Greenhouse and field experiments were conducted to study the effects of organic materials, chemical fertilizers, soil acidification and a fungicide on Phymatotrichum root rot on cotton. A Clareville cl (Pachic Argiustoll) which has a history of root rot infestation was used in both experiments. In the greenhouse experiment soil was treated with Disper-Sul (3,571, 7,143 and 14,285 lb/A) powdered sulfur (7,143 lb/A) sulfuric acid (200 and 1,600 gal/A), fresh cotton and sorghum plants (3,571 lb/A), and cow manure (8,929 lb/A). Two weeks following cotton emergence, 0.20 grams of viable sclerotia of Phymatotrichum omnivorum were placed in two point locations, approximately 2 cm below the soil surface. The order of effectiveness of different treatments on reducing plant mortality rates were:Disper-Sul > cow manure > sulfuric acid > fresh sorghum plants > powdered sulfur > fresh cotton plants. The highest dry matter yield was measured with sulfuric acid treatments while the lowest yield was obtained when powdered sulfur S was applied to the soil. Powdered sulfur (7,143 lb/A) and sulfuric acid (1,600 gal/A) application reduced the soil pH dramatically. Plant Fe uptakes were higher with sulfuric acid and Disper-Sul treatments. DTPA extractable Fe in the soil increased markedly with powdered sulfur, sulfuric acid and cow manure applications. Data in the greenhouse experiment indicated that Disper-Sul and cow manure were very effective in reducing plant mortality rates. While sulfuric acid at high rate showed moderate results. Fresh cotton plant residue appeared to encourage the disease at a level equal to powdered S. Sorghum plant residue had no effect on cotton losses to the disease.

Under field conditions ammonium nitrogen was superior to nitrate in enhancing lint yields. The significantly higher yields with NH₄+ were primarily a result of Phymatotrichum root rot suppression while nitrate apparently accentuated the disease problem. Seedrow application of tilt and elemental S applied preplant were the only additional treatments affecting yields in the field experiment.