Roses are one of the world's most popular plants, according to Clemson University, but that beauty comes at a price. Some of the most desirable types of roses are also the most susceptible to insect pests and diseases. Black spot and powdery mildew are two of the most common fungal diseases on roses. Red fungal disease includs rust and downy mildews.
Rust is a fungal problem in wet, coastal climates such as the Pacific Northwest. The fungus travels on water and thrives in the humid air. Downy mildew is found primarily along the Gulf Coast, according to Rose Magazine. Although these diseases are most prevalent in these areas, they can occur in any climate where roses grow, especially during unusually wet springs.
Rust appears as red blotches (sometimes with a raised border) on the undersides of leaves. The tops of the leaves will develop small dots of yellow or orange. Red streaks may form on newly-growing canes. Downy mildew shows up as reddish-purple blotches on the tops of rose leaves and also on the stem and sepals of the plant.
Rust is a red fungus that occurs on rose plants during very wet, cool weather. Continually rainy, misty or foggy conditions, coupled with temperatures below 70 degrees Fahrenheit, give rise to the disease. The developing spores spread on rain, on the bodies of insects, and even on the wind, according to Rose Magazine.
Downy mold also occurs during cool, wet weather when the air humidity is above 85 percent, according to Oregon State University. Infected cuttings grafted onto healthy plants and infected pruning tools can spread the disease, which usually affects young plants.
Roses should receive plenty of air circulation and sunlight, both of which will help dry any water left sitting on the plant. Infected areas are pruned off the rose if the fungus is limited to a small location on the plant. Many of these fungi can overwinter in the soil or even on the plant, so it is important to get rid of any infected plant debris in the fall. A systematic application of a fungicide can help prevent the development of these and other fungal diseases.
The best way to deal with red and other types of fungi on rose plants is to prevent them from occurring in the first place. A good way to start is to plant roses considered disease-resistant. 'Electron' is a hybrid tea rose that is resistant to all three major types of fungal diseases affecting roses. 'Tournament of Roses' is a popular, disease-resistant grandiflora cultivar, and 'Europeana' is a deep, dark-red hardy floribunda.