If rose growers had a public enemy number one, it would surely be black spot — a menacing fungal disease that poses a danger to roses.
Once black spot disease begins attacking your roses, it can be very difficult to control and manage.
Here are some tips for controlling black spot and minimizing its impact on the health, beauty and longevity of your plants.
What are the symptoms of black spot? 🖤
As the name suggests, the first signs of black spot are irregularly shaped spots up to 1.5 cm in diameter, especially on the upper surface of the leaves.
As the disease worsens, the leaves begin to yellow and eventually drop from the plant prematurely, usually starting at the bottom and working upwards.
Over time, the entire plant may become defoliated and the stems may also become infected.
What causes black spot? 🌧️
Like most fungal plant diseases, black spot (caused by the fungus Diplocarpon rosae) is caused by a damp, cool environment — conditions most likely to occur early in the growing season. If left unchecked, leaf loss can weaken your plants and inhibit flower development.
Black spot usually won't kill your plants in the first year or two, but if it continues unchecked, it will make them more susceptible to other diseases and unable to survive harsh winters.
How to prevent black spot 🌞
Black spot can be very difficult to control once it becomes well established. Here are some steps you can take to help prevent black spot infection of your roses:
- Keep the foliage as dry as possible, as the spores need a moist surface to germinate.
- Avoid overhead watering or water early in the day so the foliage has time to dry before nightfall.
- Improve air circulation around your roses by spacing them properly and pruning them regularly.
- Plant your roses where they will receive sufficient sunlight, which speeds up the drying of the foliage.
If your roses are infected, remove the diseased leaves and throw them away. Also prune and remove any infected stems. In the fall, remove all leaves and plant debris from your rose garden to prevent black spot from overwintering and returning in the spring when new growth begins. Even a harsh winter will not kill spores that may lie dormant in your garden.
Pro Tip: Denise Kelly of Variegata Studios also suggests spraying rose bushes with liquid seaweed to strengthen the leaf cuticle, making it less susceptible to the infection.
How to treat black spot 🛠️
Once you see the telltale signs of black spot, you cannot reverse the disease, but you can stop the infection from spreading to new leaves. The key to success with any treatment option is persistence:
- Start treating your roses at the start of the growing season.
- Spray them every week or two and after heavy rain.
- Be sure to spray the entire plant, including the tops and bottoms of the leaves as well as the stems.
- Apply a hibernation spray during the winter to smother overwintering fungal spores.
Fungicides 🌱
There are several eco-friendly organic products that are very effective in removing black spot, including sulfur, neem oil, and copper. Most products are available as foliar sprays or dusting powders and will control many types of fungal diseases.
Home Remedies 🏡
If needed, you can formulate your own antifungal spray to treat black spot using ingredients commonly found in the kitchen. A Cornell University researcher demonstrated that a mixture developed for powdery mildew — 1 tablespoon of baking soda mixed in 4 quarts of water, with a little horticultural oil or liquid soap to help adhere to leaves — is also effective in reducing the spread of black spot.
Some gardeners have reported varying degrees of success using sprays made from milk, vinegar, or hydrogen peroxide diluted with water. These home remedies may be worth a try, but we don't necessarily recommend them.
Recommended Post 📘
This publication from Purdue University Extension gives you tips on using organic fungicides and provides a list of commercially available products.
How is black spot spread?
Black spot spores spread primarily from leaf to leaf and plant to plant by wind, but they do not germinate without the presence of moisture. This is why damp and humid conditions, especially over a prolonged period of time, are ideal for black spot to take hold and spread freely.
Grow Disease Resistant Roses 🌹
If the thought of your roses being decimated by black spot (or another disease) seems overwhelming, you don't have to give up on growing roses altogether.
Oso Easy landscape roses from Proven Winners are highly disease resistant and are a great choice if you don't want to have to constantly worry about diseases ruining your roses. They also don't require any complicated pruning, so they are "Oso Easy" to grow!
Which Roses Are More Susceptible to Black Spot? 🌹
All types of roses can be affected by black spot, but some cultivars are much more susceptible than others.
Roses most resistant to the disease include floribundas, shrub roses and climbing roses, while the least resistant are hybrid tea roses, grandifloras and miniature roses, according to the University of Nebraska-Lincoln Extension. .
In general, rose bushes with dense foliage or those with leaves growing close to the ground are more prone to infestation than roses with open, airy foliage.
Which Roses Are More Resistant to Black Spot? 🌟
Often, the best approach to eliminating black spot in the garden is to replace your most vulnerable roses with new disease-resistant hybrids, advises Peter Kukileski, author of"Roses Without Chemicals", which lists 150 varieties of roses that excel in gardens without the use of pesticides. Disease-resistant roses are also less susceptible to other common fungal conditions, such as powdery mildew, rust and anthracnose.
The degree of resistance can vary depending on plant care, site conditions and the local environment, but in most cases your disease-resistant roses will not suffer much even if they get black spot. "Some varieties of roses tend not to defoliate even if they have a little black spot on their leaves. The genetics of the plant are strong enough to keep the leaves on the plant even when disease occurs," Kukileski says.
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