Zhaowei Liu

Document Type

Thesis - University Access Only

Award Date


Degree Name

Doctor of Philosophy (PhD)

Department / School

Plant Science


Phytophthora sojae is an oomycete which causes Phytophthora root and stem rot of soybean. Continued application of race-specific resistance has accelerated the emergence of new races of the pathogen. To better understand the population dynamics and race shifts of P. sojae, the RAPD technique and ribosomal DNA sequences were used to estimate the degree of genetic variability and relationships among isolates collected from the North Central United States. RAPD markers were also employed to identify F1 hybrids from a cross between two different races. Comparison of RAPD profiles showed that there was a relatively high degree of genetic similarity in isolates of P. sojae. The greatest difference occurred between three Wisconsin isolates and the rest of the isolates at the 82.7% similarity level. However, the majority of the isolates shared a similarity coefficient larger than 90%. RAPD profiles were generally not associated with geographic locations or virulence phenotypes. Some subgroups within major clusters, however, showed relationships with locations or races. A high degree of genetic similarity (>93%) was revealed among 27 P. sojae isolates from a single field in South Dakota. The extent of molecular variation was, however, comparable to that found in isolates from various locations in the state. Analysis of race 25, a virulence phenotype that can defeat widely deployed Rpsl-k resistance, showed that molecular variability among various isolates was widely distributed (similarity >86.7%). It almost covered the whole spectrum of genetic diversity found in isolates with different virulence and geographic backgrounds collected from the North Central United States. Virulence testing characterized 15 phenotypes in the region, and five in the single field. The correlation coefficient between virulence and RAPD similarity matrices was 0.46, indicating that genetic variability was not highly associated with virulence variability. Genetic variability may have arisen via a combination of mechanisms such as host selection, geographic isolation, clonal mutation and occasional genetic recombination through outcrossing. Among these processes, selection imposed by the host plant has probably played a major role. The fine distribution of the P. sojae subpopulations in the region found in this study would require that resistance genes be deployed in a fine scale too. Selection for resistance in a soybean breeding program should also include various races with different genetic backgrounds. The RAPD analysis showed that universally a virulent strains of P. sojae encompassed two divergent groups of isolates. One shared a similar genetic background with P. sojae. Examination of the internal transcribed spacers on ribosomal DNA by restriction analysis verified this finding. The genetic diversity revealed may suggest a new Phytophthora species or a subspecies of P. sojae. Five Fl hybrids were identified from 120 single oospores from the race 7 and race 25 cross using RAPD markers specific for each parent. This is an important step towards constructing a linkage map and detection of DNA markers linked to avirulence/virulence genes in P. sojae.

Library of Congress Subject Headings

Phytophthora sojae

Soybean -- Disease and pest resistance -- Genetic aspects



Number of Pages



South Dakota State University



Rights Statement

In Copyright