| Summary: | 碩士 === 國立中興大學 === 植物病理學系 === 93 === The main purpose of this thesis was to investigate the factors affecting the ascospore germination of Monosporascus cannonballus and to determine the ascospore densities in field soils by using circular sucrose and sedimentation tank method (CSSTM). Soil tube observation method (STOM) was employed to observe ascospore germination in rhizosphere soil and penetration into the root surface of muskmelon. Ascospore germination could reach 54.8% at 24℃, but ascospores did not germinate in the autoclaved soil. Several fungicides and antibiotics were used to treat the natural soil and the result showed that most of the tested chemicals could inhibit or reduce the ascospore germination. Several isolates of fungi, bacteria and actinomycetes isolated from different natural soils were added to the autoclaved soil and found that most tested actinomycete isolates could greatly increase ascospore germination. Using immersed ascospore tube method (IATM) to study the factors affecting the ascospore germination on agar medium, it was found that ascospores germinated at the distance of 0-2mm, especially greatly at 0-1mm, from the root surface, but rarely germinated beyond 3mm when the natural soils collected from crop fields were used as cover soil. Moreover, soils from non-crop fields, peat moss, and fish-jar sand also could stimulate ascospore germination but perlite (only 2.4%). By using IATM, when soil extract was added to perlite or muskmelon root extract was used instead of planting a muskmelon seedling, ascospores did not germinate, either. Four actinomycete isolates, A17, A21, A22 and A33, which could stimulate ascospore germination greatly by using STOM, were tested again for their abilities to stimulate the ascospore germination by operating IATM. They indeed greatly stimulated ascospore germination when autoclaved soil was used as cover soil. However, no ascospore germination was observed when their spore suspensions were added to the medium with no cover soil or with perlite as cover soil by using IATM. By using CSSTM, both ascospores and perithecia could be detected from the field soils and the ascospore densities of Monosporascus cannonballus in the top soils (depth of 0-10 cm) collected from Dongshan and Fangyuan muskmelon fields were 9.9 and 9.8 ascospores/g soil, respectively. The ascospore densities in Dongshan soil samples reduced to 1.28 from 10.4 ascospores/g soil after rotation with rice paddy once. Vertical distribution of the ascospores at Dongshan muskmelon field was also determined. It revealed that most wild ascospores existed within 30cm of soil depth and the deeper the lower ascospore numbers were detected. Few wild ascospores were detected within 30-50cm of soil depth, but sometimes intact perithecia could be observed. A carrot field at Chingchuankang and a bamboo field at Dalih were also subjected to determine their ascospore densities and yielded 5.6 and 1.8 ascospores/g soil, respectively. Furthermore, even in the soil samples collected from non-cropped fields, such as Chung Hsing Lake and Small Hall of National Chung Hsing University and Shougunuan River, the ascospore densities were 0.5, 0.1 and 0.2 ascospores/g soil, respectively.
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