How to Treat a White Powder on an Iceberg Rose

Powdery Mildew Profile

• When a white to gray powdery substance shows up on a rose's leaves, shoots, sepals or petals, blame powdery mildew disease, caused by the fungus Podosphaera pannosa. The pathogen does not require standing water on the plant's surface, but must have living tissue to survive. It may stunt new foliage growth and blossoms. Although powdery mildew thrives in humid air, the spores it produces need to attach to dry surfaces, unlike those of most fungi. The wind-borne little spore seeds come to rest on the rose's leaves and send out germ tubes that grow into white threads called mycelia. The fungus penetrates the plant's sap with its suckers. As the mycelium spore establishes chains, the plant becomes covered in a white, velvety coating within a few days. Fortunately, because it is on the surface, you can control powdery mildew more easily than many other fungi.

Home Remedies

• You can use synthetic fungicides on your roses, but they generally only work as preventatives, not as disease treatments. With so many natural remedies that do no harm and that you can mix up at home with ingredients you probably already have in the kitchen or medicine cabinet, try these to treat your sick "Iceberg." Mix a tonic of 2 teaspoons of baking soda, 1/2 teaspoon of Murphy's oil soap and 2 quarts of water to spray on the foliage. If this does not cure the affliction, try dissolving two uncoated, 325-milligram aspirin tablets in a quart of water to make another foliar spray. As a last resort, create this more-complicated garlic concoction. Puree two cloves of garlic in a blender, slowly adding 1 quart of water. After blending for about six minutes, strain and add 1/8 teaspoon of liquid soap before storing in a tightly-sealed container. To use, mix one part of the garlic solution to 10 parts water, and spray on the top and undersides of the ailing plant's leaves.

Restorative Teas

• If you have access to the wild plant, field horsetail (Equisetum arvense), use the leaves and stems to brew a potent cure for the fungal disease. The plant, growing in USDA zones 3 through 8, has a high silica content, the active ingredient to control fungi. Boil 2 to 3 teaspoons of the dried and crumbled herb in 1 cup of water for 20 minutes. Besides controlling mildew on roses, it reportedly strengthens the cells of plants sprayed with it. Where horsetail is not available, apply a chamomile tea that has fungicidal properties. Place 2 cups of chamomile flowers or 16 chamomile tea bags in 2 quarts of simmering water for 20 minutes. Allow the tea bags to steep for several hours after turning off the heat. If using dried flowers, strain them from the water. Spray the tea on the rose's foliage.

Preventative Measures

• Certain practices in caring for your "Iceberg" rose can discourage the development of powdery mildew or other fungal diseases. Garlic, onions and other alliums, when planted as companions to roses, seem to protect them from invasions by fungi. In addition, Europeans interplant the two to produce stronger perfume in the roses. Give your roses sufficient light and air circulation. Spray with jets of water early enough in the day that the plants dry out by evening. Avoid severe pruning that may stress the rosebush, causing it to be more susceptible to disease.