REFINED PROCEDURES FOR INOCULATING WHEAT SEEDLINGS WITH PYRENOPHORA-TRITICI-REPENTIS AND RATING THEIR REACTIONS

Systems for rating reaction of wheat to infection by Pyrenophora tritici-repentis, the tan spot pathogen, should be informative and reliable so that resistant cultivars can be efficiently developed. Therefore, the effects of propagule type, inoculum dosage, and spatial interaction among lesions on infection phenotype, infections per unit area, and percent disease severity were studied. Conidiophore inoculation of seedlings of wheat line ND 495, which is susceptible to tan spot, produced a lower infection phenotype, fewer infection sites per square centimeter, and less severe damage than conidia at equivalent dosages. Hyphal fragments were rarely infectious under a 24-hr postinoculation wet period. The relative reaction of the resistant wheat cultivar Eric to the three types of inoculum was similar to that of ND 495. Inoculum concentration and type particularly influenced categories of the infection phenotype scale that included coalescence as a criterion. The distal leaf half was more severely damaged and usually had more infections per unit area than the proximal half, but there was little or no evidence of interaction among lesions between leaf halves. Uniform inoculum dosage, exclusion of conidiophores from inoculum, and collection of infection type data from the middle of the uppermost fully expanded leaf at the time of inoculation should be employed to discriminate resistant and susceptible genotypes effectively.