Quantification of Infection Levels in Wheat Genotypes Varying in Stem Rust Resistance

Stem rust (Puccinia graminis f. sp. tritici) development was studied in six F₃-derived F₃ wheat (Triticum aestivum L.) lines selected from a cross of ‘Era’ (stem rust resistant) and ‘Morocco’ (stem rust susceptible). In previous tests for susceptiblity to stem rust race 15-B(TLM), two of the lines had been classified as slowrusting, two as moderate-rusting, and two as fast-rusting. The objectives of the present study were to: 1) evaluate the relationship between field measurements of stem rust resistance and grain yield and kernel weight using infected lines vs. protected controls and 2) measure stem rust development in the greenhouse at two growth stages (boot and milk) to predict field response to stem rust from greenhouse measurements. Race 15B-2(TLM) was used to inoculate all entries to measure stem rust development. Stem rust development curves (AUSRC) were calculated from percent rust on the flag leaf sheath in field plots and from number of pustules/top 10 cm of leaf sheath below the flag leaf in the greenhouse. Latent period was measured in the field and greenhouse as the number of days from inoculation to first appearance of pustules. Differential yield and 200 kernel weight reductions among protected and rusted treatments and the AUSRC differences were effective in distinguishing which lines were slow-rusting in the field. Latent period, initial infection level, and rate of increase in AUSRC were the most important stem rust variables affecting final infection level. An overall ranking system composed of percent reductions in yield and kernel weight and values for the three stem rust variables indicated only two lines (one slow-, one fast-rusting) retained their original classifications. Of the two originally classified moderate-rusting lines, one reacted as fast-rusting and one as slow-rusting. One of the slow- and one of the fast-rusting lines reacted as moderate-rusting lines. In the greenhouse, plants in both the boot and milk growth stages reacted similarly to stem rust infection. Final pustule numbers on the leaf sheath was the best measurement of stem rust severity. Prediction of field infection level using greenhouse variables was not possible.