Diagnosis and detection of host‐specific forms of Fusarium oxysporum*

Abstract

Diagnosis and detection of host‐specific forms of Fusarium oxysporum are traditionally based on the combination of diagnostic symptoms on the host with the presence of the fungus in the affected tissues. The classical approach is becoming increasingly problematic because more than one forma specialis may occur on a given host, along with non‐pathogenic strains which are common soil and rhizosphere inhabitants. Neither formae speciales nor pathogenic races within formae speciales can be distinguished morphologically. Although united by joint pathogenicity to a given host, strains belonging to the same forma specialis need not be phylogenetically related. Development of diagnostics for host‐specific groups in F. oxysporum requires monophyletic target groups. Recent studies on gene‐genealogy and AFLP‐based phylogenies show that the majority of formae speciales in F. oxysporum are polyphyletic (unnatural) and do not offer any prospects for the development of molecular diagnostics. In contrast, highly specific PCR primers have been developed for formae speciales (or races) that consist of a single clonal lineage, and for monophyletic groups of lineages within a forma specialis. Among others, specific PCR primers have thus been developed for F. oxysporum f. sp. basilici, specific races in F. oxysporum ff. spp. dianthi and gladioli, and for the EPPO A2 (EU II/A1) quarantine fungus F. oxysporum f. sp. albedinis which can reliably replace conventional isolation and pathogenicity testing procedures.