You notice small dark spots with feathery edges on the leaves of your rose bushes in cool, wet weather. The spots quickly spread up the plant, transforming into large blotches that can completely cover the leaf surface. The leaves turn yellow and die. Purple spots on young canes turn black and blister.
What you are witnessing is the most serious rose disease in the world: Diplocarpon rosae. This fungal disease, also known as black spot, can be found on roses worldwide. If left unchecked, it can cause defoliation, loss of vigor, decreased flowering, and even the death of your plant. This fungus only attacks roses; it does not affect other plants. It will also not harm you or your pets.
Fortunately, a gardener can take steps to mitigate the damage caused by this disease. Some rose varieties can withstand a minor infection, but it is critical to have a good scouting program in place, not only for black spot but also for other disease and insect problems throughout your garden. The first line of defense is knowing what to look for.
Identification and Damage
Within one day, Diplocarpon rosae can infect your roses! This fungus attacks young leaves that grow low on the plant first. Overwintering spores that have splashed up from the ground onto the leaf surface frequently cause the first infections on the underside of the leaf.
One week after infection, spots appear.
The spots are black with feathery edges and measure 18 to 12 inches in diameter. A yellow ring surrounds the spots at times. As more of the leaf becomes infected, the spots may congregate into a mass. More leaves become infected as the disease spreads.
The leaves will eventually turn yellow and fall off. The entire shrub can be defoliated in severe infestations. Infected roses lose vigor without leaves to feed on, making them more susceptible to other diseases. They also lose their ability to store reserves to last the winter.
Rose canes can also become infected. The young, first-year canes are the most susceptible, with purple dots and blotches that turn black and blister. These lesions have the potential to spread the disease to other plant parts.
If black spot is allowed to spread unchecked, the entire rose bush can die over time.
Diplocarpon rosae spends the winter in fallen leaves and occasionally in rose canes. Conidia, or spores, are released from these overwintering sites in the spring, around the time saucer magnolias (Magnolia x soulangiana) bloom. Wind and water transport these spores. The disease takes hold if a leaf becomes wet and remains wet for 7 hours.
In two weeks, fruiting bodies, or acervuli, form and produce more conidia (summer spores), completing the cycle and spreading the fungus further.
The cycle continues throughout the season, and in warm climates, it can last all year.
Diplocarpon rosae is less dangerous in dry weather and more dangerous in wet and humid weather. Temperatures between 68°F and 75°F (20°C and 24°C) are ideal for the development of black spots. Summer heat slows development. The development of black spots slows when temperatures reach 85°F (29°C).
While there is no way to completely avoid black spot, there are some steps a gardener can take to reduce the disease’s impact. Remember that Diplocarpon rosae spores require 7 hours of moisture to germinate, so keeping the foliage dry is critical. You can accomplish this by:
- Planting in full sun, and not planting your rose bushes close together.
- Watering from below, not above, and taking care not to get the leaves wet. Although there is not much you can do about rainfall.
- Watering early in the day gives the leaves time to dry more quickly.
- Pruning out inner canes to increase airflow.
Other preventative measures include:
- Providing superior sanitation. Remove infected leaves and clean up fallen leaves to prevent spores from spreading to uninfected parts of the plant. Infected canes should be pruned 6 to 8 inches below the lesion, and clippers should be sanitized between cuts. Dispose of infected parts; do not compost them.
- Planting resistant cultivars. However, because there are several races of this disease and it mutates easily, even resistant varieties will eventually develop black spot. Yellow and gold roses are more prone than pink and red roses. Black spot is a serious problem for hybrid tea roses, miniatures, and grandifloras.
- After a thorough fall clean-up, apply a 3-inch layer of mulch to help prevent lingering spores on the soil from splashing up onto the plant the following spring.
- Keep an eye out for the first signs of spots in wet and humid weather.
Some gardeners will plant a highly susceptible “sacrificial” rose bush as an indicator plant. When this rose becomes infected, the gardener knows it’s time to begin disease control measures.
There is no “cure” for this disease, only management. Control measures aim to keep Diplocarpon rosae from spreading, not to cure it. When a rose leaf becomes infected, the spots persist even after the fungus has died.
Once a black spot appears on a leaf, it should be considered dead. Remove and dispose of the infected leaves. Do not put them in your compost bin, especially if you intend to use the compost to fertilize your rose bushes!
Controls must be applied every 7 to 10 days during the spring, summer, and fall seasons. Sulfur or copper are active ingredients in many commercial products. Make sure the product you buy is labeled for use on black spots and that you follow the instructions exactly.
Neem oil can help you get rid of black spots. Because different brands have different concentrations, always read the label. On sunny days, avoid using neem oil because it can burn the foliage.
A few home remedies are recommended by some gardeners. Perhaps one of these will be useful to you.
- Baking soda and water – mix 2 tablespoons of baking soda in 1 gallon of water.
- Milk and water – 1 part milk to 2 parts water.
- Baking soda and vinegar – mix 1-1/2 tablespoons baking, 1 tablespoon vinegar, 2 tablespoons vegetable oil, and 1-½ tablespoons liquid hand soap in a gallon of water.
Fill a spray bottle halfway with the mixture; all of these remedies are applied as a spray. It is critical to spray the leaves, canes, and flowers on all surfaces. The oil and soap act as surfactants, assisting the product to adhere to the leaves.