Roses have a reputation for being disease-prone, and unfortunately, that reputation is not undeserved. There ARE quite a few diseases that tend to plague roses, particularly hybrid teas, floribundas and miniatures. Mildew is probably the most common disease encountered on roses, found throughout the United States. While some of the newer shrub roses, particularly the Knock Out series, are more mildew-resistant than many other varieties, no rose is immune to disease, and all of them tend to fall prey at one time or another to mildew.

It’s easier to prevent than treat most rose diseases, so keep your rose bed clean from fallen leaves and pruning clippings, choose rose varieties that are disease-resistant, plant your roses far enough apart to allow good air circulation, avoid watering in the evening, and plant in full sun.

Powdery mildew
If you grow roses, you are almost certainly familiar with the gray, powdery coating of mildew on the buds, new leaves and upper canes of your roses, especially during the spring when weather tends to be warm and dry during the day, damp and cool at night. Once dry summer weather arrives, mildew is likely to clear up, at least until the fall when warm dry days followed by cool, moist nights return. Powdery mildew is certainly not exclusive to roses, but tends to affect them quite strongly.

Once infected with the powdery, whitish-gray fungus, leaves first develop small, blistered lesions, then become covered with mildew. The leaves become curled, distorted and stunted, weakening the entire plant. Buds are also affected, spoiling their appearance and distorting the flower petals.

You can prevent and treat powdery mildew with fungicidal spray purchased at any garden center. You will need to spray every week or so during damp spring weather for best control. If you don’t like chemical sprays in your garden, you can try a natural mildew preventative:
Mix 1 tablespoon of baking soda, 1 tablespoon vegetable oil and 1 tablespoon gentle dishwashing liquid in a gallon of water. Add to spray bottle, and spray your roses weekly. This won’t get rid of mildew already on the rose, but will help prevent the spread of the fungus.

Downy mildew
This serious fungal infection appears as purplish brown spots and blotches on the leaves of the rosebush. As the disease progresses, the blotches become larger and turn black and the area surrounding the lesion turns yellow. Sometimes there will be white powder on the underside of the leaves, but this is not always the case. Eventually, the leaves die and fall from the bush. Untreated, the entire plant can die within days from a severe downy mildew infection.

Downy mildew is especially common in coastal areas, or anywhere with weather that tends towards cool, cloudy, moist conditions. Temperatures around 65 degrees Fahrenheit with 85% humidity are perfect conditions for downy mildew to develop. The fungal spores spread quickly through the garden, infecting nearby plants.

Downy mildew needs to be treated as soon as the symptoms appear. Unlike powdery mildew, there are no natural remedies to treat an infection, so chemical fungicides are required. Read labels carefully, selecting a spray that is specific to downy mildew. Spray in the morning, wetting leaves thoroughly top and bottom. Repeat treatment as listed on the fungicide label until dry weather appears, ending the active season of fungal infection. Cut away and discard infected leaves and stems, do not add to your compost pile or leave lying in the garden.

If you grow roses, you will need to deal with mildew sooner or later. Keep your garden clean and free of debris, make sure to space your roses for good air circulation, and try to avoid watering at night to help prevent the dreaded appearance of mildew. If mildew appears, take steps to treat the fungus right away to avoid a more serious problem.