V. ceratersperma displays intraspecific differences in pathogenicity: isolates with yellow-brown colony on PDA having strong virulence, isolates with milk-white colony having medium virulence, isolate with dark gray colony have weak virulence.
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