The first step in avoiding diseases in your rose garden is to apply good cultural practices.
- Don’t place the plants too close together. This allows plenty of space for air circulation.
- Proper watering techniques are important to follow. Usually you hear advice to not water your roses from overhead. This is permissible as long it’s done early in the day. This will allow the leaves plenty of time to dry before nightfall.
- A regular spraying programis also important. Most serious rosarians spray fungicides every 2 to 3 weeks or sooner if you have high humidity or lots of rain. You need to alternate fungicides also as fungal diseases will build up immunity to a specific brand if used repeatedly.
Onto the diseases that you are most likely to find on your roses.
Blackspot is a fungal disease that can cause almost complete defoliation of bushes by early fall. It produces a weakened bush on which cane dieback, stem canker, and winter injury can become severe.
Symptoms: As its name suggests, blackspot is characterized by circular black spots ranging from 1/16-inch to 1/2-inch in diameter, appearing generally on the upperside of leaves. The spots are frequently surrounded by a yellow halo. Infected leaves characteristically turn yellow. They fall prematurely. This leaf spot can be distinguished from others by the fringed margin and consistently black color. Cane infection produces a reddish-purple spot. In many varieties, pale flower color is also indirectly caused by blackspot.
Blackspot on rose leaf
Control: A preventive program for blackspot should begin with a thorough cleanup in the fall. After the plant goes dormant, strip off all of the leaves and give the plant a good spraying with a mixture of dormant oil and lime sulfur. Diseased leaves on the ground should be raked and destroyed. These precautions reduce over wintering fungi.
A fungicide program should start in the summer just before leaves become spotted. From then until frost, the leaves may require a protective fungicide coating. When the leaves are growing rapidly or during rainy weather, it may be necessary to spray the plants two times a week. However, if growth is less rapid and rains are less frequent, spraying at 7 to 10 day intervals is usually sufficient. Proper timing is as important as the chemical spray.
Powdery mildew appears as a white, powdery growth on rose leaves, stems, buds or flowers
Symptoms: It usually first appears on new growth in periods of warm, dry days followed by cool, damp nights. The new leaves may become curled or twisted and the shoots may look badly deformed. The upper surface of the leaves often appears normal despite extensive fungus growth on the underside of the leaf. Generally, the most favorable conditions for powdery mildew are daytime temperatures near 80 degrees F with a relative humidity of 40 to 70 percent, and nighttime temperatures near 60 degrees.
Control: Several rose varieties are more resistant to powdery mildew. When planting new roses, look for disease and insect resistant varieties - more varieties are available every year. To manage powdery mildew: Plant resistant varieties. Remove and destroy infected shoots at the end of the season and rake up and discard infested, fallen, dead leaves. Spray with sulfur dusts, neem oil (Rose Defense, Shield-All, and Triact) or systemic fungicides [i.e., thiophanates (Clearys 3336, Halts Systemic, and Greenlight Systemic), triforine (Funginex), triadimefon (Bayleton, Fung-Away, and Intercept)].
Symptoms: rust is also a fungal disease it appears as orange- or rust-colored growth on the underside of the leaves. Older leaves tend to show symptoms before younger leaves. Under favorable conditions rust can cover the entire leaf and stem of the rose plant. Severe infections can cause premature defoliation.
Control: Whenever possible plant resistant varieties. Any practice that prevents the leaves from remaining wet for extended periods of time is beneficial for control. Fungicides may also be used.
Rose Mosaic Virus
Symptoms: Yellow wavy line patterns, ring spots in leaves will occur on some varieties of roses sometime during the growing season. In general, symptoms are most evident in the spring. Yellow net and mosaic symptoms on the leaves are also associated with RMV and detract from the overall quality of the plant. Infected plants become weakened and are more sensitive to damage caused by other stresses, such as drought or low temperatures.
Rose Mosiac Virus
Control: Virus-infected plants cannot be saved. Rose mosaic spreads slowly, if at all, in established rose plantings through root grafts. Infected plants should be removed from highly prized plantings and destroyed. Avoid purchasing plants showing any mosaic symptoms.
Symptoms: Buds and flowers are most often the victims of Botrytis, but canes can also become infected. Symptoms on flowers are small tan spots surrounded by a maroon halo. Spots quickly enlarge into irregular brown blotches engulfing much of the petal. Buds will not open and appear to droop from the stem. Petals and buds soon become covered with a gray fuzzy growth (Fig 2). If disease is severe, infection can extend into the canes and cause cankers and dieback.
Control: Botrytis can be controlled by removing old flowers and fallen leaves. Space plants appropriately so sunlight and air can easily pass. Fungicides are also effective.
There are more diseases that can affect roses, but these are the most common. If you encounter one that you can’t identify, take a sample to your county extension office for help.
Photos courtesy of Michigan State University Extension amd the author.