PROGRESS WITH CHARACTERIZATION OF THE FUNGUS CAUSING SPORIDESMIN TOXICITY ON RYEGRASS

It is now certain that the fungus responsible for sporidesmin toxicity (facial eczema) in the affected areas, and probably in all of South African livestock, is Pseudopithomyces toxicarius. This fungus produces the liver toxin sporidesmin A. Until recently, the fungus has been misidentified, but through analysis of 152 fungal strains from international collections and the generation of over 900 DNA sequences, the correct identity and presence of P. toxicarius in the Eastern Cape ryegrass dominated pastures, has been confirmed.

The concerns that the fungus might have been introduced through imported ryegrass seed by screening both local and imported seed samples from New Zealand and the Netherlands have also been addressed. Comprehensive testing using culture methods and DNA-based assays found no evidence of P. toxicarius, confirming that the fungus is not seed-borne and unlikely to have arrived through international seed trade. As a result, the focus of the disease control can now be shifted from import regulation to local environmental factors that promote fungal growth and toxin production.

Fieldwork across five dairy farms in the Humansdorp region between May 2023 and April 2024, showed the year-round presence of P. toxicarius, with spore concentrations peaking in summer. The fungus is most prevalent in perennial ryegrass pastures, especially in areas exposed to western and northern winds. Phylogenetic analysis further confirmed that all isolates belong to P. toxicarius.

Genetic studies revealed both vegetative and sexual mating types among the strains, indicating potential for sexual reproduction, although sexual structures have not yet been observed under laboratory conditions. Understanding this reproductive potential is important for predicting how the fungus might persist or evolve in the field.

To develop a targeted screening method for P. toxicarius, a PCR assay amplifying region of the NRPS gene within the sporidesmin gene cluster was developed. The NRPS gene cluster is connected with the toxin expression area. This method is now being used to screen Eastern Cape isolates to confirm toxin gene presence. Isolates lacking the gene cluster may be selected for whole genome sequencing to better understand and detect potential non-toxigenic variants.

Additionally, genome sequencing of two South African P. toxicarius strains has been performed, and compared with the New Zealand reference genomes. The gene editing work has been initiated to characterize the sporidesmin biosynthesis pathway using Agrobacterium-mediated transformation methodology.