Mating Test and <i>In Vitro</i> Production of Perithecia by the Coffee Wilt Pathogen , <i>Gibberella xylarioides</i> (<i>Fusarium xylarioides</i>)

Abstract

Gibberella xylarioides Heim & Saccas, the teleomorphic state of Fusarium xylarioides Steyaert, is a fungal pathogen causing a vascular wilt disease of coffee known as tracheomycosis. Coffee wilt disease has been one of the endemic diseases of Arabica coffee (Coffea arabica) with increasing outbreaks and prevalence in Ethiopia. In the present study, in vitro production of fertile perithecia was assessed by crossing eight randomly selected monoconidial isolates of G. xylarioides (F. xylarioides) collected from C. arabica, including few strains from C. canephora and C. excelsa. Five media types and two temperature levels were compared for inducing the sexual structures in the fungus. In vivo development of perithecia on coffee trees in the field and on inoculated young seedlings in the greenhouse was also examined. More than 30% of the crosses formed perithecia in vitro including those pairings of isolates recovered from the different Coffea spp. Isolate Gx1 (BBA 71975) showed high intra- and inter-fertility with most of the strains indicating that this isolate was most probably ‘female fertile' mating type. This interfertility was successfully achieved only on carrot agar and V-8 juice medium at 20oC and 12-hr photoperiod 10 – 12 wks after plating. The fungus often produced abundant perithecia containing enormous ascospores in the barks of dead coffee trees in the field and in the artificially infected young seedlings. The in vitro and in vivo perithecial developments were most likely favoured by cool and moist (wet) conditions. Abundant production of ascospores in the field is an integral part of survival and dispersal mechanisms along with genetic exchange system. The in vitro formation of fertile perithecia enables to study inheritance of traits such as pathogenicity (virulence) and gene flow and genetic diversity in populations of G. xylarioides.

Keywords: Coffee wilt disease; Coffea arabica; Ethiopia; Gibberella xylarioides; Mating test; Perithecia

Ethiopian Journal of Biological Sciences Vol. 6 (1) 2007: pp. 63-75