Contrasting coffee leaf rust epidemics between forest coffee and semi-forest coffee agroforestry systems in SW-Ethiopia

Ethiopian Arabica coffee is produced in different agroforestry systems which differ in forest management intensity. In forest coffee systems (FC), coffee shrubs grow naturally in the understory of Afromontane forests with little human intervention, whereas in semi-forest coffee systems (SFC) thinning of the canopy and removal of the understory is applied. Coffee leaf rust (CLR) disease is a growing concern for coffee agroforestry, but to what extent infection pressure is affected by management intensity is poorly known. Here we assessed CLR infection through time across FC and SFC systems in SW-Ethiopia. CLR infection was significantly higher for SFC, with a gradual reduction of this difference during the beginning of dry season (November) through main rainy season of (July). Our findings also demonstrated that CLR infections were significantly lower in the FC system as compared to SFC system in both years 2015/16 and 2020/21. The higher CLR infection was partly explained by lower crown cover and higher human impact. We expect that reduced wind speed and droplet penetration under closed canopies and reduced human-facilitated spore dispersal are the dominating mechanisms behind lower CLR infection in FC systems, yet lower coffee density in FC may also play a role. Overall, our results indicate that although higher management intensity still generally results in higher total yields per hectare, proportionally larger losses due to CLR infection can be expected. Therefore, introducing more coffee genetic diversity, screening resistant coffee varieties and increasing canopy cover in the SFC will mitigate the CLR disease pressure and guarantee the sustainability of higher yields of the system in the future. Also, lower yields in the FC will be rewarded through providing price premiums so that farmers instantly get a higher price for their lower yield, guaranteeing livelihoods.