Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/1997
Publication Date: N/A
Interpretive Summary: Phytophthora root rot is a severe soybean pathogen which caused an estimated 560,300 metric tons reduction in soybean yields during 1994. Control of Phytophthora root rot has been accomplished mainly through the incorporation of race-specific genes from the different soybean types. New races of the fungus have emerged as a result of selection pressure due to the presence of race-specific resistant cultivars. A fast, accurate, and precise screening method was developed using race-specific resistant soybean germplasm. This method (plug-inoculation) was used to evaluate the progenitor of cultivated soybean, a wild annual species (Glycine soja) for Phytophthora root rot resistance. We tested this new method on 430 G. soja accessions for resistance to race 3 of the fungus. Nine G soja accessions were re-tested with races 1, 3, and 20. Of 430 G. soja accessions tested, 22 accessions had survival rates higher than 75% and nine had rates higher than 90% against race 3. Additionally, five of the nine accessions that were retested had greater than 60% survival against races 1, 3, and 20. These results show that the plug-inoculation method can be used as an alternataive to the traditional hypocotyl-inoculation method. This may have great impact on how the soybean industry screens for resistance. The new sources of resistance found in the progenitor of soybean may also provide longer resistance than found in cultivated soybean. Public and private researchers working on this pathogen should benefit from these findings.
Technical Abstract: New sources of soybean (Glycine max) resistance to Phytophthora megasperma f. sp. glycinea (Pmg) are needed to keep up with ever changing race patterns in the field. The objectives of our study were to: 1) develop a Pmg screening method for G. soja, and 2) screen G. soja lines for resistance to Pmg races 1, 3, and 20. An agar plug inoculation method, in which a 3 mm diameter mycelial plug of the fungus was placed mycelium side down on cotyledons of 10 day old soybean seedlings, was directly compared to the traditional hypocotyl inoculation method. There was no significant difference between the hypocotyl- and plug-inoculation methods when tested on four soybean differential lines using three Pmg races. This plug inoculation method was used to screen 430 G. soja accessions for resistance to Pmg race 3. Nine G. soja accessions were re-tested with races 1, 3, and 20. Of 430 G. soja accessions tested, 22 accessions had survival rates higher than 75% and nine had rates higher than 90% against race 3. Additionally, five of the nine accessions that were retested had greater than 60% survival against races 1, 3, and 20. These results suggest that the plug-inoculation method can be used as an alternative to the hypocotyl-inoculation method. Potential sources of new Pmg resistance and/or tolerance may be present in G. soja. However, future genetic research is needed to determine if these sources are different from the current sources found in G. max.