Powdery Mildew (Sphaerotheca pannosa var. rosae)
Damage:
White, floury, easily wiped-off deposit on all young plant parts,
primarily on leaves, ovary of the bud, as well as flowers and canes.at first
only slightly wavy, glassy leaf spots. Clear failure to thrive of affected canes
and blossom clusters. Leaves are deformed, but usually do not drop. Wood
does not mature, becomes vunerable to frost.
Cause: Powdery mildew is favored when rainfall is low or absent. Infection with powdery mildew spores from one-sided fertilizing, too little air circulation, and heat. Can also be caused by overcrowding and planting in the shade.
Prevention: Choose a well-ventilated location, fertilize with low-nitrogen. In fall, prune back affected canes to healthy wood and destroy cuttings. In spring, prune bushes to provide circulation. Do not overfertilize. Depending upon growth, spray with fungicide ever 10 to 14 days.