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 corn, and about as many attack wheat. The results of unchecked plant disease are all too obvious in countries which have marginal(贫乏的) food supplies. The problem will soon be more widespread as the population of the world increases at its frightening rate. Even in countries which are now amply(充足地) fed by their agricultural products, there could soon be critical 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(剧变)disastrous to the order of the world.
Some plants have relative immunity to a great many diseases, while others have a sus-ceptibility(易感染性) to them. The tolerance of a particular plant changes as the growing conditions change. A blight(枯萎病)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 which killed virtually every chestnut tree in North American. 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(病理学家)have made remarkable strides in identifying the patho-gens(病原体)of the various diseases. Bacteria(病菌)may invade a plant through an infes-tation(侵扰)of insect parasites( 寄生昆虫)carrying the pathogen. A plant can also be inoc-ulated(接种疫苗)by man. Other diseases might be caused by fungus(真菌)which attacks the plant in the form of a mold(霉菌病)or smut(黑穗病)or rust(锈病). 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 a primary virus infection.
36. How many diseases are known to attack wheat?
A. Around 30,000
B. Around 140
C. Around 29,960
D. Around 40
37. According to this passage, which of the following would a plant disease result in if it was left unchecked?
A. A world war.
B. Border conflicts
C. Rations of grain and meat.
D. Social upheavals.
38. What is the main idea of the second paragraph?
A. Some plants have relative immunity to a great many diseases, while others have a susceptibility to them.
B. The tolerance of a particular plant changes as the growing conditions change.
C. A blight killed virtually every chestnut tree in North American.
D. A blight may be a national infection.
39. According to the passage, some plant diseases can be prevented by
A. killing parasites
B. inoculation
C. killing insects
D. improving growing conditions
40. Which of the following statements is not true?
A. Some plant diseases may be caused by bacteria.
B. Some plant diseases may be caused by pathogens.
C. Some plant diseases may be caused by fungus.
D. Symptoms are always helpful in identifying diseases.
参考译文
农作物的病害
跟动物一样，植物也会受到各种疾病的侵袭。世界上大约有3万种侵害经济作物的疾病：已经知道有40种会侵害玉米，还有40种会侵害小麦。在食物匮乏的国家中，由植物疾病失控所引起的后果非常严重。随着世界人口以惊人的速度增加，植物疾病所引发的问题将越来越广泛。即使是在那些现在有足够的食物供给的国家，不久也将面临食物匮乏的问题。当某种主要农作物遭到灾难性病害时，我们不难想象其后果会是怎样；由此引发的饥荒将导致大量人口的死亡，而幸存的人由于度日艰难，也会引起政治上的动荡，对世界秩序产生灾难性影响。
某些植物对许多疾病都有免疫力，然而另外一些植物却很容易受到感染。一种植物对疾病的抵抗力会随着其生长环境的改变而改变。枯萎病原本只是一种易于控制的地区性传染病，但是它却能够侵袭一整片地区或整个国家中的某些特定的植物。比如，枯萎病曾经毁掉了北美洲几乎所有的栗子树。另一个例子是上个世纪发生在爱尔兰的很有名的马铃薯枯萎病，其后果是，由此引发的饥荒和疾病导致100万人丧生。
植物病理学家在鉴别多种植物疾病的病原体方面取得了长足进步。病菌侵害植物很有可能是因为寄生在植物上的昆虫携带了病菌。但是，植物疾病同样可以通过接种疫苗来预防。还有一些疾病是由真菌引起的，其病害表现为霉菌病、黑穗病和锈病。通常，一种主要的传染病会降低植物的抵抗力而使之染上其他次要的传染病。那么由此导致的症状会使诊断出现错误，人们对患病植物的治疗也许只针对了次要病菌引起的疾病，而不是针对有主要病菌引起的疾病
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