| Late blight (LB) is one of economic important devastating diseases of potato and tomato both in domestic and in the whole world, which was caused by Phtophthora infestans belong to Oomycetes fungus. Yearly average losses of LB in China are high up to 1 billion $. The main reasons for rebound of LB in recent years are that composition variation of pathogen population, appearance of A2 mating type in several areas, induced arousing of isolates of antifungicides and new physiological races. Chongqing is one of the very important cultivation areas of potato and tomato in China, but LB epidemic rate in some areas is quite severe. To seek major factors of LB rebounding, the biological characteristics including pathogensis of pathogen, isolation and purification approaches, mating types, resistance of potato varieties currently available have been evaluated. Several work resultes are listed as follows. 1. Phtophthora infestans isolation and pathogenesisRoutine isolation way, dew chamber way, disease leaf claping with potato slices and induction with potato tuber were tested, besed on semi-composed medium of RSA plus several antibiotics. Total 186 isolates from representive areas of potato and tomato in Chongqing were obtained and purified through cutting the top of mycelium of pathogen. Bioassay of typical isolate of PXX-0101 was carried out to determine the growth and sporulation of pathogen. The results showed that pathogen growth rate on tested media is as following order: RSA>RA>RSB>SVA>RTA>LVA>VA>MA>PSA>PDA; RSB is best for sporulation, then RTA.Using inoculation in vitro with detached leaves of seedling indicatedthat both isolates frompotao and tomato are able to infect potato and tomato and to cause typical symptom of LB. Also can cause re-infection to hosts.2. Determination of metalaxyl resistant isolates ofPhtophthora infestansMetalaxyl resistant isolates of Phytophthora infestans were determined using growth rate method on RSA plate. The resistant isolates of tomato pathogen from vegetable cultivation bases in Bisan and Shapingba are dominant population with 75% frequency. The resistant isolates from potato in Yunyang County are also shown high frequency with 84.2%. Otherwise the pathogen population of sensitive isolates among Kaixian, Wanzhou, Zhongxian, Beibei, Bisan, Wuxi counties were dominant, the ratios are 87. 2%, 92. 0%, 71. 4%> 66. 7%, 91. 7%, 88. 9% , respectively. Some indivdual isolates' resistant levels to metalaxyl are 10?times much higher than referance isolate of PBB-1102.Determination both sensitive and resistant isolates against fungicides, such as Metalaxyl, Oxadixyl, Cymoxanil, Fentin acetate were conducted for altenative resistance of fungicides mentioned above. The results suggested that Metalaxyl, Oxadixyl, Fentin acetate have interactive resistance among three fungicides, while Cymoxanil without interactive resistance to other three fugicides.Main bio-characteristics of Metalaxyl-resistant isolate PYY-1135 was determined on RSA medium. The temperature range of mycelium growth are 8-29 癈, optimum temperature are 13-25"C; sporulation temperature range are 11-27癈, the optimum temperature are 15-23癈. While the temperature range of mycelium growth of the sensistive isolate PKS-2142 are 8-29癈, optimum temperature are 15-21 癈, sporulation temperature range are 11-25癈, optimum temperature of sporulation are 15-21 癈.Resistant isolate to Metalaxyl (PYY-1135) was subcultured continuely and evaluated the sensitivity against Metalaxyl on RSA without fungicide or on detached leaves of potato. The results showed that the sensitivity of Metalaxyl-resistant isolate of Phytophthora infestans was not changedduring subcultured isolates. Resistant isolate (PYY-1135) and sensitive isolate (PKS-2142) to Metalaxyl were inoculated on detached leaves of potato to determine the index of parasitic fitness. Infection frequency, leasion area and sporulation capacity of the former were higher than the later's. Metalaxyl resistance frequency of subcultured mixed isolat...
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