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

Theobroma cacao (chocolate tree) is currently under serious threat from thread blight disease (TBD), which has been attributed to the causal agent Marasmiellus scandens 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 Marasmiellus scandens 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 Marasmius crinis-equi (morphotype A), Marasmius tenuissimus (morphotypes B and C), Marasmiellus palmivorus (morphotype E), and Marasmiellus scandens (morphotype D). Marasmius tenuissimus, the most frequently isolated TBD fungus in this study, is primarily an Asian fungus and not previously associated with diseases of cacao. Marasmiellus palmivorus, 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. Marasmius crinis-equi also has a broad pan-tropical distribution and host range and causes thread blight on several tropical tree crops. Surprisingly, Marasmiellus scandens, the most frequently cited agent of TBD in cacao, made up only 8% of the isolates.