Detached Leaf Assay for Palms Identifies Variable Disease Resistance to Thielaviopsis Species

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.