Taxonomy: Kingdom Fungi; Phylum Ascomycota; Class Sordariomycetes; Order Hypocreales; Family Nectriaceae; genus Fusarium. Host range: Very broad at the species level. More than 120 different formae speciales have been identified based on specificity to host species belonging to a wide range of plant families. Disease symptoms: Initial symptoms of vascular wilt include vein clearing and leaf epinasty, followed by stunting, yellowing of the lower leaves, progressive wilting, defoliation and, finally, death of the plant. On fungal colonization, the vascular tissue turns brown, which is clearly visible in cross-sections of the stem. Some formae speciales are not primarily vascular pathogens, but cause foot and root rot or bulb rot. Economic importance: Can cause severe losses in many vegetables and flowers, field crops, such as cotton, and plantation crops, such as banana, date palm and oil palm. Control: Use of resistant varieties is the only practical measure for controlling the disease in the field. In glasshouses, soil sterilization can be performed. Useful websites: http://www.broad.mit.edu/annotation/genome/fusarium_group/MultiHome.html; http://www.fgsc.net/Fusarium/fushome.htm; http://www.phi-base.org/query.php

Pathogen profile update: Fusarium oxysporum

The monophyletic origin of host-specific taxa in the plant-pathogenic Fusarium oxysporum complex was tested by constructing nuclear and mitochondrial gene genealogies and amplified fragment length polymorphism (AFLP)-based phylogenies for 89 strains representing the known genetic and pathogenic diversity in 8 formae speciales associated with wilt diseases and root and bulb rot. We included strains from clonal lineages of F. oxysporum f. spp. asparagi, dianthi, gladioli, lilii, lini, opuntiarum, spinaciae, and tulipae. Putatively nonpathogenic strains from carnation and lily were included and a reference strain from each of the three main clades identified previously in the F. oxysporum complex; sequences from related species were used as outgroups. DNA sequences from the nuclear translation elongation factor 1alpha and the mitochondrial small subunit (mtSSU) ribosomal RNA genes were combined for phylogenetic analysis. Strains in vegetative compatibility groups (VCGs) shared identical sequences and AFLP profiles, supporting the monophyly of the two single-VCG formae speciales, lilii and tulipae. Identical genotypes were also found for the three VCGs in F. oxysporum f. sp. spinaciae. In contrast, multiple evolutionary origins were apparent for F. oxysporum f. spp. asparagi, dianthi, gladioli, lini, and opuntiarum, although different VCGs within each of these formae speciales often clustered close together or shared identical EF-1alpha and mtSSU rDNA haplotypes. Kishino-Hasegawa analyses of constraints forcing the monophyly of these formae speciales supported the exclusive origin of F. oxysporum f. sp. opuntiarum but not the monophyly of F. oxysporum f. spp. asparagi, dianthi, gladioli, and lini. Most of the putatively nonpathogenic strains from carnation and lily, representing unique VCGs, were unrelated to F. oxysporum f. spp. dianthi and lilii, respectively. Putatively nonpathogenic or rot-inducing strains did not form exclusive groups within the molecular phylogeny. Parsimony analyses of AFLP fingerprint data supported the gene genealogy-based phylogram; however, AFLP-based phylogenies were considerably more homoplasious than the gene genealogies. The predictive value of the forma specialis naming system within the F. oxysporum complex is questioned.

Gene Genealogies and AFLP Analyses in the Fusarium oxysporum Complex Identify Monophyletic and Nonmonophyletic Formae Speciales Causing Wilt and Rot Disease

A 12 kDa cysteine-rich protein is secreted by Fusarium oxysporum f. sp. lycopersici during colonization of tomato xylem vessels. Peptide sequences obtained with mass spectrometry allowed identification of the coding sequence. The gene encodes a 32 kDa protein, designated Six1 for secreted in xylem 1. The central part of Six1 corresponds to the 12 kDa protein found in xylem sap of infected plants. A mutant that had gained virulence on a tomato line with the I-3 resistance gene was found to have lost the SIX1 gene along with neighbouring sequences. Transformation of this mutant with SIX1 restored avirulence on the I-3 line. Conversely, deletion of the SIX1 gene in a wild-type strain results in breaking of I-3-mediated resistance. These results suggest that I-3-mediated resistance is based on recognition of Six1 secreted in xylem vessels.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2958.2004.04177.x

A small, cysteine-rich protein secreted by Fusarium oxysporum during colonization of xylem vessels is required for I-3-mediated resistance in tomato

The innate immune system of plants consists of two layers. The first layer, called basal resistance, governs recognition of conserved microbial molecules and fends off most attempted invasions. The second layer is based on Resistance (R) genes that mediate recognition of effectors, proteins secreted by pathogens to suppress or evade basal resistance. Here, we show that a plant-pathogenic fungus secretes an effector that can both trigger and suppress R gene-based immunity. This effector, Avr1, is secreted by the xylem-invading fungus Fusarium oxysporum f.sp. lycopersici (Fol) and triggers disease resistance when the host plant, tomato, carries a matching R gene (I or I-1). At the same time, Avr1 suppresses the protective effect of two other R genes, I-2 and I-3. Based on these observations, we tentatively reconstruct the evolutionary arms race that has taken place between tomato R genes and effectors of Fol. This molecular analysis has revealed a hitherto unpredicted strategy for durable disease control based on resistance gene combinations.

https://pure.uva.nl/ws/files/4146523/62855_294714.pdf

Suppression of plant resistance gene-based immunity by a fungal effector.

To promote host colonization, many plant pathogens secrete effector proteins that either suppress or counteract host defences. However, when these effectors are recognized by the host's innate immune system, they trigger resistance rather than promoting virulence. Effectors are therefore key molecules in determining disease susceptibility or resistance. We show here that Avr2, secreted by the vascular wilt fungus Fusarium oxysporum f. sp. lycopersici (Fol), shows both activities: it is required for full virulence in a susceptible host and also triggers resistance in tomato plants carrying the resistance gene I-2. Point mutations in AVR2, causing single amino acid changes, are associated with I-2-breakingFol strains. These point mutations prevent recognition by I-2, both in tomato and when both genes are co-expressed in leaves of Nicotiana benthamiana. Fol strains carrying the Avr2 variants are equally virulent, showing that virulence and avirulence functions can be uncoupled. Although Avr2 is secreted into the xylem sap when Fol colonizes tomato, the Avr2 protein can be recognized intracellularly by I-2, implying uptake by host cells.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2009.03838.x

The effector protein Avr2 of the xylem-colonizing fungus Fusarium oxysporum activates the tomato resistance protein I-2 intracellularly

A collection of race 1 and race 2 isolates of Fusarium oxysporum f. sp. lycopersici was screened for vegetative compatibility and characterized by random amplified polymorphic DNA (RAPD) analysis to establish the identity and genetic diversity of the isolates. Comparison of RAPD profiles revealed two main groups that coincide with vegetative compatibility groups (VCGs). In addition, several single-member VCGs were identified that could not be grouped in one of the two main RAPD clusters. This suggests that F. oxysporum f. sp. lycopersici is a polyphyletic taxon. To assign avirulence genotypes to race 1 isolates, they were tested for their virulence on a small set of tomato lines (Lycopersicon esculentum), including line OT364. This line was selected because it shows resistance to race 2 isolates but, unlike most other race 2-resistant lines, susceptibility to race 1 isolates. To exclude the influence of other components than those related to the race-specific resistance response, we tested the vi...

Frits Herlaar

Papers

Biological and Molecular Characterization of Fusarium oxysporum f. sp. lycopersici Divides Race 1 Isolates into Separate Virulence Groups.