YCultivars of barley and wheat, resistant and susceptible to powdery mildew (Erysiphe graminis), were grown in perlite with added nutrient solutions in which the Ca(N0₃)₂ : KH₂P0₄ ratio was from one-third to three times the normal (1:1) amount in a balanced nutrient solution, i.e., 3:3, 3:1/3, 1/3:1/3, and 1/3:3. The degree of mildew infection was measured by planimetry, or by determining the number of conidia rer 10 g of leaf tissue or estimated visually. The Ethiopian barley cultivar CI 3926/3 was more resistant to mildew than ‘Kenia’, but the level of resistance varied depending on the Ca(N0₃)h : KH₂P0₄ ratio; both CI 3926/3 and ‘Kenia’ were more resistant when grown in 1/3:3 than in the other treatments. Small darkbrown streaking lesions were produced on CI 3926/3 and to a lesser extent on ‘Kenia’ grown in the high levels of Ca(N0}₃h. ‘Aotea’ wheat plants were also resistant to mildew when grown in 1/3:3 and to some extent in 3:3, but susceptible in 1: 1, 3:1/3 and 1/3:1/3. Chlorotic flecking lesions occurred on plants grown in Yd. ‘A Federation’ wheat plants were immune to mildew in both 1/3:1/3 and 1/3:3.