Diagnosis of tomato foot and root rot by quantification of Fusarium oxysporum in plant material.
Tomato foot and root rot (TFRR) caused by Fusarium oxysporum (Fox) is an economically important disease reported to decrease yield in both greenhouses and field in many countries. Early detection of Fox f.sp. radicis-lycopersici (Forl), which is the causal agent for TFRR, can help to predict the disease outbreak and consequently to choose defense strategy. Since non-phytopathogenic Fox strains are able to colonize tomato, Fox DNA, detected in plant material, is not the final proof for an ongoing infection which would result in TFRR. We followed the colonization of tomato by strains Fox f.sp. radicis-lycopersici ZUM2407 (a tomato foot and root rot pathogen), Fox f.sp. lycopersici 004 (causing tomato wilt) and Fox f.sp. radicis-cucumerinum V03-2 g (a cucumber pathogen) and Fox Fo47 (a well known non-pathogenic biocontrol strain). Two weeks after tomato seedling inoculation by these Fox strains, when disease symptoms were not visible yet, fungal DNA concentrations determined in tomato plantlets by qPCR with primers complementary to the Intergenic Spacer region of these four Fox strains revealed that the DNA concentration of Forl ZUM2407 is 5 times higher than that of the non-compatible pathogen Forc V03-2 g and 10 times higher than that of Fo47. In three week old plantlets the concentration of Forl ZUM2407 DNA was at least 10 times higher than those of the other strains. The fungal DNA concentration, as determined by qPCR, appeared to be in good agreement with data of the visual score of tomato foot and root rot symptoms obtained 3 weeks after incubation of tomato with Forl ZUM2407. Our results show that targeting of the multicopy ribosomal operon gives a high sensitivity to qPCR reaction for the detection of Fox. Since formae speciales of Fox cannot be distinguished by comparison of ribosomal operons, detection of Fox DNA is no conclusive evidence for plant infection by the compatible pathogen. Nevertheless, the observed difference in plant colonization between pathogenic and non-pathogenic strains strongly suggests that a fast increase of fungal DNA concentrations is due to proliferation of pathogenic Fox, which will lead to damage of the plant.