Inheritance And Variation In Virulence Of Phytophthora Sojae And Factors Affecting Viability Of Oospores

Factors affecting viability (the percentages of activation,dormancyand death) of oosporesof PhytoPhthora sOjae 597-3 were evaluated by using tetrazolium bromide. a vital stain.Single-zoospore segregated progeny isolates from successive three or four generations ofPsOjae 597-3 parent race 44 single-zoospore isolate were evaluated by using wounded excisedpetioIe inoculation method for virulence on eight differential soybean cultivars which cnyrace-specific resistant genes. The results were summallzed as follows:Oospores cuItIJre age, pretreatment temperature, chemicals, host root exthet, and soilextraCt affecting viability of oospores of Psojae were evaluated by using MTT Both of thepercentages of activation and death enhanced graduaIIy with oospores culture age prolonging,reversely, the percentage of dormancy declined gradually. The percentage of activation of45-day-o1d oospores was 24.52%, the highest one. Both of the percentages of activation anddeath of 30-day-old oospores pretreated with -l aC \ -20t. 35oC and treated with KMnO4 atdifferent concentrations were higher than control (sterile distilled water), reversely, thepercentage of dormancy was less than control. The Percentage of activation of oosporespretreated with 35C for five days was 83.55%, the highest one. The percentage of activationof oospores treated with KMnO4 at 0.4% was 58.49%. 7t pretrearment, H2O2 tretrient, hostroot extract, and soil extract lightly affected viabi1ity of oospores.The is the first report that using wounded excised petiole inoculation method to evaluatevirulence of PsOjae.Variation in virulence on cuItivars PI. l03 (hel d) and Harlon (he l a) of single-zoosPOresegregated progeny isolates of successive three or four generations of PsOja was very high,from virulence to avirulence or intermediate.Variation tendency in virulence on cultivar Pl. l03(held) was loss in virulence, and on cultivar Harlon (forla) was Perservation in virulencethrough several generaions' iItheritance and vchation. Vdriation tendency of single-zoosporesegregated progeny isoIates of successive tWo and three generations of No.l single-zoosporeisoIate from the first generation race M single-zoosPOre isolate was from race 44 to race 3,also had some single-zoospore isoIates which viruIence formula were 7 or l47. Vatiationtendency of single-zoospore segregated progeny isolates of successive tWo and threegenerations of No.30 single-zoospore isolate from the first generation race l single-zoosporeisolate was from race 1 to some single-zoospore isolates which virulence formula were 7, also to race 3 or la,7. Variation of virulence among most of single-zoospore isolates in every generation happened and produced different races or virulence formula. Basically, variation in virulence of single-zoospore segregated progeny isolates of successive three or four generations of P.sojae 597-3 can't inherit steadily.Virulence of thirty-one single-zoospore isolates of P.sojae 597-3 was not associated with their mycelial growth rates.Virulence of P.sojae 597-3 culture was evaluated in three different years .The result showed: variation in virulence happened, P.sojae could defeat two resistant genes (3a,7) in 1997, and five resistant genes (la,lb. lc,6,7) in 2000. and seven resistant genes (Ia,lb,lc,ld.lk,6,7)in2001.