Characterization Of Anti-fungal Substances Produced By The Mycoparasite Coniothyrium Minitans And Evaluation Of Their Efficacy In Suppression Of Sclerotinia Sclerotiorum

Sclerotinia stem rot caused by Sclerotinia sclerotiorum (Lib) de Bary is an important disease of oilseed rape with a worldwide distribution. Coniothyrium minitans Campbell is an effective biocontrol agent of S. sclerotiorum. The mycoparasitism of C. minitans on S. sclerotiorum was profoundly studied and exploited as a mechanism in biological control. McQuilken et al. (2003) reported that C. minitans could produce antifungal substances (shorted as AFS_cm) and an inhibitory metabolite named macrosphelide A was identified. However, the antifungal mechanisms involved in interaction between C. minitans and S. sclerotiorum are still unknown and need to be clarified. The purpose of this thesis is aimed at answering questions including: (1) the effect of AFS_cm on germination of sclerotia or ascospores of S. sclerotiorum; (2) effect of nutritional factors on production of AFS_cm; (3) basic properties about the AFS_cm; and (4) efficacy of the AFS_cm in suppression of the infection by S. sclerotiorum.The modified Czapek-Dox (MCD) was used to incubate C. minitans and the AFS-containing cultural filtrate (MCD_cm) was used to test its inhibitory effect on mycelial growth of S. sclerotiorum, on germination of sclerotia and ascospores of this fungus. The results demonstrated that MCD_cm did inhibit mycelial growth (MG) of S. sclerotiorum and the relationship between the concentration of MCD_cm (x) amended in potato dextrose agar (PDA) and the percentage of inhibition of MG (y) fitted the equation : y=19.268Ln(x)+44.152 (R²=0.9703, P<0.01). The inhibited mycelia of 5. sclerotiorum showed severe abnormal symptoms including cytoplasma granulation and outward leakage, cellular swelling and collapse. Treartment of S. sclerotiorum sclerotia with AFS_cm resulted in a delay in myceliogenic germination implying that the AFS_cm was toxic to sclerotial cells, however, without any lethal effect. On PDA plates with and without the amendment of AFS_cm, ascospores of S. sclerotiorum germinated at 99.4% and 98.8%, respectively, after incubation for 12 h. However, growth of the germ tubes were suppressed by the formation of swollen cells in the presence of AFS_cm.Strains of C. minitans, and media MCD/PDB were compared for production of AFS under the shaking incubation conditions (20 ℃ and 200r/min). The results showed that the wild-type strain Chy-1 of C. minitans and its mutated strainUv-2003 were able to produce AFS, whereas other two mutated strains Uv-202 and Uv-1731 was unable to produce AFS. Production of AFS differed greatly in the media MCD and...