The Occurrence And Epidemic Of Giant Ragweed Downy Mildew And Its Co-infection Mechanism With Rust Fungus

Giant ragweed (Ambrosia trifida Linn.) is an important alien invasive weed in China. Itscontrol work has incurred special awareness from weed scientists and public. The weed hascaused a serious problem on agricultural production, agricultural ecological environment andhuman health. Although more methods have been used to control the weed, such as artificialcontrol, chemical spraying and plant replacement, its rampant distribution has beeneffectively stopped. The measure of applying obligate parasites is a good strategy tosustainably control the weed. Puccinia xanthii Schwein. f.sp. ambrosiae-trifidae S.W.T. Batraand Plasmopara angustiterminalis Novotelnova, the two newly found fungal species on giantragweed, are obligate parasites of the weed, and have shown a good potential to biologicallycontrol the grass. In wild the co-infection of the two fungi usually contribute to the earlierdeath of the weed, showing expected biocontrol effect.The wild investigation, artificial inoculation and fixed plot observation indicated that thedowny mildew fungus is an obligate parasite, which develop vigorously under highertemperature and high humidity. It can continuously infect the plants of giant ragweed, and itoverwinters in diseased residues in form of oospores as the primary inoculums in the nextyear. The oospores were clearly observed under light microscope by methods of Commassiebrilliant blue staining, cotton blue-lactophenol staining and modified NaOH transparency, thelast method showed excellent observation effect.The host range of the downy mildew fungus was investigated by methods of field surveyand artificial inoculation in glass house. It is found that the downy mildew fungus only infectAmbrosia trifida and Xanthium strumarium, it could not infect the other plants of compositae,crops and ornamental plants. It is characterized with obligation and quick epidemic, it can beused for effective control of the weed.The genomic DNA of Plasmopara angustiterminalis was extracted by seven methods.The results showed that the methods of CTAB-steel balls and EZ-kit were better than the other methods due to their higher quality of DNA extraction with less impurity. The templateDNA samples generated from these method were detected by methods of ITS-PCR. They allproduced excepted band patterns. The research confirms that the DNA samples of the downymildew fungus generated from these two methods could be used for further molecular study.The co-infection mechanism and structures of the downy mildew and rust fungus on hostplant by means of light microscopy and electron microscopy.