| Barnyardgrass [Echinochloa crus-galli (L.) Beauv.] is a troublesome weed in rice (Oryza sativa L.) field in the world. Yield losses caused by barnyardgrass in rice are severe. The fungal pathogens, Helminthosporium gramineum Rabehn f.sp. echinochloae ( HGE ) and Bipolaris sorokiniana had been developed as mycoherbicidal agents for the control of barnyardgrass in China.Leersia hexandra Swartz is a perennial weed, has negative effect on rice yield in recent years. The fungus identified as Cochliobolus miyabeanus has the mycoherbicidal potential against Leersia hexandra Swartz.The main contents of this dissertation are as follows:1. The conidia production of HGE in solid fermentation and the comparison of bioactivity of the conidia for Barnyardgrass control in greenhouseIn the solid-state fermentation system, the most abundant conidia of HGE were produced on the soybean-mix substrate with a low cost and benefited the large-scale production of conidia. Under greenhouse condition, the effects of the pathogen on disease incidence, mortality and dry weight reduction of barnyardgrass were tested to determine the potential of this mycoherbicide. The result of bioassay in greenhouse showed that the conidia produced from soybean/wheat bran/rice bran substrate had the highest virulence.2. Screening adjuvants for the mycoherbicide formulation of the conidia of HGEThe different adjuvants showed different effects on conidial germination and mycelial growth of HGE. And the effects also varied with different concentrations. Tween-20, Tween-60, Tween-80 and sucrose stimulated the conidia germination and mycelial growth without detrimental effects. However, Sillwet L-77, Triton X-100, Tea polyphenol and Tea saponin were fungitoxic to HGE. Soybean oil stimulated the conidia germination and mycelial growth. In summary, Tween-20, Tween-60, Tween-80, sucrose and soybean oil were useful components for bioherbicide formulations to increase conidial germination and mycelial growth.3. The control efficacy of HGE conidia suspension( SC) on weeds in paddy fieldField experiments in 2007 showed that the conidia of HGE displayed excellent activity on barnyardgrass and other weed species. Compared with the untreated control, the HGE treatments increased the rice yield without negative impact on rice.This study shows that the HGE is a potential mycoherbicide for the control of barnyardgrass in agricultural cropping systems.4. Screening of some herbicides for bioherbicide formulations with HGE conidiaWhat has been studied is the effect of six herbicides currently used to control barnyardgrass and other weeds in paddy field on HGE conidial germination and mycelial growth. Chemical herbicides 250 g/L Apiro SC and FENCERTM 25 OF plus 10% Pyrazosulfuron WP stimulate the conidial germination and mycelial growth, which can be used with HGE as complex formulation at low concentration or mixed use with HGE formulation for weeds control in paddy field.5. The infection process of HGE and the alteration of cellular ultrastructure of barnyardgrassScanning- and transmission-electron microscopes were used to observe the process of HGE infection and the alteration of cellular ultrastructure in infected barnyardgrass. The mycelium of HGE not only invaded from the stomas, intercellular spaces and wounds, but also developed infection cushion and directly invaded from the barnyardgrass leaf surface. After infection, cell membranes of barnyardgrass leaves were found dramatically changed, and the ultrastructure of cells was severely deformed. Therefore, the weed was interfered and controlled effectively.6. Studies on bioactivity of Bipolaris sorokiniana and Bipolaris coicis to the control of barnyardgrass and safety to the major cropsB. sorokiniana and B. coicis isolated from diseased barnyardgrass plants collected in Kunming, Yunnan Province were evaluated for their potential against barnyardgrass as a biocontrol agent. The results of bioassay showed that the culture filtrate from two strains 5 days after ferment had excellent inhibition activity. The root length inhibition rates were 99.6% and 95.8%, and the bud length inhibition rates were 91.1% and 82.1% respectively. Under laboratory condition, both two strains have 100% disease incidence on the detached leaves on the 5th day. Under greenhouse condition, whole plant bioassay suggested that the death of plants appeared on the 7th day and that on the 21st day 82.7% and 80.0% of the plants died. What's more, crop tolerance test showed that B. sorokiniana was safer to major crops than B. coicis. As a kind of biocontrol fungi, B. sorokiniana has good application prospects.7. Studies on bioactivity of Bipolaris oryzae and Cochliobolus miyabeanus to the control of Leersia hexandra Swartz and safety to the major cropsTwo fungi from diseased Leersia hexandra Swartz plants in Xishuangbanna of Yunnan were studied for their potential against L. hexandra Swartz as a biological control agent. The fungi were identified as B. oryzae and C. miyabeanus by the spore morphology observation and sequence analysis of their internal transcribed spacers (ITS1-5.8S-ITS2). The bioactivity of the fungi against L. hexandra was evaluated by detached leaf bioassay. The crop safety was tested using pot cultivation in the greenhouse. The treatments of culture filtrate of two strains obtained disease incidence as 88.9% and 90.7% respectively. The culture filtrate with mycelium suspension of two strains got 75.9% and 100% disease incidence respectively. These two strains were safe to major crops such as rice, maize, cucumber and tomato etc. C. miyabeanus got higher disease incidence on Leersia hexandra Swartz than B. oryzae, as a kind of mycoherbicide the former has the research and development potential.In all of the research results, it is new and original achievements that resulted from the study of the infection process of HGE and the alteration of cellular ultrastructure of barnyardgrass, and the finding of new strains B. sorokiniana, B. coicis, B. oryzae and C. miyabeanus. |
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