Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Edward C. McGawley

Second Advisor

John S. Russin


Microplot experiments were established in 1992, 1993, and 1994 to investigate the relationships between the fungal pathogen Macrophomina phaseolina, and a nematode community comprised of Criconemella xenoplax and Tylenchorhynchus annulatus on grain sorghum in Louisiana. A factorial treatment arrangement was employed and consisted of two sorghum hybrids (De Kalb DK 50 and Pioneer hybrid 8333), three levels of M. phaseolina (0, 10, and 100 colony forming units (cfu)/g soil) and three nematode inoculum levels (0, X, and 2X). Nematode inoculum at X levels were 929, 1139, and 1445 for C. xenoplax and T. annulatus per pot in 1992, 1993, and 1994, respectively. Plants were harvested after 95-105 days. In all three years, grain sorghum root and head dry weights were reduced as both fungus and nematode inoculum levels increased. Reproduction of both nematode species was suppressed by M. phaseolina. Interactions between M. phareolina and nematodes were antagonistic with regard to plant dry weights, yield, and nematode reproduction. Possibility of competitive interactions between T. annulatus and C. xenoplax when grain sorghum roots were colonized by M. phaseolina were studied quantitatively in the greenhouse using a modified plant ecology technique known as replacement series. Soil in pots containing Pioneer hybrid 8333 grain sorghum were infested with 1,000 vermiform nematodes in the following T. annulatus: C. xenoplax ratios: 1.00:0.00, 0.75:0.00, 0.50:0.00, 0.25:0.00, 0.00:1.00, 0.00:0.75, 0.00:0.50, 0.00:0.25, 0.25:0.75, 0.50:0.50, 0.75:0.25, and 0.00:0.00. M. phaseolina was either absent or present at 10 cfu/g soil. Two replicative tests were conducted in 1995. Results indicated that relative yields of T. annulatus and C. xenoplax in single culture were significantly higher than hypothetical model lines representing equal inter- and intraspecific competition. M. phaseolina affected relative yields of C. xenoplax, but not T. annulatus. Root and stem weight were reduced by M. phaseolina by 38 and 31%, respectively. In the absence of M. phaseolina, T. annulatus reduced root weight by 43%, whereas C. xenoplax had no effect. As M. phaseolina colonized roots, however, root weights were not different among nematode inoculum levels.