Diseases were assessed in maize, soybean, and oat, which were growing with narrow strip intercropping. Strip intercropping may nullify the disease control benefit of crop rotation because one edge of every strip is contiguous with the debris of the same crop left in the field from the prior year. Two multifactorial strip intercrop experiments at the McNay Research Center in South Central Iowa and two strip intercrop commercial fields in Northeastern Iowa were studied in 1990 and 1991. Two fields near Ames were also assessed in 1991;The major foliar diseases were: Septoria blight of oat (Septoria avenae), brown spot (Septoria glycines), and bacterial blight (Pseudomonas syringae pv. glycinea) of soybean and eyespot of maize (Kabatiella zeae). Evidence collected would support an opinion that the source of inoculum for these diseases was maize, soybean and oat residue from the prior year;Disease development in a crop strip was always earlier and more severe in the row contiguous with the land planted to the same crop the prior year than in the row farthest from the inoculum source. Disease gradients developed across a strip and the gradients increased through the season. The disease gradients were negative and normally linear when regressed with the distance from the inoculum source. Continual increases in the disease gradients indicated that primary inoculum was important through the season. No single epidemiological model would best describe the disease progress in each row. When rainfall was not limiting, the diseases developed exponentially and when it was limiting a monomolecular model described the disease the best;Stalk rot of maize was less in the outer rows of the maize strips than in the center rows. In some instances the outer row of maize contiguous with land cropped to maize in the prior year had more stalk rot than the farthest row;Foliar diseases of crops were significant with three-crop strip intercropping rotations. Disease resistant varieties must be utilized to minimize yield losses with strip intercropping.