Cultural Characteristics and Pathogenicity of Cibberella xylarioides Isolates in Coffee

Abstract

The cultural variability in Gibberella xylarioides (Fusarium xylarioides), the cause of coffee wilt disease (tracheomycosis), was studied under standard conditions using 36 isolates collected from Arabica coffee trees in south-western Ethiopia. The isolates ranged from appressed and sparse to raised and dense in apical colony growth, but most isolates (80%) had the intermediate (slightly raised and slightly dense) colony type. The fungus isolates had generally white mycelial color although they diffused varied pigments into the media after 10 days-of incubation. The typical pigments produced by G. xylarioides were grayish white, purplish white, light bluish and light violets. The mean radial growth rate was 3.1±0.3 cm after 7 days of incubation. The isolates varied in growth rate and those isolates from Bebeka were slower type. Inoculating seedlings of nine coffee cultivars in the greenhouse tested the pathogenic variability of four representative isolates. Highly significant difference among coffee cultivars, and among the fungus isolates; and a significant cultivar x isolate interaction, was observed both in percent dead seedlings and in length of incubation periods. Thus, the results confirmed the existence of variations both in resistance level of the host and in aggressiveness of the pathogen population. Although the resistance is predominantly horizontal, the significant cultivar-byisolate interaction (differential effect) indicated the presence of some vertical resistance in Arabica coffee genotypes. The variation among G. xylarioides isolates in cultural characteristics, such as pigmentation and growth rate, coupled with variation in pathogenicity (virulence) suggests that the fungus may occur as multiple strains.