Screening And Evaluation Of Fungi Degrading Sclerotia Of Villosiclava Virens

Rice false smut is caused by the fungus Villosiclava virens that infects specific rice stamen filaments and forms curveball with a large number of chlamydospores outside. Sometimes one or several sclerotia can be produced on the surfaces. Recently our laboratory found that the ascospores produced by the sclerotia probably is the dominant primary infection inocula,and play an important role in the life cycle. Therefore control of sclerotia and ascosporeswill be animportant way to the disease management.Fungicidesnow have been used broadly in rice bootingstage to control the disease, but the effect is often influenced by environmental factors such as rainfall.Furthermore, fungicides have many potential problems, such as pesticide residue, environmental pollution and fungal resistance. In recent years, research using antagonistic microorganisms to prevent and control plant disease progress fast, but there has been nobiocontrol agent for the sclerotia of rice false smut to be reported. The sclerotia of Villosiclava virenshas been confirmed tobe able to overwinter and produce great amount of ascospores as the primary inocula in the coming year, so that the measures that can decrease the sclerotial number in paddy field will be useful in the disease control in practice. In this study, some sclerotium-degrading fungi against V. virens were screened out, and their action mechanism and efficacy in field were carried out as well.The main results are as follows:1. Sclerotium-degrading fungi against V. virens were screened from paddy field soil, the sclerotium-attached fungi, and the fungi from Tibetan plateau, respectively. Among them, six fungal strains, Bionectria ochroleuca, Gliomastix polychroma, Aspergillus fumigates, Penicillium oxalicum, Gliocladium roseum, and Myrothecium verrucariacould degraded the sclerotia rapidly.2. Most of those strains could degrade completely the sclerotia within 15-70 d. The hyphae of Bionectria ochroleuca could cover whole sclerotium within 5-7d and decompose sclerotium completely within 10-15d, whose degradating effect is best among the six strains. The hyphae of Gliomastix polychroma and Penicillium oxalicum couldembedthe entire sclerotium within 5-7d but they would take 25-30d to decompose sclerotia. The sclerotia degradation with Clonostachys spp. and Myrothecium verrucariawould take about 70d.3. Ultrastructural examinations showed that the degradation mechanisms included myco-parasitism and saprophytic modes, respectively. The myco-parasitism strain were usually employed byBionectria ochroleuca, Gliomastix polychroma, Penicillium oxalicum,and Clonostachys spp., whose hyphae grow along the intercellular space, and insert into sclerotium cell through the cell wall, then penetrate into the cells of sclerotia, and at last digestthe cell walls. Aspergillus fumigates and Myrothecium verrucariacould secrete cell wall hydrolases to degrad and destroy the whole sclerotium before the hypha invading into the sclerotium cells.4. With spreading in paddy fieldsin the spring,two of the biocontrol strains, Sc-C-1 and Sc-H, could decrease the false smut balls by 33.9% and 48.6%, respectively.In addition, the cytological interactions between resistant wheat cultivars and head blight pathogen were also carried out. The results showed that after penetrating the host cells, the pathogen dorminantly grew down along the panicle stalk. The pathogen could extend in cortex cells in cultivar Sumai 3 and just grew intercellularlly in cultivar Wangshuibai, but could not enter host vascular tissues. In the resistant lines L658 and L699, there were a few of pathogen hyphaeto be detected in the parenchyma cells around the vascular tissues. These give novel clue to future research on the relationships between host cell wall components and resistance.