and Species Associated with Yield Decline of Pyrethrum () in Australia.

Pyrethrum () cultivation in Australia, which accounts for the majority of global production of natural insecticidal pyrethrins, is affected by a persistent yield decline which in part is caused by a complex of pathogens. and species were isolated from crown and roots of pyrethrum plants showing stunting and brown discoloration of crown tissue, and from soil adjacent to diseased plants from yield-decline-affected sites in Tasmania and Victoria, Australia. Ten known species (, , , , , , , and var. ), two new species ( sp. nov. and sp. nov.) and three species (, and ) were identified through morphological studies and multigene phylogenetic analyses using ITS and sequences. var. sp. nov. and were most abundant. and were reported for the first time in Australia. Seven species were pathogenic on both pyrethrum seeds (in vitro assays) and seedlings (glasshouse bioassays), while two species and three species only caused significant symptoms on pyrethrum seeds. and var. were the most aggressive species, causing pyrethrum seed rot, seedling damping-off and significant plant biomass reduction. This is the first report of and species causing disease in pyrethrum globally and suggests that oomycete species in the family may have an important role in the yield decline of pyrethrum in Australia.