Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Soybean cultivars were evaluated during two consecutive years for field resistance to black root rot (BRR) caused by Cylindrocladium crotalariae. Evaluations were conducted in a naturally infested field with a 2-year history of BRR incidence. Densities of microsclerotia (ms) were measured at planting by a soil elutriation technique. Disease classes were based on a 0-4 scale of foliage symptoms (0 = no symptoms, 4 = 76-100 percent of leaves per plant displaying symptoms). Chronological age and physiological stage of plants were compared in the determination of disease indices. Significant (P = 0.05) differences in disease index values were detected among the cultivars for both years. Cultivars 'Dare', 'Davis', 'Forrest', 'Lancer', 'Lee 74', and 'Hood 75' exhibited the greatest degree of field resistance for both years, whereas 'Bossier', 'Centennial', and 'McNair 600' were the most susceptible. In greenhouse experiments, cultivars 'Bossier', 'Cobb', and 'Hood 75' were grown to maturity in field soil (Olivier silt loam, pH 6.0) which had been artificially infested with ms of C. crotalariae. Five-day-old seedlings were planted in 20-cm plastic-lined pots containing a soil:vermiculite mixture (1:1, v/v) with adjusted inoculum densities of 0.005, 0.05, 0.5, 5.0, and 50.0 ms/g of soil (dry weight basis). Measurements of plant growth, yield, mortality, root rot severity, and rate of fungus recovery from taproots were recorded at maturity. 'Bossier' and 'Cobb' sustained significant (P = 0.05) reductions in growth and yield at (GREATERTHEQ) 0.5 ms/g of soil; 'Hood 75' sustained likewise at (GREATERTHEQ) 5.0 ms/g of soil.