Development of a TaqMan Real‐Time Polymerase Chain Reaction Assay for Detection and Quantification of Fusarium oxysporum f. sp. lycopersici in Soil

Abstract Fusarium wilt, caused by Fusarium oxysporum f. sp. lycopersici ( FOL ), is an important disease of tomato. Pathogenicity and vegetative compatibility tests, although reliable, are laborious for the identification of FOL isolates and cannot efficiently quantify population densities of FOL in the soil. The objective of this study was to develop a rapid, sensitive and quantitative real‐time polymerase chain reaction ( PCR ) assay for detecting and quantifying FOL in soil. An inexpensive and relatively simple method for soil DNA extraction and purification was developed based on bead‐beating and a silica‐based DNA ‐binding method. A TaqMan probe and PCR primers were designed using the DNA sequence of the species‐specific virulence gene SIX 1 , which is only present in isolates of FOL , not in isolates of other formae speciales or non‐pathogenic isolates of F . oxysporum . The real‐time PCR assay successfully amplified isolates of three races of FOL used in this study and quantified FOL DNA in soils, with a detection limit of 0.44 pg of genomic DNA of FOL in 20 μ l of the real‐time PCR . A spiking test performed by adding different concentrations of conidia to soil showed a significant linear relationship between the amount of genomic DNA of FOL detected by the real‐time PCR assay and the concentration of conidia added. In addition, the real‐time PCR assay revealed a significant quadratic regression for a glasshouse experiment between disease severity and DNA concentration of FOL . The soil DNA extraction method and real‐time PCR assay developed in this study could be used to determine population densities of FOL in soil, develop threshold models to predict Fusarium wilt severity, identify high‐risk fields and measure the impact of cultural practices on FOL populations in soils.