Genetic Variability within
Fusarium oxysporum sp. lycopersici

Plant diseases, especially fungal diseases, continue to be a major obstacle to the efforts which are used to elucidate the biochemical and genetic bases of the specificity and variability of fungal-plant interactions. Here we are dealing with Fusarium wilt disease of tomato.

Tomato wilt disease caused by Fusarium oxysporum (Schlecht.) f. sp. lycopersici (FOL) (Sacc.) Snyder and Hansen is considered one of the most devastating fungal diseases which causes great losses in tomato. Three physiological races (1, 2 and 3) of FOL have been characterized and distinguished by their pathogenicity to tomato cultivars with specific dominant resistance genes.

OBJECTIVES

•  Characterization of isolates of FOL, obtained from tomato naturally affected by vascular wilt and from soil of tomato-growing fields, using pathogenicity, RAPDs and RFLPs analyses.

•  To determine 1) if the genetic diversity could be detected and distinguished within FOL isolates, and 2) if the genetic and pathogenic characteristics are correlated.

ACHIEVEMENTS

•  RAPD data revealed that the authentic ATCC races (66417 and 66605) were found to be closely related due to their high similarity coefficient (0.968).

•  The cluster analysis showed that F. oxysporum isolates could be separatd into two clusters (I & II).  Cluster I contained the pathogenic isolates, whereas cluster II contained the nonpathogenic ones.

•  The results of RFLP analysis could be summarized as follows:
1) The A1 probe hybridized with the 21 isolates of F. oxysporum used in RAPD analysis as well as both Israeli ATCC races 1 and 2 of F. oxysporum  f. sp. lycopersici.  However, the A1 probe did not hybridize to the DNAs of F. oxysporum f.sp. niveum and F. solani f. sp. pisi, the non-tomato pathogens. 
2) The B2 probe hybridized only with the seven highly pathogenic isolates (1-7) in addition to the Israeli ATCC races 1 and 2 of F. oxysporum   f. sp. lycopersici.