Roses have been grown for thousands of years for their beauty, fragrance and healing properties. They are highly symbolic flowers. Gifting a red rose expresses positive emotions such as romance, love, and courage. The emblem of England is the Tudor Rose, created by Henry VII. This symbol represents the unifying of the Houses of York and Lancaster after years of civil war.
These popular flowers are grown worldwide but face an unpleasant disease called rose black spot – which might even be in your garden right now! But don't worry; this disease cannot spread to humans! Although it is not an outrightly deadly disease, it can have devastating effects on the roses. Let's look into this fungal disease a little more, its symptoms, life cycle, what causes it, and its prevention and treatment.
What is Rose Black Spot?
Black spot is a severe disease affecting roses. It is caused by a fungus called Diplocarpon rosae. It is one of the most common diseases found in roses. Although this fungal disease does not directly kill the plant, it does, over time, cause it to be susceptible to other stresses and damage.
The fungus was first detected in Sweden in 1815. In the past 200 years, it has spread worldwide, affecting the roses' quality and life span. Black spot is most active during cooler and moist weather as this helps the fungal spores to establish infection.
Extreme heat, usually in the summer months, limits the development of the fungus.
Fungi vs Plants vs Bacteria
Fungi share similar traits to plants and bacteria, but they're classified in a kingdom of their own. What makes them different from plants and bacteria?
Table 1: some of the main characteristics that differs among fungi, plants, and bacteria.
| Characteristic | Fungi | Plants | Bacteria |
| Cell Wall Material | Chitin | Cellulose | Peptidoglycan |
| Energy Source | Heterotroph (eats other organisms) | Autotroph (produces own food) | Both heterotrophic and autotrophic |
| Reproduction | Asexual | Sexual | Asexual |
| Size | Single-celled and multicellular | Multicellular | Single-celled |
Rose Black Spot Symptoms
The symptoms of black spot can vary depending on what strain (i.e. variant) the rose is infected with.
General symptoms of black spot include:
This is how it has established its name.
The rest of the leaf tissue turns yellow.
Infected leaves fall off the stem prematurely, starting from the bottom of the plant and progressing upwards.
In the first year, purple-red scars can develop on the stem, becoming scabby black lesions as the plant grows.
Badly affected plants can lose almost all their leaves, which significantly reduces their vigour.
Vigour is the ability of a plant to produce healthy seedlings in stressful conditions.
Photosynthesis occurs in a plant's leaves, within tiny structures called chloroplasts. Inside the chloroplasts, carbon dioxide and water molecules are converted to glucose and oxygen. This reaction provides energy and resources for the plant. If a rose becomes infected with the black spot fungus, it will experience damage to its leaf tissue, making it challenging to photosynthesise. As a result, it will struggle to produce enough food. Limited photosynthesis will cause stunted growth and a possibility of early death.
Rose Black Spot Life Cycle
The black spot fungus survives the winter by staying dormant in the infected plants or their fallen leaves.
The fungus can stay dormant in rose plants for multiple winters!
Rainy spring weather triggers the dispersal of fruiting structures and their spores from the leaves, allowing them to germinate on new plants. Once the fungus has sprouted on a new plant, it penetrates the leaf's cuticle and grows between cells to infect the rose.
Fruiting structures are bodies that contain spores, which are individual reproductive units.
A cuticle is a waterproof, waxy cover on the top of leaves to prevent water loss by evaporation.
Rose Black Spot Causes
The dispersal of black spot reproductive bodies is triggered by a wet environment and cool temperatures – typical during the UK spring. The fungus typically requires seven hours of wet conditions to disperse its fruiting structures.
Most UK summers are also sufficiently wet, enabling the life cycle to repeat numerous times throughout the growth period.
Susceptibility
Although all roses can be affected by black spot, some cultivars are more susceptible than others.
These include hybrid tea roses, Grandiflora roses, and miniature roses.
Rose cultivars with dense foliage, or leaves that grow close to the ground, are more prone to infection. These plants are more likely to remain moist than roses with an open, airy canopy.
Resistance
Newer-disease resistance hybrid cultivars are less likely to suffer from black spot. However, resistance bred into new cultivars does not always last long because new strains of the fungus can appear quickly.
Rose Black Spot Prevention
It's easier to stop the fungus from infecting the roses than to treat infected plants. As a result, prevention is considered the first line of defence.
Keeping Foliage Dry
The fungus spores need a wet surface to germinate, so keeping roses as dry as possible will limit the spread.
Water roses early in the day. It gives excess water enough time to evaporate before nightfall, so the plants remain dry overnight.
Grow your roses where they will receive a high level of sunlight. Exposure to direct sunlight speeds up evaporation and drying.
Fig. 2 - Keeping roses spaced out limits the spread of black spot, unsplash.com
Garden Hygiene
Good gardening practices can reduce the accidental spread of pests and diseases.
- Keep the garden (and gardening tools) clean and tidy.
- Apply mulch to the soil for protection.
Mulch is a layer of organic material applied to the soil's surface. Other mulch uses include moisture conservation, soil fertility improvements, and weed growth reduction.
- Monitor plants – early detection makes it easier to control the spread of the fungus.
- Deal with garden waste appropriately. Dispose of infected leaves through composting or incineration.
Rose Black Spot Treatment
Once a rose plant has started showing signs of black spot, it is impossible to reverse the disease. However, it is possible to prevent the infection from spreading to new leaves.
Chemical-free treatments focus on cutting out the infected plant material and removing it from the garden.
Visible stem lesions can be pruned out in early spring before the leaves grow. Later in the growing season, infected leaves can be removed from the plant. Careful disposal of infected material is essential; the fungus spores can lie dormant throughout harsh winters.
However, natural treatments are not foolproof, as spores can blow in from elsewhere.
If chemicals are used to treat black spot, they should be used minimally and carefully targeted.
Excessive fungicide use can lead to water pollution and disrupt the natural soil community.
Fungicides, or fungicide-based products, can be sprayed onto infected plants weekly. Dormant sprays can be applied to the roses over winter to kill any dormant fungus spores.
Fungicides are chemicals used to kill parasitic fungi or their spores.
When using fungicides, pay attention to the weather. Rainfall can remove fungicide layers from roses – and create moist conditions perfect for fungus dispersal. Spray the roses after a period of heavy rain.
Rose Black Spot - Key takeaways
Black spot is a severe rose disease caused by a fungus called Diplocarpon rosae.
Symptoms include dark spots on infected leaves, which later fall off the plant. Scabby lesions appear on the stem before the leaves begin to grow.
The black spot fungus stays dormant over winter. Prolonged wet and cool conditions trigger the dispersal of fruiting structures. As a result, germination is standard during the UK spring.
New strains of black spot appear rapidly, making it difficult to breed resistance into rose cultivars.
Keeping foliage dry and maintaining good garden hygiene can prevent black spot infection.
Treatment includes removing infected parts of the plant and applying fungicides.
References
- Anne Balogh, How to Treat and Prevent Black Spot on Roses, Garden Design, 2022
- CGP, AQA A-Level Biology, 2015
- Dan Gillman, Black Spot of Rose, Center for Agriculture, Food, and the Environment, 2011
- Harlow Gardens, Symbolism and Meaning of Roses, 2004
- Jim Chatfield, Infectious Disease Host Range: How Wide Can It Be?, Buckeye Yard & Garden Online, 2020
- Marvin R. Paulsen, Soybeans: Chemistry, Production, Processing and Utilization, 2008
- Natalia A. Peres, Black Spot of Rose, Plant Pathology, 2018
- Nikita, Red roses and white: A brief history of the Tudor Rose, Royal Central, 2015
- The Royal Horticultural Society, Protect your garden, 2022
- The Royal Horticultural Society, Rose black spot, 2022
- Fig. 2. Image (https://unsplash.com/es/fotos/EMQlFWKASTg) by Billy Cox, free use under Unsplash License (https://unsplash.com/license).
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