|ROD, KATHERINE - University Of Illinois|
|BRADLEY, CARL - University Of Kentucky|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/28/2017
Publication Date: 5/22/2018
Citation: Rod, K.S., Walker, D.R., Bradley, C.A. 2018. Evaluation of major ancestors of North American soybean cultivars for resistance to Pythium species that cause seedling blight. Plant Disease. 102:2241-2252. https://doi.org/10.1094/PDIS-09-17-1341-RE.
Interpretive Summary: Pythium seedling blight, a soybean disease caused by Pythium ultimum var. ultimum and several other species of Pythium, causes reduced plant stands and sometimes yield losses in the northern United States and in Canada. Out of 102 major ancestors of modern North American soybean cultivars, 14 were found to have resistance to an isolate of Pythium ultimum var. ultimum. Four of the lines were confirmed to be resistant to P. ultimum var. ultimum, and were found to also be resistant to isolates of Pythium irregulare and Pythium sylvaticum. Three of these resistant soybean lines were from or were descended from cold-tolerant cultivars that were developed in Sweden and used extensively in Canadian soybean breeding programs. These sources of Pythium resistance, or improved modern cultivars developed from them, could be used in soybean breeding programs to develop high-yielding, resistant cultivars adapted to the northern soybean production regions of the United States.
Technical Abstract: Pythium seedling blight, which is caused by a number of oomycete Pythium species, is a disease that affects soybeans (Glycine max (L.) Merrill) grown in the United States and Canada. Pythium ultimum var. ultimum, one of the most common species, is favored by cool, wet conditions that are most likely to occur in early spring, and it causes seed decay, root rot, and seedling damping off. One hundred and two major ancestors of modern North American cultivars and “first progeny” cultivars developed directly from ancestral lines were evaluated for resistance to P. ultimum var. ultimum and two other Pythium pathogens. Fourteen ancestors and first progeny cultivars had varying levels of partial resistance to an isolate of P. ultimum var. ultimum from Illinois. In a subsequent experiment, a subset of four of the most resistant lines (PI 84637, ‘Maple Isle’, ‘Fiskeby III’, and Fiskeby 840-7-3) and the susceptible cultivar Kanro were screened for resistance against isolates of P. irregulare, and P. sylvaticum, and resistance to P. ultimum var. ultimum was confirmed. The lines that were partially resistant to P. ultimum var. ultimum in the first experiments were also partially resistant to P. irregulare and P. sylvaticum. Among the isolates tested, the P. ultimum var. ultimum isolate was the most aggressive, followed by the P. irregulare and P. sylvaticum isolates. Modern cultivars descended from the soybean lines with partial resistance to these pathogens could be useful sources of resistance to Pythium seedling blight if they have similar levels of resistance.