| Fifty-eight single spore strains, from 345 samples of seedling blight of rice in dry-raised nursery, have been identified as 6 species of Fusarium according to their asexual stage. Isolations of Fmoniljforme were majority with the ratio of 60.3 5%; the ratio of F. solani, Foxysporum, F semitectum, F lateritium and F. lateritium was 5.17, 13.79, 6.90, 10.35 and 3.45%, respectively. The result of inoculation showed Fmonil~forme had the highest pathogenicity .The percentage infected by pathogenic Fusarium spp. from middle and base of root was higher than that from tip of root and base of stem. Seedlings of 6-8day rice were the most sensitive to pathogenic Fusarium. spp. The rice seedlings treated with filtrates of different liquids medium to culturing Fusarium spp. could lead to the same or more serious symptoms of blight. Toxin secretions were produced by some isolates of Fusarium spp such as FY3A3X3 and DYA3X2 cultured by PD. The filtrate of PD cultured them has pathogenicity and the symptom is similar to that caused by the pathogens. That is to say, the pathogens can secrete toxin secretions to culture. The secretions have two function, one is stimulating rice growing up just as Gibberellin, another is inhibiting rice growing up as DON toxin. The secretions were demonstrated as Gibberllin-A3 and DON analysed by HIPLC. Seven of eleven strains of Pythium spp., identified as Pythium ultimum Trow, P. mamillatum Meurs, P.salinum Hohnk, P.vexans de Bary, P.acrogenum YU, P.intermedium de Bary , P.spinosum Saw, were of pathogenicity to rice seedling. P.salinzim Hohnk P.Qcrogenum YU , Pintermedium de Bary showed higher pathogenicity to rice seedlings than the other species of Pythium spp.The ratios of root tip and stem base of seedling infected by 96 pathogenic Pythium spp.were higher than that of root middle and base. The peroxidase in rice buds and conduction ratio in filtrate which culturing the buds rise up after 24h inoculated PYO3(Pythium .ca/imum )-~ PYO6 (Pythiuni intermedium) ,and they reach the tip in 3 days. At the same time, pectin lyase(PL) rises up gradually and reach the tip in 5 days. The result show that the main mechanism of Pythium spp. to inroad hosts is to product PL to destroy hosts?cells wall, causing protoplasm gone out, tissue destroyed, host blighted. Another main basis is the action of PL in mycelium has been monitored. Rhizoctonia spp. are most sensitive to tridemefon and the EC50 of the two isolates RH1 and RH2 to the fungicides is 0.23 and 0.62 11 gIml respectively. Correspondingly, the EQ0 of the two isolates to tebuconazde and tachigaren is 4.88, 5.43 i-i glml and 13.91, 14.16 1-? g/ml. Compared with the post three fungicides, Rhizoctonia spp. are not sensitive to xiuxiaochun relatively. The EQ0 of Pythium spp. to tebuconazde, xiuxiaochun, tridemefon is 5,9.09,12.79 Li gIml arranged from lower to higher. It is interested that Pythium spp is not sensitive to Tachigaren in vitro and the EC50 up to 20.38 1~ gIml. MIBC is the best fungicide to control growing of Fusarium spp., the EQ0 of Fusarium spp to the fungicide below 4.67 ~t glml. Great advantage have gain in controlling seedling blight of rice by fungicides such as tachigaren, ridomil, fenaminosulf, propionazole.The effect is more befter used in mixture fungicides than used in sole. Three methods, seed soaking, watering or spraying after discover p...
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