Identification and Characterization of Fungi Causing Thread Blight Diseases on Cacao in Ghana

Abstract

<i>Theobroma cacao</i> (chocolate tree) is currently under serious threat from thread blight disease (TBD), which has been attributed to the causal agent <i>Marasmiellus scandens</i> in other regions of the world. TBD in Ghana has similar symptomology but variable signs. This study sought to determine whether TBD in Ghana was caused by a single agent and whether <i>Marasmiellus scandens</i> was a significant agent of TBD. Forty-eight isolates were collected from eight geographical locations in Ghana for morphological and molecular characterization. Disease signs occurred as vegetative rhizomorphs or hyphal aggregates, which were classified into five morphotypes: A, abundant thin, black, "horse hair"-type rhizomorphs; B, scattered brown rhizomorphs; C, whitish to brownish-white; D, faint cream or dull white; and E, aggregates of shiny or silky white hyphae. Sequencing and analyses of three loci-the internal transcribed spacer region of the nuclear ribosomal repeat, nuclear large subunit, and mitochondrial small subunit-detected four species, all members of the Marasmiaceae, causing TBD-like disease. These were identified as <i>Marasmius crinis-equi</i> (morphotype A), <i>Marasmius tenuissimus</i> (morphotypes B and C), <i>Marasmiellus palmivorus</i> (morphotype E), and <i>Marasmiellus scandens</i> (morphotype D). <i>Marasmius tenuissimus</i>, the most frequently isolated TBD fungus in this study, is primarily an Asian fungus and not previously associated with diseases of cacao. <i>Marasmiellus palmivorus</i>, the second most frequently isolated fungus, is a pan-tropical pathogen with a broad host range; this is the first report of the fungus causing TBD on cacao. <i>Marasmius crinis-equi</i> also has a broad pan-tropical distribution and host range and causes thread blight on several tropical tree crops. Surprisingly, <i>Marasmiellus scandens</i>, the most frequently cited agent of TBD in cacao, made up only 8% of the isolates.