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 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.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 model or smut or rust.Frequently such a primary infection will weaken the plant so that a secondary infection may result form 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. According to the passage, some plant diseases can  _  .
infect people be prevented by inoculation be prevented by killing insects be prevented by improving the growing conditions