Diversity of pathogenic Fusarium populations associated with asparagus roots in decline soils in Spain and the UK

Asparagus decline is a disease associated with several species of Fusarium. In order to assess the relative significance of causative species, single-stranded conformational polymorphism (SSCP) analysis of the ITS2 (internal transcribed sequence) region of the ribosomal DNA was used to rapidly and objectively identify the fusarial populations associated with the roots of two intensively sampled asparagus crops, one in the UK and the other in Spain. Over 360 fusarial isolates were obtained from fields showing symptoms of asparagus decline, and most were easily differentiated by SSCP into four principal species, F. oxysporum f. sp. asparagi, F. proliferatum, F. redolens and F. solani. Fusarium oxysporum f. sp. asparagi (Foa) was most frequently isolated from the UK site (69%), whilst Foa and F. proliferatum were found in similar proportions overall (40 and 39%, respectively) from the Spanish site, although individual fields showed considerable intraregional variation. Other minor populations, such as F. culmorum, were also found. Most isolates were highly pathogenic to asparagus in vitro, although F. solani isolates comprised both pathogenic and nonpathogenic populations. Two populations of Foa were distinguished by a single ITS2 base transition, and the dominance of these two populations differed between Europe and the USA. Fusarium proliferatum was more abundant in Spain than in the UK. Phylogenetic analysis using EF1 alpha sequences indicated that isolates of F. oxysporum pathogenic to asparagus are spread across a number of clades within the species complex, supporting the hypothesis that pathogenicity to asparagus in this species is a relatively unspecialized trait.