The Identification Of The Pathogenic Fusarium Spp. And The Preliminary Study On Their Pathogenic Mechanisms In Apple Replant Disease

Apple Replant Disease (ARD), distributed in all over the apple producing areas around the world, is the main problem that affects the replacement of the apple orchards. It is also a serious threat to the sustainable development of apple industry.Based on the isolation of the pathogen of the ARD, the Fusarium which had a highest isolation frequency were tested. In this study, 21 isolates were identified by observation of the morphological characteristics and determination of molecular features. Begonia seedlings were used to test the pathogenicity of the strains; the radicle of the Begonia were also used to test the pathogenicity of the strains, and this maybe a new method to test the pathogenicity of the isolates. Five Extracellular wall degrading enzymes (PG, CX, PMG, PGTE and PMTE) were tested in this study, with the help of the CODEHOP. Degenerate primers were designed for cloning the fungal polygalacturonase gene fragment and the polygalacturonase gene fragments were cloned and analysed. The results are as follows:1. In this study, five different species of pathogenic Fusarium include F. oxysporum, F. equiseti, F. acuminatum, F. proliferatum and F. solani were identified. Among them, F. proliferatum, F. equiseti, F. acuminatum were the first report as the pathogens of the ARD.2. The results of the pathogenicity to the Begonia seedlings showed that different strains had different pathogenicity, and the pathogenicity had no correlation with the species and the geographic origins of the strains. The corrected mortality of the seedlings treated by the strongest virulent isolate was 50% while it was just 3.3% treated by the lowest virulent isolate.3. After infected by the test fungi the radicles became brown and short. The result showed that it had a positive correlation between the browning rate and the mortality of the seedlings, and it had negative correlation between the radicle length and the mortality of the seedlings. Thus, using radicles maybe a new method to test the pathogenicity of the isolates.4. 5 Extracellular wall degrading enzymes (PG, CX, PMG, PGTE and PMTE) could be produced by the strong virulent isolates and the low virulent isolates, the activities of PG, CX, and PMG produced by these strains showed a postive correlation with the pathogenicity.5. With the help of CODEHOP, degenerate primers were designed for cloning the fungal polygalacturonase gene fragment. The DNA fragments had high similarity with the PG5 from GenBank, This indicated that the product was the fragment of the PG5.