Diseases as a Factor in Plant Evolution

Abstract

Disease epidemics are among the most spectacular of biological phenomena. Not only can mortality be enormous but it is selective, sparing the occasional resistant or escapt(d individual and destroying the susceptibl; ones. Crops, domesticated animals, and man himself have suffered repeated epidemic disasters throughout recorded history. Of the many plagues that raged across Europe and Asia, the one in the winter and spring of 1347 -1348 was especially virulent and the death rate exceptional. In many communities, less than 10% of the population survived and there were not enough healthy people to bury the dead (16). In 1633, a smallpox epidemic swept through the New England Indian population with such virulence that some villages were carried off entirely (14). Even measles, a com­ paratively mild disease in Europe, took a devastating toll of Polynesians in Hawaii (61). From the havoc wrought on the forces of Sennacherib (2 Kings 19:35) and the plagues visited on Egypt to the 20th century, history has been punctuated by the dread and terror of epidemic disease. Most of us have seen towns and cities stripped of their best shade trees by Dutch elm disease, and some of us can recall the great, stark, grey hulks of dead chestnut trees that once dotted our eastern forests. We can, somehow, get Py without shade and timber trees, but when epidemics strike the food plants we li.ve by, famine and starvation may result. We all know of the great potato famines of 1846-1851 which had such devastating effects on the population of Ireland. Perhaps we are not so well informed on the great famine in Bengal of 1943 (65) caused primarily by Helminthosporium oryzae (Cochliobolus miyabeanus). Even a nation producing substantial food surpluses like the United States can suffer heavy financial loss in epidemic years. In countries that normally import some food and that have limited exchange resources, epidemics of crop disease are utter disaster. Plant diseases can cause humans to die (of starvation) as well as the black death or cholera. One must suppose that any agent that selectively destroys genotypes, populations, or whole species as efficiently as epidemic disease surely has played a role in the evolution of modern faunas and floras.