阅读下面这篇短文，短文后有2项测试任务：(1)第23～26题要求从所给的6个选项中为第2～5段每段选择1个正确的小标题；(2)第27～30题要求从所给的6个选项中选择4个正确选项，分别完成每个句子。
Plants and Diseases 1 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 diseases are all to 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 unrest disastrous to the order of the world. 2 Some
plants have relative immunity to a great many diseases, while others have a
suceptibility(易感性) 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 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 diseases. 3 Plant scientists have made
remarkable strides in identifying the pathogens(病原菌) of various diseases.
Bacteria may invade a plant through an infestation(大批出没)of insect parasites
carrying the pathogen. Other diseases might be caused by fungus(真菌)which attacks
the plant in the form of rust. 4 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. Plant diseases can be prevented by ______.