Study of Phylogenetic Relationships Among Fusarium oxysporum f. sp. dianthi Isolates: Confirmation of Intrarace Diversity and Development of a Practical Tool for Simple Population Analyses

Fusarium oxysporum as an Opportunistic Fungal Pathogen on Zinnia hybrida Plants Grown on board the International Space Station

A plant production system called Veggie was launched to the International Space Station (ISS) in 2014. In late 2015, during the growth of Zinnia hybrida cv. 'Profusion' in the Veggie hardware, plants developed chlorosis, leaf curling, fungal growth that damaged leaves and stems, and eventually necrosis. The development of symptoms was correlated to reduced air flow leading to a significant buildup of water enveloping the leaves and stems in microgravity. Symptomatic tissues were returned to Earth on 18 May 2016 and were immediately processed to determine the primary causal agent of the disease. The presumptive pathogen was identified as Fusarium oxysporum by morphological features of microconidia and conidiophores on symptomatic tissues; that is, by epifluorescent microscopy (EFM), scanning electron microscopy (SEM), metabolic microarrays, and ITS sequencing. Both EFM and SEM imaging of infected tissues showed that germinating conidia were capable of stomatal penetration and thus acted as the primary method for infecting host tissues. A series of ground-based pathogenicity assays were conducted with healthy Z. hybrida plants that were exposed to reduced-airflow and high-water stress (i.e., encased in sealed bags) or were kept in an unstressed configuration. Koch's postulates were successfully completed with Z. hybrida plants in the lab, but symptoms only matched ISS-flown symptomatic tissues when the plants were stressed with high-water exposure. Unstressed plants grown under similar lab conditions failed to develop the symptoms observed with plants on board the ISS. The overall results of the pathogenicity tests imply that F. oxysporum acted as an opportunistic pathogen on severely high-water stressed plants. The source of the opportunistic pathogen is not known, but virulent strains of F. oxysporum were not recovered from unused materials in the Veggie plant pillow growth units assayed after the flight.

Current Status of Fusarium oxysporum Formae Speciales and Races

The Fusarium oxysporum species complex includes both plant pathogenic and nonpathogenic strains, which are commonly found in soils. F. oxysporum has received considerable attention from plant pathologists for more than a century owing to its broad host range and the economic losses it causes. The narrow host specificity of pathogenic strains has led to the concept of formae speciales, each forma specialis grouping strains with the same host range. Initially restricted to one plant species, this host range was later found to be broader for many formae speciales. In addition, races were identified in some formae speciales, generally with cultivar-level specialization. In 1981, Armstrong and Armstrong listed 79 F. oxysporum formae speciales and mentioned races in 16 of them. Since then, the known host range of F. oxysporum has considerably increased, and many new formae speciales and races have been identified. We carried out a comprehensive search of the literature to propose this review of F. oxysporum formae speciales and races. We recorded 106 well-characterized formae speciales, together with 37 insufficiently documented ones, and updated knowledge on races and host ranges. We also recorded 58 plant species/genera susceptible to F. oxysporum but for which a forma specialis has not been characterized yet. This review raises issues regarding the nomenclature and the description of F. oxysporum formae speciales and races.