The Australian pyrethrum (Tanacetum cinerariifolium) industry faces significant threats from fungal disease, causing decreased yields of pyrethrins, natural insecticidal products. Chemical control as well as understanding pathogen biology help minimise losses1. This study completed genetic analysis of two key ascomycete pathogens Stagonosporopsis tanaceti (ray blight) and Didymella tanaceti (tan spot) to understand the risk they pose to the industry. Populations of the two pathogens were collected from seeds harvested from six fields across north-west Tasmania in 2022. Pathogen incidence ranged from 1.5-63.0% for S. tanaceti and 22.0-58.8% for D. tanaceti (n = 200 per field). Approximately sixty isolates of each species were collected from each field. As both species are heterothallic, the frequency of mating types was estimated to infer the potential for sexual reproduction. Results were consistent with previous observations. For S. tanaceti only mating-type-1 (MAT1-1) individuals were identified. For D. tanaceti, isolates of both MAT1-1 and MAT1-2 were identified and were consistent with a 1:1 ratio (P > 0.10) in four of six fields, indicating mating-type diversity across the growing region and the possibility of reproductive cycles. To evaluate mutations which result in decreased fungicide efficacy, gene sequencing was completed for the cytochrome-b (Cytb) and Succinate dehydrogenase B, C and D subunits (Sdh), which interact with QoI (Group 11) and SDHI (Group 7) fungicides, respectively. The partial (S. tanaceti) or full (D. tanaceti) sequencing of the Cytb was found to have no evidence of the G143 mutation which could confer resistance to QoI fungicides. No unknown Sdh substitutions were identified in the D. tanaceti isolates (n= 384). However, majority of isolates had a mutation that has been linked to resistant to the SDHI fungicide boscalid2. The SdhB-H277Y, SdhC-H134R and SdhD-D113E were the most frequent substitutions, occurring in 37.5, 33.0 and 19.3% of the population, respectively. In addition, a few isolates contained a SdhC-S135 and SdhC-H134Q. The high frequency of substitutions is consistent with previous studies with only 14 isolates identified as wild type, without any known mutation that could confer resistance. For S. tanaceti (n = 288), the SdhB-H277Y and SdhC-S73F were the most frequent substitutions occurring in 28.5 and 18.1% of the population, respectively. Previous research indicated S. tanaceti was highly susceptible to SDHI fungicides3. The in vitro fungicide response of these S. tanaceti isolates to a range of SDHI fungicides will be evaluated. Additionally, these populations are undergoing genetic diversity analysis, using microsatellite markers, to understand how resistance has spread through populations. These results also highlight the importance of the effective seed treatment strategies employed to remove pathogens from seeds, thereby, limiting the inoculum source for future crops, enhancing crop health and productivity.
Keywords: Stagonosporopsis tanaceti, Didymella tanaceti, fungicide resistance, mating type genes