Inheritance And Infection Process Of Phytophthora Sojae

Phytophthora root rot of soybean (Glycine max(L.) Merr.), caused by Phytophthora sojae, is a major disease of soybean worldwide, and occurs at local region in China. P. sojae was A1 import quarantine fungus and quarantine fungus in China. The highest economic losses arise as a result of both direct plant loss and the costs of replanting when favorable environmental conditions occur just after planting. P. sojae has been managed successfully over the years with single gene resistance (Rps genes). But sometimes the Rps genes can not resist the P. sojae infection because of the occurrence and development of new races of P. sojae. It was a homothallic species, of which inheritance research was not reported in China. The study on inheritance, variation of growth rate, colony morphology, sporulation, homothallism, virulence and resistance to metalaxyl using classic genetics method open out the genetics fundament of the inheritance characteristics, diversities, moreover to control disease with Rps genes efficiently. The results were obtained as follows: 1. The production condition of zoospore of P. sojae was studied. The results showed that the mycelia could be induced to produce sporangia that released zoospores by changing intermittently. The isolates of P. sojae were cultured on LBA or V8A medium for 4-8 days. Five or more culture blocks (8 mm diameter) were cut and transferred to a 7 cm plate with distilled water just covering the surface of culture blocks. 15 ml Petri nutrient solution was added into the culture dish after having changed water four times at 30 minutes interval. After being incubated under 25 ℃ in darkness for 18-20 hours,mycelia could produce a large number of sporangia that released zoospores.2. The germination condition of oospores of P. sojae was studied. The results showed that the oospore could germinate on water agar(WA)or on thin coat of soil. On WA only germ tubes were produce. On the coat of soil both germ tubes and sporangias were produced. The optimum temperature for oospore germination was 18-24 ℃. Light stimulated germination of oospores but restrained them to produce sporagia. Germination of oospores did not need external nutrient. Oospore germination rate differed significantly among different isolates.3. The inheritance and variation of biological characteristics including growth rate, colony morphology, sporulation and homothallism of P. sojae were studied in vitro. The results showed that the colony morphology and the growth rate were all steadily inherited in both single-zoospore and single-oospore progenies, which indicated that the genetic factors controlling these two characteristics appeared to be homozygous. However, sporulation of zoospore was markedly different in asexual and sexual progenies, which suggested that the genetic factor controlling sporulation might be heterozygous, and liable to be affected of environment factors. The homothallic characteristic of the isolates tested was steadily inherited in both asexual and sexual progenies, which suggested that the genes controlling the homothallic characteristic appeared to be homozygous.4. The single–zoospore progenies from a wild-type of P. sojae was selected as the parent isolate, from which two successive single-zoospore progenies and one single-oopore generation were established to evaluate the inheritance of the virulent of P. sojae. The successive variation of some virulence genes was observed in the both two single-zoospore and one single-oospore progenies, and the inheritance of other virulence genes was stable. The results indicated that the virulence genes controlling the pathogenicity of P. sojae were located difference inheritance site. Some virulence was steadily inherited, which suggested that the virulence genes controlling the pathogenicity appeared to be homozygous nuclear gene. Some virulence was not steadily inherited, which suggested that the virulence genes controlling the pathogenicity appeared to be heterozygous nuclear gene or cytoplasmic gene, and the variation of virulence of...