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  3. Diseases of Agricultural Plants Plants, like animals, are subject to diseases of various kinds. It has been estimated that so

Diseases of Agricultural Plants Plants, like animals, are subject to diseases of various kinds. It has been estimated that so

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问题                                Diseases of Agricultural Plants
   Plants, like animals, are subject to diseases of various kinds. It has been estimated that some 30,000 different diseases attack our economic plants; forty are known to attack com, and about as many attack wheat. The results of unchecked plant diseases are ail too obvious in countries that have marginal food supplies. The problem will soon be more widespread as the population of the world increases at its frightening rate. Even in countries that are now amply fed by thek agricultural products there could soon be food shortages. It is easy to imagine the consequences of some disastrous attack on one of the major crops; the resulting famines could kill millions of people, and the resulting hardship on other millions could cause political upheavals disastrous to the order of the world.
   Some plants have relative immunity to great many diseases, while others have a susceptibility to them. The tolerance of a particular plant changes as the growing conditions change. A blight may be but a local infection easily controlled; on the other hand it can attack particular plants in a whole region or nation. An example is the blight that killed virtually every chestnut tree in North America. Another is the famous potato blight in Ireland in the last century. As a result of that, it was estimated that one million people died of starvation and related ailments.
   Plant pathologists have made remarkable strides in identifying the pathogens of the various diseases. Bacteria may include invade a plant through an infestation of insect parasites carrying the pathogen. A plant can also be inoculated by man. Other diseases might be caused by fungus which attacks the plant in the form of a mold or smut or rust. Frequently such a primary infection will weaken the plant so that a secondary infection may result from its lack of tolerance. The symptoms shown may cause an error in diagnosis, so that treatment may be directed toward bacteria which could be the result of a susceptibility caused by primary virus infection.
选项 A、Around 30,000.
B、Around 140.
C、Around 29,960.
D、Around 40.
答案D
解析 该题问“人们知道多少种小麦疾病?”第1段中的forty are known to attack corn，and about as many as attack wheat，“即大约与玉米疾病数目一样多的疾病会发生在小麦的身上”。
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