I am trying to grow Rose powdery mildew on detached rose leaves, but when I inoculate leaves from mildew free plants by spore suspension spray or by dusting spores from severly infected plants, there is little to no growth of mildew, and some times mold starts to grow on the leaves (by microscopic observation appears to be penicillum ssp.). Leaves are kept in petri plates (picture), placed in seetrough plastic boxes with wet tissue paper, humidity - ~ 98 %, T ~ 21 C, light 14/24. Mildew infected and mildew free plants are the same cultiwar (Sparkling jewel), bought at different times, could these plants have different resistance to powdery mildew?