Pathogenicity variation and genetic structure differentiation of Fusarium oxysporum f. sp. conglutinans in soil under successive cultivation of Brassica oleracea
Abstract: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 2nd cultivation to 1.608×105 cfu·g-1 soil after the 5th 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 1st to 0 after 3rd cultivation, in contrast with increment of proportion of highly pathogenic isolates (DI>50) from 6.7% to 16.7% after 4th 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 3rd cultivation. Thirty isolates obtained after the 3rd 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.