In the paper, the systemic research on the biological characteristics of downy mildew is conducted. The results show that the preference temperature range for its sporangium germina-tion is from 15 to 20℃ with the optimal temperature of 18℃. Sporangium germination needs higher humidity because humidity is below 95% sporangium fails to germinate. Soluble starch has a strong role in promoting the sporangium germination;after being treated with soluble solu-tion for 3h, the germination rate of sporangium increases from 37. 7% to 90. 9%. The treatment of sporangium under the condition of low temperature reduces the sporangia germination ability. the longer the processing time is, the lower the germination ability is. Under the condition of constant temperature, within the scope of 10 ℃ ~20 ℃, with the increase of temperature, downy mildew infection ability enhances. When the moisturizing time is less than 2h, downy mildew can' t infect Ambrosia trifida, which shows humidity condition is the important factor af-fecting the morbidity of Ambrosia trifida.

In this paper, lignin deposition and the chloroplast structure were observed and initial-ly studied by means of light microscopy, Transmission electron microscope ( TEM) after Plas-mopara angustiterminalis Novotelnova infecting on the host giant ragweed. The result shows that P. angustiterminalis could cause the damage of chloroplast membrane and inclusion outflow of leaf, decline of photosynthesis and eventually inhibit the growth of the giant ragweed. Lignin deposition increases significantly due to infection of downy mildew.

The characteristics of melanin,extracted from cell wall and fermented filtrate of Ascochyta anemones, were determined by diagnostic test and infrared spectroscopy. The results indicated that they have similar physical and chemical properties,and they are soluble in KOH,H2 O2 and NaClO,but not in water,ethanol and acetone. Infrared spectrum analysis showed that the wall-bound melanin and extracellular melanin belonged to the same type. Tricyclazole,a specific inhibitor of DHN melanin synthesis,could inhibit the fungal melanin’s biosynthesis of A.anemones. The homologous fragment of polyketide synthase gene (AaPKS)was amplified by PCR using A.anemones genomic DNA as a template. The results suggested that the melanin in A.anemones might be synthe-sized from DHN pathway.