Probably no species of plant is exempt from the attacks of some kind of animal or vegetable enemy or parasite. Under natural conditions, however, it is rare to find any disease developing the proportions of an epidemic and killing off large numbers of plants in a particular area. In the case of tropical trees like Hevea, one very good reason for the absence of epidemics under the ordinary conditions of forest growth is readily discernible. Unlike the uniform woods characteristic of temperate climates, a tropical forest almost invariably consists of a varied mixture of different species of trees. This mixed character of tropical vegetation extends so far that it has been estimated that as many as three hundred different species of trees are often to be found on a single acre of ground, whilst two individuals of the same species practically never stand side by side.
The conditions obtaining in a plantation are precisely the reverse of those natural to a tropical forest.
Hundreds of thousands of trees of the same species are here arranged in regular rows, their leaves and roots actually intermingling. The conditions are therefore ideal for the development into an epidemic of any disease which may make its appearance.