The survey on spreading of Erysiphe flexuosa (Peck) U. Braun & Takamatsu (syn. Uncinula flexuosa Peck) was performed on horse chestnuts in different locations in Wielkopolska and Kujawsko-Pomorskie provinces. It was stated that powdery mildew occurred commonly on trees. In comparison to 2008 more infected by E. flexuosa chestnuts were found in 2009. The percentage of white chestnuts (Aesculus hippocastanum) with the disease increased from 1.1% in 2008 to 19.4% in 2009. The infection level of pink chestnuts (A. xcarnea) was similar in both years (69.2 and 66.1%).