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

Systems for rating reaction of wheat to infection by Pyrenophora triti ci-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 interactio n among lesions on infection phenotype, infections per unit area, and percent disease severity were studied. Conidiophore inoculation of see dlings of wheat line ND 495, which is susceptible to tan spot, produce d a lower infection phenotype, fewer infection sites per square centim eter, and less severe damage than conidia at equivalent dosages. Hypha l fragments were rarely infectious under a 24-hr postinoculation wet p eriod. The relative reaction of the resistant wheat cultivar Eric to t he three types of inoculum was similar to that of ND 495. Inoculum con centration and type particularly influenced categories of the infectio n phenotype scale that included coalescence as a criterion. The distal leaf half was more severely damaged and usually had more infections p er unit area than the proximal half, but there was little or no eviden ce of interaction among lesions between leaf halves. Uniform inoculum dosage, exclusion of conidiophores from inoculum, and collection of in fection type data from the middle of the uppermost fully expanded leaf at the time of inoculation should be employed to discriminate resista nt and susceptible genotypes effectively.