Roses are beautiful to look at, and their scent is euphoric, but all that elegance necessitates a lot more TLC than do other plants. This includes disease control.
Rose stems turn black because of either Black Spot, Powdery Mildew, Stem Canker or Cane Borers.
How to Manage Blackened Stems
Black spot, powdery mildew, stem canker, and cane borers are the four most prevalent diseases that affect roses. They are all caused by fungi.
Botryosphaeria, Leptosphaeria, Coniothyrium, Cryptosporella, or Botrytis, the latter of which can also damage the petals and foliage, could be the cause of stem canker.
The organisms that produce stem canker are resistant to fungicides, thus the only treatment is to prune and remove the diseased areas of the plant.
Which should be done in the spring rather than the fall.
Make a 45-degree cut with sharp bypass pruning shears well below the problematic region where there is no sign of blackening.
Examine the cut region to ensure that the inner section of the stem contains only whitish, healthy material, and then disinfect the shears with a bleach-to-water solution.
Stem canker is produced by fungi that enter the woody stem material through a wound caused by improper pruning, wind or hail damage, or flower harvesting.
The fungal colony causes raised, dark spots to form on the stem, which spread and enlarge as the colony expands. Eventually girdling the stem and shutting off the flow of water.
As a result, the upper half of the stem wilts and dies, as does anything growing on it.
If the blackening of your rose stems is caused by cane borers, depending on their size, you may be able to physically remove them.
To do so, you would use a needle to probe into the holes, hoping to capture the larvae and pull them out. It’s a pesky job.
If you are successful, use carpenter’s glue to close the holes and keep their relatives out.
Use an insecticide sparingly because it will not reach them while they are inside the stem.
If physical extraction fails, the affected parts of the plant must be pruned and discarded.
Cane borers are insect larvae that dig into the stems and feast on the juices and cause major destruction.
Borer damage is easy to differentiate from stem canker damage because borers leave visible tracks as they wend their way through the stem.
There is typically a visible hole and the discolored area of the stem swells. The final consequence is the same: the growth at the stem’s tip receives little or no nutrition and wilts or dies.
When a gardener hears the term “roses,” the words “black spot” generally spring to mind. Fortunately, many modern roses are devoid of problems.
But there are still lots of roses that are sensitive, so keep a watch out for symptoms.
Black spot (Diplocarpon rosae) is a fungal illness that is exacerbated by warm, wet, or humid conditions.
If a black spot has previously been present in the area, the spores are most likely still present, waiting for the right conditions.
Black spot spores grow in around 7 hours of warm wetness, but symptoms may not appear for several days. Act swiftly once you’ve done so because new spores are created every three weeks.
Small black spots appear on the leaves, which grow and become ringed with yellow, eventually turning the entire leaf yellow.
When the leaves turn yellow, they begin to fall off the plant, and a severely affected plant will completely defoliate.
How to Prevent and Treat Black Spot
Maintain Good Sanitation
Sanitation techniques are crucial in preventing the spread of disease in the future. Eliminate all dead leaves on the ground, as well as any mulch where sick leaves have fallen, in the fall or winter.
Before new rose growth begins in the spring, replace with a fresh layer of mulch.
Remove & Destroy Infected Canes
Dark or reddish patches appear on black spot-affected canes (lesions). Depending on the variety and cultivar, severely affected plants should be trimmed down to within 1 to 2 inches of the bud union in the winter or early spring.
Remove and discard contaminated leaves when they develop during the growing season.
Keep Leaves Dry
Watering plants using overhead irrigation is not recommended, especially in the late afternoon or early evening. Soaker hoses are a great method to water flowers while conserving water.
Planting roses in broad sun promotes quick drying of the leaves. New plants should be spaced far enough apart to allow for adequate air circulation.
Fungicide sprays can be used to efficiently control black spots, even on resistant cultivars.
During conditions that encourage disease growth in susceptible cultivars, a strict fungicide regimen must be followed.
If the illness is serious enough, use one of the following fungicide sprays: chlorothalonil, mancozeb, myclobutanil, propiconazole, or copper fungicides.
All chemicals should be used by the instructions on the label.
Considering fungal infections and stem borers are difficult to treat, the best method is to prevent them from damaging your roses.
Fungal diseases spread more quickly when canes grow together and contact, so prune carefully to avoid this.
When pruning, use sterilized shears to make clean, 45-degree cuts, and seal the wound with carpenters’ glue for further protection.
Borers will be discouraged as well as the fungi that cause canker.
If the stems of a rose shrub are becoming black, insects known as cane borers may be to blame.
Most fungus and parasitic insects are tough to eradicate once they have established themselves, so you will have to prune and destroy the infected portions of the plant.
If you decide to use a fungicide, the optimum time to do it is before you notice any symptoms. When it comes to rose illnesses, prevention is the best therapy.
Enjoy your beautiful roses, care for them properly and they will reward you. You’ll have blooms beyond your dreams and perfumes that transport you to heaven and can never be bottled – your rose garden is your patch of paradise.