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Colletotrichum tanaceti , the causal agent of anthracnose, is an emerging pathogen of commercially grown pyrethrum ( Tanacetum cinerariifolium ) in Australia. A microsatellite marker library was developed to understand the spatio-genetic structure over three sampled years and across two regions where pyrethrum is cultivated in Australia. Results indicated that C. tanaceti was highly diverse with a mixed reproductive mode; comprising both sexual and clonal reproduction. Sexual reproduction of C. tanaceti was more prevalent in Tasmania than in Victoria. Little differentiation was observed among field populations likely due to isolation by colonization but most of the genetic variation was occurring within populations. C. tanaceti was likely to have had a long-distance gene and genotype flow among distant populations within a state and between states. Anthropogenic transmission of propagules and wind dispersal of ascospores are the most probable mechanisms of long-distance dispersal of C. tanaceti . Evaluation of putative population histories suggested that C. tanaceti most likely originated in Tasmania and expanded from an unidentified host onto pyrethrum. Victoria was later invaded by the Tasmanian population. With the mixed mode of reproduction and possible long-distance gene flow, C. tanaceti is likely to have a high evolutionary potential and thereby has ability to adapt to management practices in the future.

A complex of foliar diseases can affect pyrethrum in Australia, but those of greatest importance are ray blight, caused by Stagonosporopsis tanaceti, and tan spot, caused primarily by Didymella tanaceti. Isolation of fungi from pyrethrum seed lots produced over 15 years resulted in recovery of six known pathogens: S. tanaceti, D. tanaceti, Alternaria tenuissima, Colletotrichum tanaceti, Stemphylium botryosum, and Botrytis cinerea. The incidence of S. tanaceti and D. tanaceti isolated from seed varied between 0.9 and 19.5% (mean = 7.7%) and 0 and 24.1% (mean = 5.3%) among years, respectively. Commercial heat treatment of pyrethrum seed via steaming reduced the incidence of D. tanaceti from 10.9 to 0.06% and the incidence of S. tanaceti from 24.6% to nondetectable levels (<0.18%). In a second experiment, both species were reduced to nondetectable levels (<0.20%) from their initial incidences of 22.4 and 2.4%, respectively. In a field study in 2013, colonization of pyrethrum foliage by S. tanaceti was reduced from 21.1 to 14.3% in early winter when heat-treated seed was planted. However, isolation frequency of D. tanaceti was not affected significantly by seed treatment in this year. In a related experiment in 2015, the isolation frequency of D. tanaceti in plots planted from heat-treated seed depended on both prior application of an industry-standard fungicide program and proximity to another pyrethrum field in autumn. The fungus was recovered at a similar frequency in fungicide-treated and nontreated plots located near other pyrethrum fields (13.8 versus 16.3%, respectively), whereas recovery of the pathogen was reduced by fungicide applications in geographically remote pyrethrum fields (6.7 versus 1.4%, respectively). However, these differences in isolation frequency of D. tanaceti in autumn did not obviate the need for later fungicide applications to suppress foliar disease intensity in spring or flower yield in summer, independent of the proximity to other pyrethrum fields. This study suggests that steam treatment of seed can delay development of the foliar disease complex on pyrethrum, although an extremely low level of remaining infected seed or exogenous sources of inoculum necessitates the use of foliar fungicide applications in spring.