Variation of Phakopsora pachyrhizi isolates on soybean

Phakopsora pachyrhizi Sydow, the causal fungus of soybean (Glycine max (L.) Merrill) rust, occurs in most soybean-growing areas of the world except continental North America. Initial studies on soybean rust isolates from the Western Hemisphere indicated that they were different than isolates from the Eastern Hemisphere. In 1992, the Eastern Hemisphere species, P. pachyrhizi, and the Western Hemisphere species, P. meibomiae, were established for the soybean rust fungi based on morphological differences. The first molecular differentiation of the two species was reported in 2002. A number of studies have reported the occurrence of race in P. pachyrhizi. either on soybeans or on other hosts. In 1984, a set of four native Australian Glycine species were used to identify six different virulence combinations of P. pachyrhizi. Much of the research on differentiating isolates on soybean was completed in a containment facility at in the U.S. Genetic characterization on four plant introductions (PIs) indicated the occurrence of four independently inherited dominant genes. These genes are known to be effective to a limited number of isolates. There are many studies that need to be completed to determine if all isolates respond equally in terms of survival, urediniospore production, telia formation, and host range under different environments. Over the next few years, our understanding of pathogen diversity will increase as more concerted research efforts take place in different parts of the world.