Institut National de la Recherche Agronomique, Unité de recherches intégrées en grandes cultures, Le Rheu cedex, France

Tapesia yallundae and Tapesia acuformis are the causal agents of eyespot, the major foot disease of cereals. Many authors have suggested that the two species differ in epidemiological behaviour, based on the later appearance of lesions with T. acuformis than with T. yallundae . However, the reasons for this difference in natural conditions are unclear in the absence of a method to distinguish the two species in the plant before the symptoms appear. Methods of detection [visual assessment, enzyme‐linked immunosorbent assay, polymerase chain reaction (PCR) test] and characterization (isolation on artificial medium, PCR test) were used to investigate the behaviour of Tapesia spp. in the field under various epidemiological conditions. The PCR test was used to determine infection dates and to identify the species involved early, in order to demonstrate that infection occurred at the same time for T. yallundae and T. acuformis . Symptoms of T. acuformis infection occurred later because the pathogen developed more slowly in the tissues. A triticonazole seed treatment was used to evaluate the effectiveness of the PCR test for estimating fungicidal effects. This molecular tool for determining the period of activity of the chemical and changes in the proportions of the two species of Tapesia involved in the disease was demonstrated to be accurate. The PCR test either had advantages over or was complementary to the other methods tested.