Characterization of Gibberella xylarioides isolates and testing the reactions of some coffee collections in the forest populations in southwest and southeast Ethiopia

Abstract

Coffee wilt disease which is caused by Gibberella xylarioides (Fusarium xylarioides) is one of the highly destructive diseases that affect plantations and smallholder coffees resulting in yield reduction, and also affecting forest coffee. However, the characteristics and variability of the pathogen, and the reactions of forest coffee accessions against the pathogen are not well studied in the forest coffee areas of southwest and southeast Ethiopia. Thus, the objectives of this study were to characterize and determine the variability of Gibberella xylarioides populations and also to test and determine the reactions of some coffee accessions collected in the major forest coffee areas of southwest and southeast Ethiopia. These studies were conducted both in the field, laboratory and greenhouse. Disease assessment and sample collection was conducted in four rainforest coffee areas (three fields/location) of Bonga (Keffa), Sheko (Brehane-Kontir), Yayu (Ilubabor) and Harenna (Bale) in southwest and southeast Ethiopia and the laborartory and greenhouse study works were conducted. The experimental design of this experiment was laidout as complete randomized design (CRD) with three replications in laboratory study while evaluations of coffee accessions and the host-pathogen interaction studies were also conducted in randomized complete block design (RCBD) with 5 isolates x 13 accessions factorial treatment combinations replicated three times. Coffee wilt was prevalent in most assessed forest coffee trees, semi-forest and plantation coffee trees and its incidence significantly increased with coffee tree losses that ranged from 12.8% to 27.6% at Bonga (Kaffa) and at Sheko (Brehan-kontir) in the southwest. The disease was more severe in Harenna (Bale) varying between 26.2 and 28.5% in southeast forest coffee areas. In cultural and morphological characterization of Gibberella xylarioides populations, most of the isolates from the southwest forest coffee areas had similar appearances in colony pigmentations, aerial and radial growths but slightly different from those isolates collected in the southeast forest coffee (Harenna). Morphologically, most of the isolates were similar in features like shape and size of both conidia (macro and micro) and septations. The reactions of 14 accessions collected in some forest coffee areas of Bonga and Yayu (southwest) and Harenna (southeast) were tested at seedling stage by inoculating with Yayu isolate (Y21) in the greenhouse at JARC. The result indicated highly significant differences among accessions ranging between resistant and susceptible host reactions. Accessions of Harenna HA1, HA2 and HA6 had very low (0 – 10 %) seedling deaths being highly resistant while accessions of Bonga and Yayu (BO1, BO2, YA1 and YA2) showed complete seedling death (90%) being highly susceptible to the pathogen isolates in the forest. There were highly significant differences (p < 0.01) both in percent seedling deaths and incubation periods in the host-pathogen interactions study where by seedlings of 12 coffee accessions were inoculated with four Gibberella xylarioides isolates representing forest populations of both organisms. Based on the results, the accessions of Harenna (P4 and P6) revealed resistance reaction to most isolates with low mean percent seedling deaths (21.9 and 21.5%); whereas Sheko (P38) and Bonga (P27) accessions were susceptible to most isolates and showed high mean percent seedling death (85.7 and 79.2%), respectively. Harenna isolate was more aggressive than most of the isolates including the one from plantation coffee (Gera). In comparing the combined forest coffee accessions by isolate interactions (differential effects), Harenna (H11) isolates induced a higher rate of percent seedling death on accessions P41, P47 (90%) and P49 (85.97%) than Sheko (SH21) isolate on the same accession of P41 (9.47%), P47 and P49 (0.0%). In conclusion, coffee wilt is an important disease of forest coffee impacting the conservation of wild coffee genetic resources although there exist variations in host reactions to the pathogen. The fungus populations collected in these forest coffee areas showed basically similar cultural and morphological structures of the species Gibberella xylarioides although clear differences in some features like growth nature, pigmentation and aggressiveness. The pathogen strains in the forest coffee areas are as aggressive as those isolates in plantation coffee.