First Report of Fusarium oxysporum Causing Wilt on Hoodia gordonii in South Africa
Authors: Philippou O. A. , Minnaar-Ontong A. , Swart W. J. , van Biljon A. 
Published: November 2012

Traditionally the San people of southern Africa used Hoodia species as an appetite suppressant and various medicinal purposes (1). Hoodia gordonii (Masson) Sweet ex Decne therefore became a commercially sought-after species due to the claim of its anorectic activity. During 2004, extensive wilting was observed on H. gordonii in commercial plantings near Kakamas and Pofadder (northern Cape, South Africa). The wilting gradually increased, which caused stems to rot at the base and shrivel up, causing plants to collapse and die. Affected plants exhibited discoloration in the stems' vascular tissues. Vascular tissue excised from stems and excisions of roots were surface sterilized for 3 min in 70% ethanol followed by 3 min in 1% NaOCl, rinsed with sterile water and plated onto Van Wyks Agar, a Fusarium-selective medium (3). Isolates were grown on potato dextrose agar (PDA) and carnation leaf agar (CLA) for 14 days at 25°C. The morphological features were examined (2); identification was based on colony and sporodochia color as well as conidial morphology from single-spore colonies. The conidial morphology includes the presence or absence of macro- and microconidia and chlamydospores as well as the shape, number of septa, and basal cell of the macroconidia. The shape, characteristics, and phialides of the microconidia was also included in this analysis. To confirm pathogenicity, 18 1-year-old H. gordonii plants, 18 H. pilifera (L.f.) Plowes subsp. annulata (N.E.Br.) Bruyns plants, and 18 carnation seedlings were planted into autoclaved soil amended with 1% finely grounded oats inoculated with isolate CBS 132482 (PREM 11783), while control plants were planted in sterile soil. After 30 days, tissue was dissected from each stem, surface sterilized, rinsed, and plated on CLA and PDA for recovery of fungi. Control plants and carnations remained healthy and no fungi were recovered. All Hoodia plants displayed wilt symptoms and F. oxysporum were reisolated from the infected plants. DNA was extracted from the representative isolate (CBS 132482) and a fragment of the translation elongation factor 1-alpha (EF-1α) gene was amplified using primers EF-1/EF-2 by the polymerase chain reaction assay (4). After the isolate was sequenced and aligned, BLAST analysis of the 603-bp fragment (GenBank Accession No. JX003858) showed a 100% homology with F. oxysporum (GenBank Accession No. GU226828). The beta tubulin gene sequenced (GenBank Accession No. JX003859) was amplified using the primers Bt-2a/Bt-2b. BLAST searches with the resulting 311-bp fragment showed a 99.4% homology with several isolates of F. oxysporum in the GenBank database (JQ265753; FR828825; DQ092480). The fungus had a specific host preference because it did not infect carnations as well as previously tested plants, which included beans, pumpkin, tomato, and watermelon. To our knowledge, this is the first report of F. oxysporum causing wilt in H. gordonii in South Africa.

References: (1) B. Hargreaves and Q. Turner. Askelpios 86:11, 2002. (2) J. F. Leslie and B. A. Summerell. Page 369 in: The Fusarium Laboratory Manual, Blackwell Professional, Ames, IA, 2006. (3) P. S. van Wyk et al. Phytophylactica 18:67, 1986. (4) P. Vos et al. Nucleic Acids Res. 23:4407, 1995.