Abstract

Fusarium oxysporum f. sp. lycopersici (FOL) is a soilborne pathogen causing vascular wilt disease of tomato. F. oxysporum is also an ubiquitous soil resident and non-pathogenic strains (NPF) are frequently isolated from tomato tissues and rhizosphere. Our working hypothesis is that NPF has acquired pathogenicity factors during their non-pathogenic association with tomato allowing FOL strains to emerge. The origin of edible tomato (Lycopersicon esculentum) is considered to be in the Andean Plateau and several wild Lycopersicon spp. can be found there. Several Lycopersicon spp. were transported to Mexico where cultivated L. esculentum is thought to have been selected. After the 16th century, L. esculentum was spread worldwide. To address our hypothesis, we sampled fungi from wild, symptomless Lycopersicon spp. and the soil in which these plants were grown in Chile, Ecuador, and Mexico. From tissues and rhizosphere soil we obtained approximately 200 F. oxysporum isolates. Pathogenicity of these isolates was tested and all did not produce wilt symptoms on tomato. Approximately 600 base pairs of the nuclear ribosomal intergenic spacer was sequenced from each isolate and used to estimate a phylogeny which included FOL and other forma speciales of F. oxysporum. Isolates sampled from Lycopersicon spp. and rhizosphere soil were distributed randomly throughout the phylogeny. Some of the isolates had identical sequence with FOL isolates. Thiese support our hypothesis.