Phylogenetic characterization, virulence, and propiconazole sensitivity of <i>Thielaviopsis ethacetica</i> , causal agent of pineapple sett rot in Florida sugarcane

Pineapple sett rot, caused by Thielaviopsis species, constrains sugarcane production worldwide. Mechanical planting systems facilitate pathogen infection through increased wounding of planting material, exacerbating disease incidence. In Florida's Everglades Agricultural Area, the recent transition from manual to mechanical planting has intensified pineapple sett rot problems, threatening the United States' most important sugarcane-growing region. This study characterized Thielaviopsis isolates from symptomatic sugarcane billets collected during the 2023-2024 Florida planting season to establish species identity, virulence, and baseline fungicide sensitivity. Phylogenetic analysis of concatenated ITS and tef1-α sequences confirmed all 93 isolates as Thielaviopsis ethacetica, representing the first molecular characterization from North American sugarcane. Phylogenetic analysis resolved the Florida population in study into the two previously described intraspecific lineages of T. ethacetica, one isolate assigned to the African lineage and 11 to the South American lineage; the latter exhibited intra-lineage polymorphism resolvable into at the least three subclades. MAT1-1 mating type idiomorphs were detected in 38.7% of isolates; MAT1-2 could not be confirmed due to primer amplification failure. Virulence testing on cultivars CP96-1252 and CPCL05-1201 revealed that all representative isolates were highly pathogenic, reducing survival by 50-98% and plant height by 64-96% at 38 days after inoculation. Among the tested cultivars, CPCL05-1201 displayed higher quantitative resistance to T. ethacetica. Baseline propiconazole sensitivity testing of 14 representative isolates spanning the phylogenetic diversity observed in the population showed uniform susceptibility, with EC₅₀ values from 0.0081 to 0.0115 μg/mL (mean = 0.0095 μg/mL, CV = 12.2%). These values were substantially lower than typical field application rates, suggesting strong propiconazole efficacy. These findings provide critical baseline data for proactive resistance management and breeding programs targeting pineapple sett rot in sugarcane.