| Water hyacinth, Eichhornia crassipe~ (Mart.) Solms, is one of the worst weeds in the world. In the middle of 1990, International Organization of Biological Control organized a Global Working Group to develop technologies for bio-control of water hyacinth. In China, two species of phytophagous insects were introduced for control of water hyacinth in 1995. However, development of bio-herbicide for control of this weed is on the initial stage in China. The requirement for integrated management of this weed to make the work forward is urgent. Therefore, this study focused on screening of effective pathogen against water hyacinth. Pathogenicity of twelve fungi strains, belonging to three genera and five species respectively, was compared in greenhouse screening test. On the basis of the screening results, the most virulent fungus strain, Alternaria eichhorniae C4 16, was selected to be a potential agent for developing bioherbicide. The host-specificity, biology and sporulation characteristics of A. eichhorniae C416 were studied. By using scanning electron microscope (SEM) and transmission electron microscope (TEM), the process of infection and ultrastructural host response were observed. The results are as follows:(l)The results of pathogenicity tests indicated that A. eichhorniae C416 was more virulent than others. The mean disease severity (DS) on leaves was up to 3.78 in 12 days after inoculation with mycelium of A. eichhorniae C416. All the inoculated leaves were killed within 6 weeks, and the whole mean DS on leaves was 7.04.(2)None of 14 economically important plants and cultivars evaluated in host range test was susceptible to A. eichhorniae C4 16 except for water hyacinth. The result further affirmed A. eichhorniae is host- specific.(3) In the greenhouse tests, temperature and moisture were main influent factors of infection and pathogenicity. A. eichhorniae C4 16 caused high DS and rapid rate of disease development in 30 0C. The optimal period with high humidity for the initial infection of A. eichhorniae C416 was 16h. Lower light intensity (l200Lux) cause 46% DS on leaves more than that of higher light intensity (3000Lux). The optimal inoculum density (ID) was 25% (wet weight of mycelium: water).(4) For mycelium growth, the optimal growth temperature range was from 25 0C to 30 0C. The best growth temperature was 28 0C. The best initial pH of mycelium media was 7. The best carbon source for mycelium growth wassucrose, and the best nitrogen source was peptone. The oPtimal carbon-tO-nitrogen ratio is 60:l. Adding Vitamin B. and BI, in the media improved theproduction of mycelium significantIy, but adding Mn, Co and biotin did notshow significant differences. Whereas, adding IAA in the media inhibited thegrowth ofmyceliurn significanly.(5) Light is necessary for A. eichhorniae C4l6 to sPOrulate. ExPOsure tonear-ultraviolet light and at initial pH4 or pH9, C4l6 accelerated conidia grOWthand produced more sPOres. The best carbon source for sporulation media wasmaltose, and the best nitrogen was NaNO,. The oPtimal carbon-to-nitrogen wasl0:l. Adding Viboin B,, B,,, biotin and IAA, esPecially IAA, improved theproduction of conidia significantly, whereas adding Mn, Zn did not show muchsignifican differences. According to these results, the optimal solid-phasesPOrulation media consisted of Maltose, NareO,, KCl, MgSO., KH,PO,,Na:HPO., adding l0mg/L IAA, and carbon-to-nitfogen waS l0: l.(6) The infechon process was observed by SEM. Penetaling waerhyacinthleaves by mycelia occured not only through the stomata, but also through theleave sdse directly. The whole infection finished within l6 to 20 hours. ThetyPical ultrchCture host responses were that the membrane strUcbe wasdrarnatically changCd, with starch particle in chloroplasts disaPPCaring,cytoplastic connection growing thickee lots of phenolic comPounds produced byinfected cells.All the above results provided basic data and theoretic foundation fordevelopment of bioherbicide a...
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