Catastrophic Plant Diseases

Many plant disease agents cause epidemics and pandemics, particularly if they spread from their endemic area of origin into countries where they have not existed before. These events often have had disastrous conse­ quences for man. Historians record the marching and countermarching of "all the king's horses and all the king's men"-the intrigues and counter-intrigues of the politicians and statesmen, but they generally neglect the factors of pests and pestilence, food and famine. In this article we propose to discuss some of the classic plant disease epidemics and pandemics of history and to assess their dramatic impact on the affairs of men. And the end is not in sight. In our time, nearly every country in the world is threatened by exotic plant diseases, especially because of greatly expanded international travel. There is a great diversity in the way that plant diseases occur and pro­ gress. We know that certai� pathogens appear in a sporadic manner, infect­ ing only single plants. Often no connection can be noticed between isolated disease foci. If the conditions of mili eu and season are favorable for the parasite, a mass infection may result, i.e. an epidemic. An epidemic is called epidemic if the disease is spread over an area in which its causal agent has been present for a long time. When the pathogen expands from this area into others, the disease is called a progressive epidemic. Often such epidem­ ics spread over several continents, causing mass infections and thus becom­ ing pandemic. It is decisive for this process that there be an adequate epi­ demic potentiality.