Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 21, 2005
Publication Date: June 1, 2006
Citation: Bonde, M.R., Nester, S.E., Hartman, G.L., Miles, M.R., Austin, C.N., Stone, C.L., Frederick, R.D. 2006. Evaluation of virulence of phakopsora pachyrhizi and p. meibomiae isolates. Plant Disease. 90:708-716. Interpretive Summary: Two closely related fungi, neither of which has ever been known to be present in the U.S. cause soybean rust. Disease producing propagules of the pathogens, collected in the 1970's and 1980's, and preserved alive in refrigerators at a very cold temperature, were compared to samples of the more aggressive pathogen recently collected from southern Africa and South America. Pathogen propagules of each collection were sprayed onto the leaves of popular U.S. soybean cultivars, in addition to elite soybean lines possessing an inherited ability to withstand infection from rust. Two weeks later, plants were compared. Results showed that some of the recently collected pathogen samples are more damaging than older samples collected 2 to 3 decades earlier. Today, o soybeans appear to be able to withstand all strains of the more damaging rust fungus, and entirely new approaches are needed to develop soybeans with the ability to withstand rust.
Technical Abstract: Soybean rust is incited by Phakopsora pachyrhizi and P. meibomiae. In 2001, P. pachyrhizi, the more virulent organism, was discovered in Paraguay, and the following year in Brazil and Argentina. Because no soybean cultivars presently exist in the U.S. that are resistant to all isolates of P. pachyrhizi, and screening is difficult and laborious, we initiated a project to develop a better method to screen for resistance. In greenhouse tests during non-winter months, P. pachyrhizi caused tan lesions typically containing 1to14 uredinia that varied greatly in size on individual soybean accessions susceptible to rust. In contrast, resistance was typified by 0 to 6 small uredinia with little size variation in reddish-brown lesions. P. meibomiae, during optimum periods for disease produced reddish-brown lesions with 1to7 small uredinia. During December and January, a period less favorable for disease, the average numbers and sizes of uredinia for both pathogen species were smaller; however, susceptible and resistant responses still could be differentiated. Using appropriate rust resistant and susceptible genotypes as standards, histological examination of inoculated soybean accessions 14 days after inoculation can be used as a method to quantitatively measure and screen for specific rust resistance.