Shade reduces the incidence of the Arabica coffee berry disease (Colletotrichum kahawae) in Cameroon

The coffee berry disease (CBD), caused by Colletotrichum kahawae, is a major constraint for coffee (Coffea arabica L.) cultivation in the main producing countries of Africa. This disease is specific to green berries and can lead to 60% harvest losses under conditions that are particularly favourable to its development. In Cameroon where small farms predominate, mixed cropping systems of coffee with fruit trees or intercropping with food crops together with low intensity management and pruning of coffee trees are widespread agricultural practices that can affect CBD epidemics. Fruit trees are commonly planted at random in coffee farms allowing a heterogeneous shading pattern for coffee trees growing underneath. Consequently, an epidemiological study was conducted to assess the effect of shade on disease development on smallholders' farms in Cameroon. The disease was monitored over four successive years (2002-2005), on coffee trees cultivated under diverse agricultural managements. To assess the specific effect of a permanent shade under homogeneous conditions, the disease development in-situ and in-vitro was compared in 2005, between coffee trees shaded artificially by a net and those located in full sun. In the field, assessments carried out on a weekly basis enabled to identify agricultural practices likely to reduce losses due to CBD. These observations showed that shade-grown coffee trees were significantly less attacked than those grown in full sun. Berries on the leafless branch parts, near the main trunk of the coffee tree, were less infected than those on leafy and peripheral branch sections. However, artificial inoculations in the laboratory showed that shade did not have any significant effect on an intrinsic susceptibility of coffee berries to CBD. They also indicated an ascending gradient for disease severity, whatever the location of coffee trees under the heterogeneous shade pattern. These results show that mixed cropping with shade plants is an agricultural practice which creates environmental conditions that limit CBD development. While reducing yield losses, this agricultural practice may also contribute to reduce the chemical control of the disease and to diversify coffee growers' revenues. (Résumé d'auteur