ABSTRACT
The devastating effects of plant disease have been known for many centuries, and the actual diseases involved can often be recognized from the original texts. Epidemics might be regarded as acts of God, and proper ceremonies had to be performed if disaster was to be avoided. The connection between microorganisms and disease was established about the middle of the 19th century and opened the way to a scientific study of plant disease and its control. Fungi are probably the most important pathogens, but virus diseases are very important and cause severe losses. The effects of infections with bacteria and mycoplasmas 1 are severe when they occur but, overall, they are less important as pathogens. A few algae, usually Cephaleuros spp, cause serious bark diseases of tree crops, such as cocoa, coffee, tea, and rubber in the humid tropics. 2
