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 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 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 disastrous to the order of the world.
Some plants have relative immunity to a 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 which 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 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 a primary virus infection.
单选题
How many diseases are known to attack wheat?
【正确答案】
D
【答案解析】[解析] 本题问的是小麦的疾病数量。原文第一段的第二句话明确说到，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.由此可知，经济作物大约有30000多种疾病，有40种是针对玉米的，并且针对小麦的疾病数量与玉米的差不多。也就是说，小麦的疾病数量也是大约有40种。因此，本题的正确答案为D。
单选题
According to this passage, which of the following would a plant disease result in if left unchecked?
【正确答案】
D
【答案解析】[解析] 本题问的是原文中认为植物的疾病没有及时检查出来的后果。原文第一段的第三句话明确说到，The results of unchecked plant disease are all too obvious in countries which have marginal food supplies.表明这种后果在食物供应不足的国家最为明显。然后接着介绍说，这会引起食物短缺，导致饥荒，并且最后一句话说到，the resulting hardship on other millions could cause political upheavals...表明，最终的后果就是会引起政治动乱。可见，只有D选项的“社会动乱”与原文中的“政治动乱”意思最为接近。因此，本题的正确答案为D。
单选题
What is the main idea of the second paragraph?
单选题
According to the passage, some plant diseases can be prevented by ______.
【正确答案】
B
【答案解析】[解析] 本题问的是原文认为有些植物疾病可以通过什么途径预防。原文的最后一段介绍的是植物疾病的治疗。该段的第三句话说到，A plant can also be inoculated by man.表明植物也可以由人嫁接。可见，B选项符合原文内容。而A选项“把寄生虫杀死”；C选项“把所有昆虫都杀死”；D选项“改善作物的生长条件”的说法在原文中均没有提到。因此，本题的正确答案为B。
单选题
Which of the following statements is not true?
【正确答案】
D
【答案解析】[解析] 本题问的是哪一项的说法是错误的。根据原文最后一段的内容可知，植物的疾病可能是由病原体(pathogen)、细菌(bacteria)和真菌(fungus)引起的。因此，A、B、C三个选项的说法均是正确的。原文的最后一句话明确说到，The symptoms shown may cause an error in diagnosis，...表明，症状有时候可能会导致错误的诊断。由此可知，D选项说“症状总能帮助我们确定疾病”的说法显然是错误的。因此，本题的正确答案为D。