First report of Neofusicoccum australe causing dieback of honeybush in the Western Cape, South Africa

• Published in: Plant disease
• Main Authors: Halleen, Francois, Havenga, Minette, McLeod, Adèle, Mostert, Lizel
• Format: Journal Article
• Language: English
• Published: United States 01.03.2023
• Subjects: Botryosphaeriaceae; Crop Type; Pathogen detection; Causal Agent; Fungi; pathogenicity; Ornamentals; canker; Etiology; woody ornamentals; Cyclopia subternata; Subject Areas; dieback
• ISSN: 0191-2917
• DOI: 10.1094/PDIS-06-22-1429-PDN

Honeybush ( spp.) is an indigenous, leguminous member of the Cape fynbos biome growing in the coastal winter rainfall districts of the Western and Eastern Cape Provinces of South Africa (Joubert et al. 2011). Honeybush is used for the production of herbal teas and is harvested from wild-growing and cultivated plantations (du Toit et al. 1998). Very little is known regarding diseases caused by pathogens on this indigenous plant. Only one report of twig dieback on honeybush caused by several Nitschke species have been reported in South Africa (Smit et al. 2021). Several honeybush producers reported poor growth and dieback in their plantations in the Western Cape Province, South Africa. Symptoms included twig dieback, branch dieback, death of branches as well as death of entire plants. In April 2008, branches from 8-year-old cultivated plants with dieback symptoms were collected in Stellenbosch. Fungal isolations were carried out from affected material as described by Van Niekerk et al. (2004) which consistently revealed the presence of a Botryosphaeriaceae species. Two isolates were grown on water agar with sterile pine needles and incubated at 25˚C using a 12-hour day/night cycle and near-ultraviolet light. Pycnidia formed after two weeks. Morphological characteristics similar to (Slippers, Crous & Wingfield) Crous, Slippers & Phillips were observed (Phillips et al. 2013). Conidia were hyaline, aseptate, fusiform with subtruncate bases (16.8-)18.8-22.1(-24.6) × (4.8-)5.3-6.1(-6.4) µm (n=50). Conidiogenous cells were holoblastic, hyaline and subcylindrical to flask-shaped tapering to the apex (11-15 × 2 µm) (n=10). Colonies on potato dextrose agar were light primrose turning olivaceous grey after 7 days with a light-yellow pigment diffusing into the medium. Mycelia was moderately dense with an appressed centre mat. The identity of the isolates was further confirmed by sequencing the ribosomal RNA Internal Transcribed Spacer (ITS) and the elongation factor 1-alpha (EF-1α) gene regions using primer pairs ITS4-ITS5 (White et al. 1990) and EF1-728F-EF1-986R (Alves et al. 2008), respectively. Sequences had a 100% similarity to ex-type CMW6837 isolate (accessions AY339262 and AY339270) (Slippers et al. 2004). Two isolates (STEU6554 and STEU6557) were deposited in the culture collection at the Department of Plant Pathology at Stellenbosch University and the sequences were submitted to GenBank with accession numbers ON745603, ON745604, ON746573 and ON746574. Pathogenicity tests using the two isolates were conducted by inoculating two shoots each of three field-grown plants with a 4mm colonised potato dextrose agar (PDA) mycelium plug of each isolate on wounds made by a 4mm cork borer (Van Niekerk et al. 2004). A third shoot was inoculated with a uncolonized PDA plug as the negative control. After 12 weeks, brown-black lesions that were significantly longer (average 55.2 mm) than the uncolonized agar plug control (16.1 mm) were observed. Lesions were observed in all three plants. was re-isolated (van Niekerk et al. 2004) from all inoculated shoots confirming Koch's postulates. The economic impact and damages caused by as well as its incidence and severity on honeybush in South Africa is unknown. However, the pathogen caused dieback of entire branches and death of plants indicating that it could be an important pathogen of honeybush. Additionally, is one of the most important disease-causing Botryosphaeriaceae pathogens on a wide range of economical fruit and vine crops globally (Mojeremane et al. 2020). This is the first report of as a known pathogen causing decline and dieback of in South Africa. : Alves, A. et al. 2008. Fungal Divers. 28:1. du Toit, J. et al. 1998. J. Sustain. Agric. 12:67. Joubert, E. et al. 2011. S. Afr. J. Bot. 77:887. Mojeremane, K. et al. 2020. Phytopathol. Mediterr. 59:581. Phillips, A. J. et al. 2013. Stud. Mycol. 76:51. Slippers, B. et al. 2004. Mycologia 96:1030. Smit, L. et al. 2021. Eur. J. Plant Pathol. 161:565. van Niekerk, J. M. et al. 2004. Mycologia 96:781. White, T. J. et al. 1990. Pages 315 in: In PCR Protocols: A Guide to Methods and Applications. Academic Press Inc, USA. . The author(s) declare no conflict of interest . This work benefitted from the financial support of the Agricultural Research Council, Infruitec-Nietvoorbij, South Africa.