Description:
<jats:p>Substantial differences in the rates of mycelial growth and sporulation of different isolates of <jats:italic>Puccinia striiformis</jats:italic> were observed when seedling and upper leaves of five barley cultivars with susceptible reaction types were inoculated. The highest rate of mycelial growth was usually on cv. Sultan, followed by Proctor, Astrix, Senta and Zephyr in that order, but more spores were produced on Senta and Astrix than on Proctor, Zephyr or seedlings of Sultan. Host‐specific differences between isolates were slight, but some isolates were more aggressive than others, with generally higher mycelial growth and sporulation rates and shorter latent periods. The effect of temperature on sporulation varied little between cultivars or isolates, but there was some evidence of high temperature resistance in Senta and Zephyr. Latent periods were longer on flag leaves than on seedling leaves, and spore production per unit area of leaf infected was lower on the flag leaf of all cultivars except Sultan. Low sporulation rates seemed due to adult plant resistance in Astrix and Senta, and overall resistance, more strongly expressed in adult plants than in seedlings, in Proctor and Zephyr. Sultan exhibited slight seedling resistance, but lacked the adult plant resistance of other cultivars.</jats:p>