Coffee Leaf Rust Epidemics (Hemileia vastatrix) in Montane Coffee (Coffea arabica L.) Forests in Southwestern Ethiopia

Abstract

Coffee ( Coffea arabica L.) is native to southwestern Ethiopia growing as understory of the rainforests that harbor huge floral and faunal diversities. Besides drastic reduction in the forest cover and low average yield, the crop is attacked by several diseases among which coffee berry disease, coffee wilt disease and coffee leaf rust caused by Colletotrichum kahawae , Gibberella xylarioides and Hemileia vastatrix , respectively, are the major fungal diseases contributing to reduced yield in the country. The epidemics of coffee leaf rust (CLR) was monitored between July 2007 and April 2008 in Bonga, Berhane-Kontir and Yayu montane coffee forest populations of southwest Ethiopia to determine the incidence and severity of CLR and its seasonal variation in the forest coffee populations and their reaction to leaf rust in the natural habitat. Thirty coffee trees were selected from each forest (three sites within a forest) coffee population to record incidence (percent rusted leaves), severity (percent leaf area damaged) and sporulated lesion density (number of lesion per leaf, SLD) from selected six branches per tree. An average of 10-12 leaves per branch was considered to determine each disease parameter. The data were analyzed using nested design (tree under location) using SAS statistical package. The occurrence of CLR significantly varied with locations and seasons. Overall mean incidence of 31.1, 21.4 and 7.9 and SLD of 2.7, 1.8 and 0.86 occurred in Yayu, Berhane-Kontir and Bonga montane coffee forest populations, respectively. Leaf rust was low (13.9%) in July 2007 and high (29.6%) in January 2008. Significant variation observed among each coffee tree within a location and season significantly interacted with both location and coffee trees within a location. The mean rust incidence varied from 0.36 to 18.5% in Bonga, 1.8 to 49% in Berhane-Kontir, and 11.8 to 62.6% in Yayu forest coffee populations. The corresponding severity ranged from 0.08 - 1.2%, 0.24 - 1.7% and 0.91 - 3.3% whereas the SLD varied from 0.08 - 1.9, 0.33 - 3.65 and 1.5 - 5.9% in that order. The observed heterogeneity of forest coffee populations to leaf rust in the field under native agro-ecology provides an opportunity to develop resistant varieties among the enormous forest coffee genetic resources and at the same time calls for strategic multi-site in situ conservation to rescue and maintain the present genetic variation and enhance co-evolutionary processes. The selected forest coffee trees that showed promising results should be further investigated for their possible value for future utilization. The location-season and coffee tree-season interaction effects necessitate characterization of Hemileia vastatrix races prevalent at each location and insist strategic variety development for contrasting environments. Keywords: Coffee Leaf Rust; Ethiopian Coffee; Epidemics; Hemileia Vastatrix ; Incidence; Seasonal Variation; Severity