Pest and Disease Management (African Organic Agriculture Manual Booklet Series)

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It is important to know which pathogens create damage in the fields. Proper determination of the pathogen is the first step to effective pest and disease management.
Farmers consider organisms which reduce their crop yields as pests and diseases. Insects, birds or other animals are considered as pests, whenever they cause damage to crops or to stored produce. Fungi, bacteria and viruses are recognized as diseases, when they interrupt or modify the vital functions of growing plants or stored produce. But basically, all organisms are part of nature, irrespective of what they do, and deserve our respect.
Pest damage on crops is commonly caused by insects, mites, nematodes and mammals.
Insect damage is usually easy to identify: Leaves with holes or missing parts are damage caused by caterpillars or weevils; curled leaves are the result of aphids sucking sap; damaged or rotten fruit are common after being eaten by larvae of fruit flies; withering plants can also be caused by larvae of noctuids or stem borer; and branches or trunks with holes may be the result of an attack by wood-eating insects.
Mites are very small and cannot be seen with the naked eye. Spider mites, however, weave a typical tissue on attacked plant parts that is easily detected. Infested plants, leaves and fruit turn yellowish.
Nematodes are also very small and are not easily observed by the naked eye. Nematodes mostly attack plant roots resulting in yellowish plants that wither and die.
Mammals like elephants, monkeys or voles, and birds like sparrows, starlings and crows can also damage crops.

FiBL, Research Institute of Organic Agriculture