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

Abstract

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.

Mitochondrial Genomics of Six Cacao Pathogens From the Basidiomycete Family Marasmiaceae

“…Fungal isolates were isolated from TBD samples collected across Ghana as reported previously (Amoako-Attah et al, 2020). Based on that previous study (Amoako-Attah et al, 2020), one strain each from M. crinis-equi, M. palmivorus and Mar.…”

“…The whole genome sequence analysis of these isolates which is ongoing in our lab should shed more light on this subject. The rRNA ITS sequence is the only molecular marker so far used in Marasmiaceae phylogeny (Koch et al, 2018;Amoako-Attah et al, 2020). The mitogenome sequence provide more genetic information than the ITS sequence alone, and is a better choice for analyzing the origin and phylogeny of this lesser known family of Basidiomycetes.…”

“…scandens (faint-cream or dull-white hyphae). Among the four species, M. tenuissimus was the most frequently isolated TBD-associated fungus in Ghana (Amoako-Attah et al, 2020). When considered with the Moniliophthora spp.…”

“…scandens (syn: M. byssicolor) in Ghana and in other cacao growing areas of the world (Opoku et al, 2007;Amoako-Attah et al, 2016). However, recent morphological and molecular analyses identified four different fungal species, of the Marasmiaceae, causing TBD-like symptoms on cacao in Ghana (Amoako-Attah et al, 2020). The four species were M. crinisequi (abundant thin, black, "horse hair"-type rhizomorphs), M. tenuissimus (scattered brown or whitish to brownish-white rhizomorphs), M. palmivorus (aggregates of shiny-or silkywhite hyphae), and Mar.…”

“…scandens. As M. tenuissimus was the major causal agent TBD in Ghana and reported to have different morphotypes (Amoako-Attah et al, 2020), we also sequenced and assembled the complete mitogenomes of six additional M. tenuissimus isolates and performed a comparative mitogenome analysis along with Mon. roreri and Mon.…”