Identification and characterization of fungi causing thread blight diseases on cacao in Ghana

Authors: AMOAKO-ATTAH, ISHMAEL - Cocoa Research Institute Of Ghana; ALI, SHAHIN - University Of California, Davis; AIME, CATHERINE - Purdue University; ODAMTTEN, GEORGE - University Of Ghana; CORNELIUS, ERIC - University Of Ghana; NYAKU, SALOME - University Of Ghana; KUMI-ASSARE, E - Cocoa Research Institute Of Ghana; YAHAYA, HENRY - Cocoa Research Institute Of Ghana; Bailey, Bryan

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2020
Publication Date: 11/1/2020
Citation: Amoako-Attah, I., Ali, S., Aime, C.M., Odamtten, G., Cornelius, E., Nyaku, S., Kumi-Assare, E., Yahaya, H., Bailey, B.A. 2020. Identification and characterization of fungi causing thread blight diseases on cacao in Ghana. Plant Disease. 11:3033-3042.

Interpretive Summary: Ghana is a major grower of cacao (Theobroma cacao L.), a valuable crop yielding the essential component of chocolate. Thread blight, an emerging disease on cacao in Ghana, is caused by fungi that grow on the stems and leaves, causing severe defoliation if not managed. Surprisingly little is known about the fungi causing thread blight on cacao. The objective of this research was to identify the species of fungi causing thread blight on cacao in Ghana. The fungi causing thread blight were isolated in pure culture and their DNA extracted and sequenced. When comparisons were made to the DNA sequences of known fungi, four species were identified as causing thread blight on cacao in Ghana: Marasmius crinisequi, Marasmius tenuissimus, Marasmiellus scandens, and Marasmiellus palmivorus. Although M. scandens is cited in the literature as the causal agent of thread blight of cacao, Marasmius tenuissimus was the most commonly recovered species. We now know that thread blight of cacao is caused by a complex of related fungi that cause similar symptoms. Based on these results, scientists and farmers can adapt field practices directed at managing thread blight in the field making it easier to limit the crop losses resulting from the emergence of thread blight as a significant disease on cacao in Ghana.

Technical Abstract: Theobroma cacao L. is cultivated extensively in Ghana for the beans which are used in the production of chocolate and other confectionaries. The crop is currently under serious threat from thread blight disease (TBD) which is a complex of morphological forms of causal fungi but similar symptomologies. This study sought to find out whether the different forms of the casual fungi on cacao represent the same or distinct pathogenic species. Fifty-four TBD isolates were collected from eight geographical locations in Ghana for morphological and molecular characterization. Five morphotypes of TBD fungi were identified and designated; A (black), B (brown), C (whitish to brownish-white), D (faint cream or dull white) and E (shiny or silky white). Sequencing and analyses of internal transcribed spacer (ITS), mitochondrial small subunit (mtSSU) and nuclear large ribosomal subunit (nLSU) genomic regions from DNA of 47 selected isolates supported the morpho-typing. Morphotype A was confirmed as Marasmius crinis- equi F. Muell. Ex Kalchbr causing ‘horse-hair blight’ disease of cacao. Morphotype B and C were a single species belonging to Marasmius tenuissimus (Junghuhn) Singer. Meanwhile morphotype D was identified as Marasmiellus scandens (Massee) Dennis and Reid Comb. Nov., previously considered the main causal agent of TBD on cacao in Ghana. However, during sampling of TBD fungi for this study M. tenuissimus was most commonly recovered. Morphotype E was identified as Marasmiellus palmivorus (Sharples) Desjardin and this represents the first report of M. palmivorus causing thread blight disease on cacao. These species were also involved in dieback symptoms on twigs and young branches.