(a) Despite all the natural conditions of climate, relief and soil that may favour crop cultivation, the presence of biotic agents such as weeds, parasitic plants, diseases and insect-pests will damage crops and hamper farming in many parts of the world.
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(b) Many fungus diseases and insect pests do great damage to crops.
For example, the boll-weevil is a dreaded pest in the Cotton Belt of U.S.A. and has completely annihilated the Sea Island species of cotton in south-eastern U.S.A.
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i. The Coffee Blights of Ceylon destroyed practically all the coffee trees in the Central Highlands in 1878 and farmer s had to turn to tea growing instead.
ii. Even greater havoc was created by the fungus parasite Phytophthora infestations in Ireland between 1845 and 1848. It attacked the potatoes and caused the Potato famine, which drove many Irish farmers to leave their homes and migrate to England and the United Siates of America.
iii. Millions of hectares of vineyards in the U.S.A. (California) and Mediterranean Europe (especially France) were destroyed in the 1870s by an insect pest Phylloxera which bored through the roots of the vine plants and destroyed them.
iv. The Monilia disease and the Witches’ broom disease plagued the cacao industry of Ecuador in the early twentieth century.
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v. The temperate wheat lands often suffer from attacks by Smut and Bunt fungi and the rust disease.
vi. In Monsoon Asia, the paddy fields are not immune to attack. Insect pests like the pianggiang fly, as well as birds and animals such as rats, help to reduce the paddy yield.
vii. In Africa, the locust swarms, which eat up every leaf and blade of grass, are dreaded by all, while in many areas cattle rearing is almost impossible because of the prevalence of the tsetse fly, which spreads disease to the animals.
(c) Preventive measures against fungus diseases and insect pests include hand-picking, destruction of diseased stock and the spraying of selected insecticides and fungicides, e.g., DDT. Regular inspection and care are very necessary if farm crops are to be kept healthy and free from pests and diseases.