| Fitness of Phytophthora sojae, which causes Phytophthora root rot of soybean, was evaluated in Jiangsu Province, China. The fitness strongly depends on the production of oospores of P. sojae in that oospores are not only the primary origin of infection, but the only stage which can survive in soil for a long time. Accordingly, sexual reproduction plays an essential role in the life cycle of Phytophthora and in the pathogenesis. To study the factors effecting the sexual reproduction of Phytophthora and relative oomycetes may have some constructive meanings. Several experiments about the effect of lipids on sexual reproduction of Phytophthora and Pythium were also conducted.The pathogenicity of four isolates of P. sojae, including S317 from China and Pmg-1, Pmg-2, Pmg-3 isolated from soil mixed within commercial soybean imported from America, to 30 popular soybean cultivars in Jiangsu Province was studied by wound-inoculation in the hypocotyl of seedlings. The results showed that all the cultivars were susceptible to isolate S317, and isolates Pmg-1, Pmg-2, Pmg-3 were pathogenic to 27 out of 30 cultivars with 3 cultivars being resistant. The survival potential of oospores of 4 isolates in the field of Nanjing, Jiangsu Province was also studied. The results indicated that 6.6%~13.3% of the oospores could still germinate in laboratory conditions after burying in soil for 9 months. So it could be concluded that P. sojae had strong fitness in Jiangsu Province, and the pathogen possibly could threaten soybean production in the area in case of being introduced.The effect of lecithin on sexual reproduction of P. boehmeriae, P. sojae, P. cactorum and Pythium aphanidrematum on basal medium were tested. The sexual reproduction of all species tested were stimulated by lecithin. And extracts from mycelia of P. boehmeriae, P. sojae and P. cactorum grown in liquid basal medium were tested for their ability to support oospore formation. The fact that the three species were responsive to their own mycelial extracts respectively indicated that the strains tested were able to synthesize substances needed for sexual reproduction. Nutrient deprivation on water agarose medium was used as a stress factor to trigger the process of sexual reproduction of P. boehmeriae, P. sojae, P.cactorum and Pythium aphanidrematum. Only Pythium aphanidrematum fonned oospores on water agarose medium.Three kinds of palmitate glycerides were tested on basal medium, after purification by Florisil column chromatography, for their ability to stimulate oospore formation in P. boehmeriae (strains Ec-8, Jc-20, Js-2), P. sojae (strains S317, CN-1), P. medicaginis (strain 44390) and Pythium myriotylum (strain 357F). For strains Ec-8, Jc-20, Js-2 and S317, 1-monopalmitoyl-rac-glyceral was stimulatory to sexual reproduction, while Dipalmitin and Tripalmitin were slightly stimulatory. For strains CN-1 and 44390, the three palmitate glycerides all have no effect. |
