根据材料请回答 36～40 Diseases of Agricultural PlantsPlants, like animals, are subject

根据材料请回答 36～40

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.

第 36 题 How many diseases are known to attack wheat？

A．Around 30,000

B．Around 140

C．Around 29,960

D．Around 40