Date of Graduation


Document Type


Degree Name

Master of Science in Plant Pathology (MS)

Degree Level



Plant Pathology


Craig S. Rothrock

Committee Member

John C. Rupe

Second Committee Member

Nathan Slaton


Increasing salinity is an important factor limiting agricultural productivity worldwide. In addition to direct effects on growth and yield, diseases also may be affected. This study characterized the effects of soil salinity on seedling disease of soybean and rice caused by Pythium spp. Controlled environment experiments on soybean used two cultivars which differed in chloride tolerance and soil treated with a CaCl2 solution to create a range of electrical conductivity (EC) levels. For soybean, soil was either not infested or infested with Pythium sylvaticum or P. aphanidermatum (pathogenic to soybean), or P. oligandrum (not pathogenic to soybean). Twenty-one days after planting, seedling stand, growth, and development were assessed. Salinity reduced seedling stand at or above 2.057 dS/m. Electrical conductivity averaged 0.640 (control), 1.060, 1.632, and 2.039 dS/m in the first experiment and 0.930 (control), 1.483, 2.057, and 2.570 dS/m in the second experiment using the soil dilution method (2:1) to measure EC. Leaf number, shoot weight and root altitude also decreased at or above 2.057 dS/m. Root volume and root tips decreased at 2.570 dS/m (Exp 2) but not at lower EC levels. Shoot growth decreased with P. aphanidermatum and P. sylvaticum at moderate salinity increasing growth reductions compared to the control. Cultivars which differed in chloride tolerance responded similarly in these experiments. Root development was stimulated by pathogen infestations at the base EC levels. Salinity had no effect when the nonpathogenic species were used. Controlled environment experiments on rice used two genotypes which differed in Pythium resistance, Wells (susceptible), and PI 560281 (having moderate resistance). For rice, soil was either not infested or infested with Pythium irregulare or P. torulosum (pathogenic to rice) or P. ultimum (not pathogenic to rice). Thirty-five days after planting, seedling stand, growth, and development were assessed. Electrical conductivity averaged 0.651 (control), 1.113, 1.658, and 2.190 dS/m for calcium chloride treatments. Salinity significantly reduced stand at 1.113 dS/m. Shoot growth and root development also decreased at 1.113 dS/m. P. irregulare and P. torulosum decreased stand across all EC levels. These pathogens decreased shoot growth and root development at low EC levels (including the base salinity), but this effect was overwhelmed as salinity increased. P. ultimum slightly decreased emergence and stand at the base salinity, but had a protective effect against increasing salinity levels at and above 1.113 dS/m. P. ultimum also increased root altitude across salinity levels (0.651 and 1.658-2.190 dS/m). Pythium resistance for PI 560281 was only evident as greater emergence after 14 days. In vitro, experiments were conducted on Pythium spp. over a range of electrical conductivity levels using CaCl2 solutions. Zoospore production, discharge, motility, and chemotaxis; oospore germination; and mycelial growth were used to assess salinity effects. EC levels in zoospore experiments ranged from 0.3 (control) to 4.3 dS/m. Salinity significantly decreased zoospore production and motility at EC levels as low as 1.3 dS/m, while zoospore discharge was reduced at 3.3 dS/m, and zoospore taxis was not significantly affected. EC levels in oospore and mycelium experiments ranged from 2.3 (control) to 12.8 dS/m. Mycelial growth and oospore germination were not significantly affected by increased EC. Abiotic interactions are important considerations in understanding and managing diseases. This research suggests that Pythium spp. caused greater seedling disease in the presence of a stress such as occurs with increasing soil salinity. Pythium spp. which were not virulent at the base soil salinity caused disease at moderate EC levels (soybean experiment), while Pythium spp. which are virulent at base salinity levels have an additive effect that diminishes at high EC levels. Understanding the biology of seedling pathogens and how they are affected by environment is important for the management of seedling diseases over the range of planting environments. This research indicates that Pythium spp. are able to grow and reproduce at EC levels which limit seedling stand establishment. Results suggest that increased Pythium disease by increasing EC levels is not the result of conditions which favor pathogen development.