Oospores Formation And RDNA-ITS Sequence Analyses Of Pseudoperonospora Cubensis

Cucumber downy mildew is one of the most common and destructive diseases of cucumber. The spread of the pathogen are rapidly, and the disease always destroys entire plants of susceptible varieties. In this study, the formation of oospores was investigated in 10 cities of china, the factors affecting oospores formation and germination were studied and rDNA-ITS sequence was analyzed. The purpose of this study is to understand the genetic variation of Pseudoperonospora cubensis and primary infection. The results were summarized as follows:(1) Oospores were discovered in Mohe, Harbin, Shenyang, Changchun, Baoding, Liaocheng, Yinchuan, Xining except Haikou and Yangling. Oospores usually concentrates in superior and inferior parts of plant, rarely exists in plant middle region. The quantity of oospores from Shenyang is most and that from Baoding is fewest; the size of oospores come from different city is dramatic different. The oospores from Shenyang are largest and that from Xining are smallest.(2) The levels of factors affecting oospores formation arranged from high to low are temperature, contactant, maturity, humidity and time. The eligible condition of oospores formation is that temperature is 4℃, relative humidity is 90%, rhizosphere leaf contact with soil for 50 days. Oospores could not form when the temperature is higher than 36℃and relative humidity is lower than 15%. Illumination does not influence the formation of oospores.(3) The study of viability of oospores during overwintering indicate that activation rate of oospores gradually rise from January to May and the peak appeared in last third of May, the percentage of activation is 14.01%, and then fall-off.(4) Temperature, KMnO4, H2O2, leaf tissue fluid and soil maceration extract could significantly enhance the activation rate of oospores. The optimal condition for activation of oospores is that 36℃, 0.3%KMnO4, 0.7%H2O2, dealed with leaf tissue fluid or soil maceration extract for 9 days.(5) Through rDNA-ITS analysis, the sequence length of 34 Pseudoperonospora cubensis from 12 cities of China is 802bp and there are 866 mononucleotide polymorphism sites . There is correlation between the source and geographic areas. The similarity among 34 isolates is 47.87% to 100%, average genetic distance is 1.231±0.200.(6) All strain could be divided into 3 groups: RG1,RG2 and RG3. RG1 includes Pc-qh1, Pc-qh2, Pc-qh3, Pc-qh4, Pc-nx1, Pc-nx2, Pc-nx3, Pc-nx4, Pc-bd1, Pc-bd2, Pc-bj1, Pc-bj2, Pc-bj3, Pc-bj4, Pc-sd1, Pc-sd2, Pc-sd3, Pc-yl; RG2 includes Pc-hrb1, Pc-hrb2, Pc-hrb3, Pc-hrb4, Pc-mh1, Pc-mh2, Pc-cc1, Pc-cc2, Pc-cc3, Pc-cc4, Pc-cc5, Pc-shy1, Pc-shy2, Pc-shy3, Pc-shy4; RG3 includes Pc-hz1 and Pc-hz2 .