Temporary partial breakdown of Mlo‐resistance in spring barley by the sudden relief of soil water stress

Seedlings of spring barley cultivars expressing mlo genes for resistance to mildew (Erysiphe graminis f.sp. hordei) in different genetic backgrounds were subjected to degrees of water stress under different nutrient and soil conditions. The temporary breakdown in the host of the expression of Mlo‐resistance, reported occasionally under field conditions, was successfully simulated when stress treatments were relieved. Differences in the degree of temporary breakdown were attributable to nutrient status, soil compaction and genetic background of the mlo gene.