Pests and diseases in agroforestry systems of the humid tropics

Few studies have included detailed investigations of the interactions of ag roforestry techniques with pests and diseases, although the relevance of su ch interactions has long been recognized. The objectives of this review are to provide basic information on pests and diseases in tropical agroforestr y systems and to develop concepts which can assist in the future in the sys tematic data collection and analysis in this field. The emphasis is on simu ltaneous agroforestry systems with annual and perennial crops, although rot ational systems are also discussed. Crop rotation is an important pest and disease control strategy in annual cropping systems, and the principle of a ltering host with non-host plants can also be applied in improved fallow sy stems, provided that hosts of crop pests and diseases are avoided when sele cting the fallow species. When annual cropping systems are transformed into simultaneous agroforestry, the control strategy of frequent disturbance of pest and disease populations is to some extent substituted for the strateg y of increased stability and internal control mechanisms. However, reduced pest and disease risk is not automatically achieved by introducing perennia l plants and increasing the plant diversity in a system. If plant species a re introduced that harbor pests or diseases of other species in the system, the risk of pest and disease outbreaks may actually increase. For evaluati ng such risks, it is important to consider host-ranges of diseases on the p athovar instead of the species level. Beside the selection of compatible pl ant species, their spatial arrangement may be important for reducing the sp read of pest and disease organisms through the system, although little info rmation is available on such effects, and they may be largely irrelevant fo r organisms with efficient dispersal mechanisms such as wind-dispersed fung i. In addition to the species-specific, 'biological' effects of plants on p ests and diseases, their unspecific, 'physical' effects can be of major rel evance for pest and disease development as well as the susceptibility of th e affected plant species. Increased pest and disease incidence has often be en observed directly at the tree-crop interface, caused by the humid microc limate, physical protection of mammal and bird pests by the trees and event ually reduced pest and disease tolerance of competition-stressed crops. Lin ear tree plantings and hedgerows affect the wind transport of small insects and disease propagules, the active immigration and emigration of pest orga nisms as well as the populations of natural enemies. Similarly, overhead sh ade has a major effect on the micro- climatic conditions under which pest a nd disease organisms, their natural enemies and the crops themselves develo p, and its optimization is a highly efficient control strategy for many pes ts and diseases. On infertile soils, the susceptibility of crops to pests a nd diseases is strongly affected by the availability of plant nutrients, wh ich may be influenced by agroforestry techniques in various ways. Soil mana gement measures such as mulching and planting cover crops may affect crop h ealth by improving soil fertility and by directly acting on pest and diseas e populations. The importance of a more systematic collection of pest and d isease related information for agroforestry, e.g., in a central database, a nd of the development of strategies for reducing pest and disease risks in agroforestry in cooperation with farmers is stressed.