Traditionally, plant pathologists have attempted to determine the presence and the activities of pathogens that invade and necrose plant roots. This has been done by observing the pathogens that develop from diseased root tissue when it is placed in a suitable growth environment or by assessing the damage accruing to diseased roots. Members of the Cotton Disease Council have used such methods for many years to identify the main seedling disease pathogens. Based on their performances in greenhouse and field trials, fungicides have been selected to control these pathogens. For example, Lesan is active against Pythium ultimum and PCNB inhibits Rhizoctonia solani. When both of these pathogens were added to a pastuerized soil neither Lesan nor PCNB alone provided enough disease control. When the two fungicides were combined, however, excellent seedling survival resulted. Excellent seedling survival also often resulted with this fungicide combination in field trials. In such cases we may not assume that both P. ultimum and R. solani were present and active as pathogens in that soil. Trials can be designed, however, so that each of these fungicides can be compared when they are used as single seed treatments and when they are combined as a treatment. Certain assumptions can be made from such trials. For example, if seedling survival is as great with Lesan as with the combination of Lesan and PCNB it is assumed that P. ultimum is probably present and active. In addition, if non-treated seed in that trial produces as many surviving seedlings as seed treated with PCNB it is assumed that R. solani is not very active. Confidence in these assumptions is much greater if P. ultimum is also the main pathogen isolated from diseased seedlings from this trial.