The ‘forma specialis’ issue in Fusarium: A case study in Fusarium solani f. sp. pisi

Abstract

The Fusarium solani species complex (FSSC) has been studied intensively but its association with legumes, particularly under European agro-climatic conditions, is still poorly understood. In the present study, we investigated phylogenetic relationships and aggressiveness of 79 isolates of the FSSC collected from pea, subterranean clover, white clover and winter vetch grown under diverse agro-climatic and soil conditions within Temperate and Mediterranean Europe. The isolates were characterized by sequencing tef1 and rpb2 loci and by greenhouse aggressiveness assays. The majority of the isolates belonged to two lineages: the F. pisi comb. nov. lineage (formerly F. solani f. sp. pisi) mainly accommodating German and Swiss isolates, and the Fusisporium (Fusarium) solani lineage accommodating mainly Italian isolates. Based on the results of aggressiveness tests on pea, most of the isolates were classified as weakly to moderately aggressive. In addition, using one model strain, 62 accessions of 10 legume genera were evaluated for their potential to host F. pisi, the species known mainly as a pathogen of pea. A total of 58 accessions were colonized, with 25 of these being asymptomatic hosts. These results suggest a broad host range for F. pisi and challenge the forma specialis naming system in Fusarium.Fusarium solani (sexual morph Nectria haematococca; syn. Haematonectria haematococca) is a filamentous fungus of significant agricultural importance that has been accommodated as a single species in the section Martiella within the genus Fusarium 1 . Re-evaluation of species taxonomy based on molecular phylogenetic analyses has revealed that F. solani is a species complex (FSSC) which includes at least 60 distinct phylogenetic species 2,3 . Members of the complex are globally distributed and of considerable ecological plasticity, causing infections in both plants and humans 4,5 . Phytopathogenic species within the FSSC include some of the most economically important plant pathogens associated with vascular wilts and root rots in over 100 crops 6 . Despite the broad host range of the complex as a whole, individual species are often associated with only one or a few plant hosts. Consequently, populations of F. solani pathogenic on plants have been divided into 12 formae speciales and two races [7][8][9][10] . Early studies on sexual compatibility of special forms and races has already shown that F. solani represents at least seven biological species classified as mating populations (MPs) I-VII, with Nectria haematococca as the most commonly referred sexual morph. Biological species correlated with a host range, as successful sexual crosses were found only among heterothallic isolates within each special form or race 11 . However, the designation forma specialis may lead to incorrect assumptions concerning the aggressiveness and host specificity of individual isolates. For example, studies on the host range of F. solani f. sp. pisi (MP VI), named by its specific pathogenicity on pea (Pisum sativum), revealed that the speci...

Fungi between extremotolerance and opportunistic pathogenicity on humans

“…The Hypocreales contains species causing both human infections (only 64 out of 4479 described species) as well as major plant diseases, as an example of cross-kingdom pathogenicity (van Baarlen et al 2007;Segorbe et al 2017) or more accurately, crosskingdom opportunism. Other such examples include species of Fusarium, where both plant and human infection is frequent and strains have an unusually wide host range (Al-Hatmi et al 2016;Š išić et al 2018). Similarly, Sharma et al (2014) noted the plant-virulence of clinical strains of Exserohilum rostratum (Pleosporales), but this species is not host-specific, it has a wide host range on diverse grasses, and is also found in soil and on rotten materials (Ellis 1971).…”

Morphology, phylogeny and pathogenicity of Fusarium species from Sansevieria trifasciata in Malaysia

“…This suggested that additional genes might be required for better resolution within the species. Nevertheless, the classification and naming system of formae speciales within Fusarium species has been challenged with the discovery of polyphyly and the wide host range of a forma specialis (Pinaria et al , ; Šišić et al , ). In addition, the nomenclature of formae speciales lack standardization and some formae speciales even have multiple names (Edel‐Hermann and Lecomte, ).…”

<p>Multiresistant <em>Fusarium</em> Pathogens on Plants and Humans: Solutions in (from) the Antifungal Pipeline?</p>

“…lycopersici, F. keratoplasticum , and F. falciforme are known for their pathogenicity to plants, but have also been reported from humans and other mammals 11–13. Similarly, F. pisi, F. temperatum, F. ramigenum, F. musae, Fusarium solani sensu stricto , and F. volatile have been recovered from living plants, while also their clinical relevance has been underlined 14,19–21. Cross-kingdom pathogenicity is in obvious conflict with plant host specificity.…”