Risk assessment of the Acacia cyclops dieback pathogen, Pseudolagarobasidium acaciicola, as a mycoherbicide in South African strandveld and limestone fynbos

•Pseudolagarobasidium acaciicola causes dieback of Acacia cyclops in South Africa.•In a field survey 0.9% of indigenous plants were dead around 100 killed A. cyclops.•Of these P. acaciicola was confirmed in 45% by use of a species-specific primer.•None of these species were killed in inoculation trials.•This is the first time this approach to undertake a risk assessment has been used.

Acacia cyclops, an invasive weed in South Africa, has become a major threat to the fragile biodiversity of strandveld and limestone fynbos vegetation. The locally occurring A. cyclops dieback pathogen, Pseudolagarobasidium acaciicola, is under consideration as a mycoherbicide to control A. cyclops. To determine the level of risk posed to surrounding indigenous plants when using P. acaciicola as a mycoherbicide, a field survey was performed to record dieback and mortality incidence among indigenous woody plant species around diseased A. cyclops trees. Subsequently, DNA extractions were made from the roots of all dead or dying indigenous woody plants recorded, and A. cyclops trees, to verify if P. acaciicola was present. A mortality rate of 0.9% of indigenous plants was recorded and P. acaciicola was confirmed as present in 45% of these dead plants. Presence of P. acaciicola was confirmed in 77% of the sampled A. cyclops trees. Pathogenicity trials revealed susceptibility of some native Fabaceae species to stem inoculations in the nursery and, to a lesser extent, in the field. The optimum growth temperature for P. acaciicola was determined as 30–35 °C. Aside from being pathogenic to A. cyclops, results from this study suggest that P. acaciicola is primarily a saprophyte and possibly a weak opportunistic pathogen on some indigenous Fabaceae. Due to the very low incidence of mortality, and demonstrated limited pathogenicity, it is concluded that the use of P. acaciicola as a mycoherbicide on A. cyclops poses a sufficiently low risk to any indigenous vegetation to warrant its widespread use.

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