Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Plant Science

First Advisor

Roy A. Scott


One of the most destructive fungal diseases of soybean [Glycine max (L.) Merr.] is Phytophthora root rot, (PRR), caused by Phytophthora sojae (Syn. P. megasperma Drechs. f. sp. glycinea T. Kaun & D.C. Erwin). The most common method of protecting soybean from PRR is the utilization of specific resistance genes; however, continuous selection pressure exerted on the pathogen can lead to these resistance genes becoming ineffective. Infected plants that exhibit tolerance are able to yield nearly as well as uninfected resistant plants and may prevent the buildup of races capable of overcoming resistance genes. Two experiments were initiated to obtain information concerning the genetics of tolerance to PRR. The objectives of the first experiment were to: 1) determine frequency distributions of tolerance among F2 soybean populations and 2) compare F2 genotypes with their mid-parent values. The objectives of the second experiment were to: 1) compare parent-offspring regression and realized heritability estimates for tolerance, 2) determine the impact of number of parents on the heritability of tolerance, and 3) on progeny performance with respect to mid-parent values. Seven F2, and ten F3:4 soybean populations were screened for tolerance in all three generations in the greenhouse using the inoculum layer method to determine their relative levels of tolerance. F2 progeny scores for rot symptoms were continuously distributed. There were four populations in which some plants had lower root rot scores than the mid-parent values (1-44%). Extremely low heritability estimates which ranged from a high of 0.17 to a low of-0.10 were obtained through the use of parent-offspring regression. Standard errors associated with each estimate were larger than the individual estimates. Realized heritability was estimated by selecting the 20% highest and lowest tolerant lines from each population to determine selection response. These results were generally much higher, and ranged from 0.80 to -0.13. The number of parents used to create each population did not influence heritability estimates among populations nor the performance of progeny within populations. Large environmental variances of F2 populations indicated that this trait is sensitive to environmental influences. Low parent-offspring regression heritability estimates, in conjunction with large standard errors, further verified this finding. Realized heritability estimates, however, suggested that under selection, some relatively high tolerant lines may be identified. However, it appeared that for breeders to generate highly tolerant lines, large populations should be maintained prior to selection. The most progress would likely be made if selection for tolerance was practiced in later generations.

Library of Congress Subject Headings

Phytophthora sojae
Soybean -- Disease and pest resistance -- Genetic aspects




South Dakota State University



Rights Statement

In Copyright