The incidence of plant diseases even in the prehistoric period can be inferred from fossils about 250,000 years old, and it has been estimated that about 80,000 diseases may affect various crops, resulting in losses as high as u.s. $60 billion throughout the world annually (Klausner, 1987; Agrios, 1969; Chu et aI., 1989). Crop diseases are caused by distinct groups of organisms, predominantly fungi, bacteria, mollicutes, viruses, and viroids. Nematodes and protozoa are able to cause disease in some crops. Effective management of crop diseases depends essentially on rapid detection and accurate identification of the pathogens causing them.