Components of partial resistance to Erysiphe graminis in spring barley

The development of powdery mildew on seven spring barley varieties lacking effective major resistance factors was compared in a small‐plot field trial. Differences were observed at later growth stages indicating partial resistance in some varieties. Latent period and sporulation capacity were examined on the fifth or sixth leaf of these varieties plus several known to be susceptible, using plants raised under spore‐free conditions. Latent period was extended in most partially resistant varieties compared with susceptible controls, more so at 5°C than at 10°C. Cumulative total spore production per colony over the 16‐day sporulation period differed markedly between varieties, the most resistant yielding one‐third as many spores as the susceptible controls. There was evidence that these resistance components were correlated with each other and with previous estimates of infection frequency. The significance of this finding, in relation to selection for improved levels of partial resistance, is discussed.