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Clematis is subject to infection by several diseases including powdery mildew ( Erysiphe ploygoni ) , crown gall ( Agrobacterium tumefaciens ) , and various leaf spots ( Cercospora squalidula , Septoria clematidis ) . In Minnesota , Aster Yellows follow an outbreak of the 6 spotted , Aster Leafhopper . Trees planted on M.26 generally require staking for the first few years of growth or , on windy sites , for the life of the tree . Trees also have survived well in harsher climates at Morris and Grand Rapids . Powdery Mildew shows as a white powdery dusting on leaves , stems and buds of many flowers and shrubs in Minnesota . Powdery Mildew is caused by the fungus Sphaerotheca pannosa . Trees grafted to dwarfing rootstocks require only 3 to 4 years to begin bearing fruit , while standard-sized trees may require eight years or more . Trees if Alderman are vigorous and reach 12 to 15 feet in height when mature . Trees planted on M.7a generally do not require staking , except on windy sites . They've also been seen in St. Paul , Stillwater , Hastings , Rochester and Winona , according to Doree Maser of the Minnesota Department of Agriculture .