Zea maysin Latin America is affected by a disease complex known as tarspot caused byPhyllachora maydis, Monographella maydisandConiothyrium maydis.Two series of pictorial standard diagrams were developed for field assessment, one forP. maydisand the other forP. maydis + M. maydis.The disease severity depicted on the 72 diagrams was assessed for the number of lesions and the percentage disease severity by 36 volunteers of diverse professional backgrounds without prior experience in assessing crop diseases. The assessment error for theP. maydisdiagrams included a 17–38% underestimation of number of lesions and a 184–330% overestimation of disease severity. For theP. maydis × M. maydisdiagram, overestimation for the classes with 1.0–4.5% disease severity was relatively high, whereas the error diminished with a higher proportion of larger lesions. In general, regular distribution of disease tends to be overestimated, in this case by 10–71%. The greatest assessment error was, however, related to the inappropriate choice of assessment scale, particularly for lesions smaller than 3.5 mm.