Thielaviopsis paradoxa sensu lato is a soilborne fungal pathogen that causes Thielaviopsis trunk rot and heart rot in palms. The loss of structural integrity resulting from trunk rot can cause the palm trunk to collapse suddenly and poses a serious threat to life and property. Even though rudimentary knowledge about the Thielaviopsis infection process in palms is available, nothing is known about the T. paradoxa species complex in the US. The aim of this study was to characterize T. paradoxa s. lat. isolates collected from diseased palms grown in Florida. Multi-locus phylogeny using three genes, ITS, β-tubulin, and tef1-α, revealed that the isolates separate into two distinct clades with high bootstrap support. The majority of the isolates clustered with the species T. ethacetica, while two isolates formed a separate clade, distinct from T. musarum, and might represent an undescribed Thielaviopsis species. One representative isolate from each clade, when grown on three distinct media and at four different temperatures, showed differences in gross colony morphology, as well as growth rates. The T. ethacetica isolate TP5448 and the Thielaviopsis sp. isolate PLM300 grew better at opposite ends of the temperature spectrum tested in this study, i.e., 35 °C and 10 °C, respectively. In pathogenicity assays on whole plants, the T. ethacetica isolate proved to be more aggressive than Thielaviopsis sp. isolate PLM300, as it produced larger lesions when inoculated on wounded leaflets. An unequal distribution was observed for the mating-type locus of T. ethacetica, as 12 isolates carried the MAT1-1-1 allele, while the status for four isolates remained undefined. Variation in mycelial growth in response to different fungicides was also observed between the two clades. These results demonstrate the existence of two Thielaviopsis clades that can infect palms in Florida and underscore the need for targeted sampling to help uncover the diversity of Thielaviopsis species across palm-growing regions in the US.

The Thielaviopsis paradoxa species complex comprises fungal pathogens that cause both bud (heart) and trunk rot of palms. Thielaviopsis spp. are soilborne pathogens that infect all plant parts (i.e., roots, stem, leaves, and fruits) of primarily monocot hosts, including palms, banana, sugarcane, and pineapple. However, very little is known about the T. paradoxa species complex in the United States and the palm-Thielaviopsis pathosystem. Our previous analysis showed that the T. paradoxa complex comprises at least two species in Florida, T. ethacetica and a novel, undescribed Thielaviopsis species. To study the interaction, a detached leaflet assay for palms was established to test the pathogenicity of the species in the T. paradoxa complex. Lesion size was measured quantitatively and used to assess pathogenicity of the fungal isolates and host response. Wounding the detached leaflets promoted lesion development, and disease symptoms were similar to whole plants. A comparison of the two Thielaviopsis species showed that all T. ethacetica isolates consistently produced larger lesions and were more aggressive than the undescribed Thielaviopsis species in detached leaflet assay on foxtail palms (Wodyetia bifurcata). Subsequently, T. ethacetica was used to screen 262 individuals from 29 species across 17 genera of palms using the detached leaflet assay. Lesion development in individuals from at least five palm species across four genera was significantly different from the negative control, suggesting that these palm species may be susceptible to T. ethacetica. Additionally, variation in lesion development was also observed at varietal level in coconut palms. This detached leaflet assay would facilitate dissection of the Thielaviopsis-palm interaction and accelerate screening of palm germplasm repositories to identify sources of resistance to Thielaviopsis.