连作条件下土壤中甘蓝枯萎病菌的致病力变化和种群遗传结构分化

 枯萎病菌致病力的变化可能是连作条件下甘蓝枯萎病严重发生的重要原因之一。本研究在建立适度感染的人工病圃的基础上连续种植甘蓝5茬,每茬收获后随机采集土样。利用驹田氏培养基通过稀释平板法对连作土壤中尖孢镰刀菌种群数量的监测结果表明,连作后尖孢镰刀菌的数量由第二茬后的3.047×10⁴ cfu·g^-1土壤增加到第五茬收获后的1.608×10⁵ cfu·g^-1 土壤。对各茬后所分离30株甘蓝枯萎病菌（Fusarium oxysporum f. sp. conglutinans, Foc）的培养性状的观察结果表明,连作后Foc菌株在菌落形态、菌落扩展速率和产孢量等方面均发生明显的变化。用浸根法进行的致病力测定结果表明,随着连茬次数增加,弱致病力菌株占总供试菌株的比例逐渐变小,到第三茬后由第一茬的6.7%下降为0;而强致病力菌株的比例逐渐上升,由第一茬后的6.7%上升到第四茬后的16.7%。利用11条寡聚核苷酸随机引物对受试菌株进行PCR-ISSR扩增,结果显示从第三茬后Foc群体遗传结构出现分化。UPGMA聚类分析结果表明,第三、四和五茬后的Foc菌株都分为A和B两个类群,每个类群又分为Ⅰ和Ⅱ两个亚类群,但同一类群和亚类群中包含不同致病力的菌株,未发现病菌的致病力变化与遗传结构分化之间的相关。

Pathogenicity variation and genetic structure differentiation of Fusarium oxysporum f.sp.conglutinans in soil under successive cultivation of Brassica oleracea

Fusarium oxysporum f.sp. conglutinans (Foc) causes serious wilt disease of Brassica oleracea. The objective of this research was to investigate the mechanisms of pathogenicity variation under successive cultivation system frequently presented in China. After gaining a moderately infested field by inoculation of Foc, the cabbage was planted successively for five times and the soil samples were taken after each cultivation. Quantitative assay by soil dilution method on Komada’s medium plates indicated that the population was increased from 3.047×10⁴ cfu·g^-1 soil after the 2^nd cultivation to 1.608×10⁵ cfu·g^-1 soil after the 5^th cultivation. Comparison among the original Foc isolate and the isolates obtained after each cultivation indicated that the variation occurred in pigment production, growth rate and conidial quantity as the cultivation succeeded. Pathogenicity tests by root dipping in the 30 isolate cultures demonstrated that proportion of the Foc isolates with weak pathogenicity (DI=0-20) declined from 6.7% after the 1^st to 0 after 3^rd cultivation, in contrast with increment of proportion of highly pathogenic isolates (DI>50) from 6.7% to 16.7% after 4^th cultivation. Investigation on genetic structure of Foc populations after each cultivation using inter-simple sequence repeat (ISSR) with 11 pairs of primers indicated that population genetic differentiation was obvious after the 3^rd cultivation. Thirty isolates obtained after the 3^rd cultivation could be clustered into two distinct clades with two sub-clades under each by UPGMA analysis. However, there was no obvious corresponding relationship between the clades and pathogenicity of the tested Foc isolates.