Abstract: A study was conducted to elucidate the relations between the effects of humic acid applications and incidents of root rot diseases of tomato in greenhouse soils contaminated with soil-borne pathogens. For this purpose, commercially available powder preparations containing 80% humic acid and fulvic acid were used in the experiment. After the application of three different doses (80, 160, 240 mg active ingredient, a.i., plant-1), with five replications, healthy and diseased plants were counted and root weights were recorded to determine the disease development followed by fruit maturity. The disease percentage of the control group was found as 30.4 while those of the treated groups were 29.6, 30.8 and 34.4 with respect to doses of 80, 160 and 240 mg a.i. plant-1, respectively. Average root fresh weight in the control group was found 110 g plant-1 while those of the treated groups were found as 135, 156, 215 g plant-1, respectively. The results clearly showed that the application of humic acid caused an increment in root fresh weight while making plants to susceptible to the root rot pathogens.