POPULATIONS OF NONPATHOGENIC FUSARIUM-OXYSPORUM ASSOCIATED WITH ROOTSOF 4 PLANT-SPECIES COMPARED TO SOILBORNE POPULATIONS

The effect of the plant on the diversity of soilborne populations of F usarium oxysponrum was assessed after successive cultures of flax, mel on, tomato, and wheat in separate samples of the same soil. Forty soil borne isolates of F. oxysporum and forty root-colonizing isolates of e ach plant species were sampled during the first (T0) and fourth (T1) c ultures. The population structures were assessed by a genotypic method based on restriction fragment analysis of polymerase chain reaction-a mplified ribosomal intergenic spacer (IGS) DNA. Sixteen IGS types were defined among the four hundred isolates analyzed. The distributions o f soil isolates among IGS types were similar at both sampling times. T he structure of F. oxysporum populations associated with the roots of flax or melon did not differ from the structure of soilborne populatio ns. In contrast, the structure of F. oxysporum populations associated with roots of wheat or tomato differed from the structure of soilborne populations. High frequencies were found for IGS type 4 among wheat i solates at both T0 and T1 and for IGS type 11 among tomato isolates at T1. Moreover, a high level of genetic divergence was obtained between IGS types 4 and 11. Our results suggest that tomato and wheat have a selective effect on soilborne populations of F. oxysporum and that thi s effect seems to be plant specific.