Effects of different copper fungicide application rates upon earthworm activity and impacts on cocoa yield over four years

Blackpod disease, caused by Phytophthora fungi, is the major cause of yield loss in cacao systems worldwide and is most commonly controlled by copper (Cu) fungicides. However, fungicide use might have negative impacts on soil-faunal mediated nutrient cycling, a crucial process in perennial systems in the tropics as fertilizer is rarely used. An experiment was established in 35 year old cacao farms in southern Cameroon to assess the effects of different fungicide application rates, none, reduced (‘low’), and manufacturerʹs recommended rate (‘high’), upon earthworm cast production for 4 years and this was also compared with cast production in a secondary forest of the same age. Supplementary measurements were made to assess the impacts of fungicide treatment on litter decomposition and, through potentially lower earthworm activity, on soil compaction. Cocoa yields were 2.5 times greater in the high spray treatment than in the low spray treatment. There were no significant differences in cast production between cacao spray treatments in the first 3 years. In year 4 only, cast production was significantly lower in the high spray treatment than in the no-spray control. With no evidence of any negative effect on either litter decomposition or soil bulk density, the high spray regime can be recommended to farmers, perhaps accompanied by measures such as adding leguminous cover crops or mulch to hasten litter decomposition rates, provide soil coverage and thus possibly reducing the efficacy of P. megakarya inoculum.