Date of Graduation

7-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Plant Science (PhD)

Degree Level

Graduate

Department

Plant Pathology

Advisor/Mentor

John C. Rupe

Committee Member

Craig S. Rothrock

Second Committee Member

Pengyin Chen

Third Committee Member

Burton H. Bluhm

Fourth Committee Member

Brad Murphy

Keywords

Biological sciences, Plant pathology, Pythium aphanidermatum, Pythium resistance, Soilbourne pathogens, Soybeans

Abstract

Pythium spp. are an important group of pathogens causing stand losses in Arkansas soybean production. New inoculation methods and advances in molecular techniques allow a better understanding of cultivar resistance and responses of Pythium communities to cultural practices. The objectives of this research were to i) characterize the resistance of soybean to P. aphanidermatum with two phenotyping assays that evaluated the seed rot phase of the disease; and ii) understand the effect of long term crop rotation on species diversity and iii) to determine the effect of location, temperature and continuous soybean and soybean-rice rotation on Pythium spp. diversity in several locations in Arkansas. For objective one, resistance to seedling disease caused by P. aphanidermatum, was characterized in 84 F2:6 soybean lines derived from a cross of ‘Archer’ and ‘Hutcheson’ cultivars using a seed plate assay and an infested vermiculite assay. The lines were assayed with 5,403 SNP markers and genetic maps and QTL mapping was performed. With both inoculation methods, two quantitative trait loci (QTL) were identified on chromosomes 4 and 7. In objective two, the effect of crop rotation and location on species diversity was determined by using soybean seed to bait Pythium spp. from soil of plots following a ten year rotation. Of the 320 isolates, 12 species identified, P. spinosum, P. irregulare, P. pareocandrum, and P. sylvaticum were the most common. There were significant differences in number of Pythium isolates from the ten rotation systems with the rice (wheat)-soybean (wheat) rotation having the highest recovery of these six Pythium spp. In objective three, soils from a soybean-rice and a soybean- soybean rotation were collected from three locations in 2012. A total of 275 isolates were identified representing 25 species. The most frequently recovered species were P. irregulare, P. pareocandrum, P. sylvaticum, P. corolatum and P. spinosum. Location had a large effect on Pythium population composition and diversity. Distinctive species prevailed in each of the three locations across two temperature and two rotations. Overall, populations of Pythium spp. varied the most among locations, but were not influenced by the previous crop or the isolation temperature.

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