Powdery mildew, caused by Blumeria graminis f. sp. tritici, is an important disease of bread wheat (Triticum aestivum L.). Little is known about the powdery mildew population structure and the economic loss that occurs on commercial wheat cultivars in Western Australia. Eighteen wheat lines (including sixteen commercial wheat varieties, line 7HRWSN58 and a susceptible control) and a set of differential lines with known powdery mildew (Pm) resistant genes were evaluated using a cumulative pool of powdery mildew (Pm) inoculum each year to examine the changes in their responses over the years. Trials with a randomised block design were conducted under controlled environment over the years 2011 to 2014. Resistance genes Pm2, Pm3a, Pm3e, Pm4a, Pm13 and Pm27 were postulated to be effective in WA while genes Pm1a, Pm3c, Pm4b, Pm5a, Pm7, Pm17, Pm24 and Pm28 were ineffective. Differential lines carrying Pm6 and Pm8 genes were moderately resistant at adult plant stage but susceptible at seedling stage. Twelve out of sixteen commercial wheat varieties were susceptible or highly susceptible at seedling and adult plant stages lacking gene/s for resistance to powdery mildew. Most of these varieties were stable but some of the differential lines showed a breakdown in resistance over the four years. The cultivars Fortune, Magenta and Yitpi were moderately susceptible (MS) at adult plant stage but susceptible (S) at the seedling stage. The variety Arrino and line 7HRWSN58 were moderately resistant (MR) and resistant (R) to powdery mildew respectively.