Thermal Tolerance Data and Molecular Identification Are Useful for the Diagnosis, Control and Modeling of Diseases Caused by Thielaviopsis paradoxa

Abstract

Several economically important diseases of forest trees and agricultural crops in many parts of the world have been linked to the ascomycete fungal pathogen Thielaviopsis paradoxa. This study compared the growth rate of 41 isolates of T. paradoxa sourced from different hosts and two countries (Nigeria and Papua New Guinea (PNG)) under six temperature levels (22 °C, 25 °C, 30 °C, 32 °C, 34 °C and 35 °C). Phylogenetic relationships were obtained from the analysis of their nuclear ribosomal DNA internal transcribed sequence (ITS) data. While all the isolates from PNG and few from Nigeria grew optimally between 22 °C and 32 °C, the majority had their highest growth rate (2.9 cm/day) between 25 °C and 32 °C. Growth performances were generally low between 34 °C and 35 °C; no isolate from the sugar cane grew at these high temperatures. The oil palm isolate DA029 was the most resilient, with the highest growth rate (0.97 cm/day) at 35 °C. Phylogenetic analysis delineated five clusters: a very large clade which accommodates the majority (30 Nigerian and 3 PNG oil palm isolates) and four small clades containing two members each. To a large extent, the clustering pattern failed to address the temperature-isolate relationship observed. However, only the four small clades represent isolates with similar temperature tolerances. It is most likely that wider and robust analyses with more diverse isolates and genetic markers will provide better insight on thermal resilience of T. paradoxa. Additionally, future research to establish relationships between vegetative growth at different temperatures and of different pathogenicity and disease epidemiology merits being explored. The results might provide useful information for the formulation of effective management and control strategies against the pathogen, especially in this era of climate change.

Keywords: Ceratocystis paradoxa; disease management; growth rate; optimum temperature; pathogen.

Figure 1

A dry basal rot-infected oil palm showing fracture of the lower fronds (A). A cross section of a dry basal rot-infected oil palm showing fruit bunch rotting symptom (B). A pure culture of Thielaviopsis paradoxa (C). Microscopic view of two ascospore types produced by T. paradoxa (D).

Figure 2

Phylogenetic tree constructed by Bayesian analysis of ITS rDNA sequences of 41 Thielaviopsis paradoxa isolates and two related species, Ceratocystis fimbriata and T. thielavioides (serving as outgroup; written in red colour). Numbers indicated above the branches are the relevant %PP values for the consensus tree. The scale bar represents the number of nucleotide substitutions per site. Isolates are written with black (oil palm isolates from Nigeria), purple (oil palm isolates from PNG), green (date fruit isolate from Nigeria) and blue (sugarcane isolates from Nigeria) colours.