Early infection biology and disease impact of Didymella tanaceti on pyrethrum

Didymella tanaceti is an important foliar pathogen of pyrethrum ( Tanacetum cinerariifolium ) in the Australian production ecosystem, but its infection process and effects on plant growth remain poorly understood. This study investigated the early infection biology of D. tanaceti and its impact on pyrethrum whole-plant growth. Conidial germination increased progressively between 12 and 24 h after inoculation (HAI), reaching 75% of spores germinating at 20 HAI. Germ tubes penetrated host tissues directly through the epidermis or via stomata, with no specialised penetration structures observed. A biofilm-like extracellular matrix surrounding both conidia and germ tubes was ​observed during early infection, suggesting it plays a potential role in surface adhesion and host colonisation. Fluorescent microscopy enabled visualisation of pathogen colonisation within inoculated leaf, petiole and crown-region ​tissues. Whole-plant phenotyping showed that infection reduced total plant biomass. These findings offer​ new insights into the infection strategy and disease development of D. tanaceti and may contribute to the refinement of disease management strategies.