Document Type

Dissertation - Open Access

Award Date


Degree Name

Doctor of Philosophy (PhD)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Emmanuel Byamukama

Second Advisor

Febina M. Mathew


interaction, partial resistance, pathotype, Phytophthora, soybean


Phytophthora root and stem rot, caused by Phytophthora sojae Kaufmann and Gerdemann, is an important disease of soybean (Glycine max L.) in South Dakota. To gain a better understanding of the importance of P. sojae in South Dakota, specifically pathotype diversity, identification of new resistance sources and the interaction with the soybean cyst nematode (Heterodera glycines Ichinohe, SCN), this research was undertaken with the following objectives - 1) to characterize the pathotype diversity of P. sojae causing Phytophthora root and stem rot on soybean in commercial fields in South Dakota; 2) to compare inoculation methods to evaluate for partial resistance to P. sojae on soybean and identify new sources of resistance to two virulence pathotypes of P. sojae in a recombinant inbred line (RILs) population derived from the cross between cultivated Glycine max (cv. Surge) and wild Glycine soja (PI 468916); and 3) to study the interaction between SCN and P. sojae on soybean. In order to achieve the objectives, a total of 114 isolates of P. sojae were recovered from soil samples covering 30 counties in South Dakota during a three year survey (2013 - 2015), of which 70 P. sojae isolates were pathotyped using 13 standard soybean differentials. Results suggest that mean complexity of the P. sojae pathotypes have increased over time and over 85% of the P. sojae isolates were able to defeat Rps1a, Rps1c and Rps1k that are commonly deployed Rps genes in the commercial cultivars of South Dakota. In order to find new sources of partial resistance to P. sojae, a qualitative comparison among three inoculation methods (inoculum layer test, tray test and rice grain inoculation) was accomplished in the greenhouse. Based on the recovery of P. sojae isolates (%), inoculum layer method was adopted to screen 100 recombinant inbred line (RIL) for partial resistance to two virulence pathotypes of P. sojae identified in South Dakota [PS-15-TF3 that is virulent on all 13 soybean differentials and PS-14-F14 that is virulent on only one differential (Rps7)]. As compared to the parents of the RIL population, [Glycine max (cv. Surge) and wild Glycine soja (PI 468916)] we found 9 RILs that had relatively shorter lesion length (0 to 5 mm) when inoculated with either of the P. sojae isolates. To study the interaction between SCN and P. sojae on soybean, a greenhouse experiment was set up in a completely randomized design in a factorial arrangement with four soybean cultivars (Jack, Surge, William 82 and Williams). Two isolates of P. sojae representing two different virulent pathotypes (PS-15-TF3 and PS-14-F14) and SCN HGtype 0 representing the most commonly found HG-type in South Dakota was used to perform inoculations. For all the cultivars, we observed that the lesion length was caused by P. sojae was increased in the presence of SCN relative to P. sojae treatment. However, SCN population was reduced in the presence of both the pathogens. The findings of our study highlight the high pathotype diversity of P. sojae and and increased lesion size when P. sojae co-infects with SCN. This information will help with the development of effective and improved strategies for managing Phytophthora root and stem rot through deployment of resistant genes in commercial soybean varieties that are likely to be more durable, managing SCN to reduce severity of Phytophthora root rot, and incorporation of identified resistance to P. sojae in RIL population for future breeding efforts.

Library of Congress Subject Headings

Soybean -- Disease and pest resistance -- South Dakota -- Genetic aspects.
Phytophthora sojae.
Soybean cyst nematode.


Includes bibliographical references



Number of Pages



South Dakota State University



Rights Statement

In Copyright