Part of regular rose care is treating the bush for mildew. Caused by various types of fungi, many of these mildews are named for their appearance. Powdery mildew covers the leaves, stems and buds with a white, powdery substance. Black spot causes black spots with a yellow halo on the rose leaves and canes. Downy mildew is a serious fungus that causes red to brown spots that might be surrounded with a yellow halo. If left untreated, these mildews can weaken and even kill the rose bush.
Clean and sharpen the pruning shears. Remove any dirt from the shears and sharpen them using either a file or sharpening tool. Pruning a rose with dull shears can result in stems and canes being ripped rather than cut, which damages the rose bush.
Mix a sanitising solution of 1 part bleach and 10 parts water in a clean bucket. Dip the sharpened shears in the bleach/water solution.
Remove all parts of the rose bush affected by mildew. Some leaves and buds will fall off easily when touched, while stems and canes require pruning. Prune the cane below the infected part. Dip the shears into the bleach/water solution between cuts to prevent spreading the mildew to other parts of the bush or to other plants.
Seal the pruned rose canes with a white household glue to prevent insects from attacking the rose through the pruned site. Drip or smear the glue onto the cut edge of the canes, covering the surface completely.
Clean the rose beds of rose debris; this reduces the chance for insects and fungi to grow. Bag up all rose clippings and debris and place in the trash. Do not add to a compost pile, as many fungal spores can survive in a compost pile.
Treat the pruned rose bush with a fungicide, either organic or chemical. Apply all fungicides in the morning, when the wind is lowest and there is sufficient time for a liquid fungicide to dry before evening.
Reduce the chance of reinfestation throughout the growing season. Keep the soil beneath the rose cleaned of fallen leaves and blossoms. Apply regular applications of fungicide, according to the instructions on the container.
When purchasing new roses, select bushes that are certified free of diseases.
Treat the rose bush at the first sign of mildew. If left untreated, the fungus will often spread to other plants. Wear protective gloves and clothing when using chemicals.