rose diseases

Black Spot and Other Common Rose Diseases and Pests Every Gardener Should Know

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Many people all over the world refer to roses as the “queen of flowers” because their timeless beauty and elegance make them stand out amongst the rest of the world’s flora. The mostly upright, regal growth and stunning, fragrant blossoms are the defining characteristics of roses that make us want them in our gardens, patios, bouquets, and vases.

But just like any other plants, roses are not exempted from the attacks of pests and diseases. In fact, certain varieties are some of the plants that are highly susceptible to these attacks although breeding techniques today have helped them become more resistant. Nevertheless, to keep the beauty of rose plants, they must be protected from pests and diseases.

colorful rose bush

It is important to know the common pests and diseases roses are vulnerable to in order to understand how to prevent their occurrence and effects on the plants. This comprehensive guide will discuss the nature of these rose garden problems and some of the easy curative and preventive maintenance practices to counter them.

Rose Plant and Garden Problems

Pests are usually insects and small animals that feed on plant tissues while diseases are what prevents plants from performing their functions, often brought about by pathogenic microorganisms. These organisms grow naturally and many times occur on plants at the same time.

They attack different parts of the plant and when left untreated, they cause the plant to deteriorate and die. They can spread to other plants in the garden and for farmers, this causes a decline in yield.

yellow leaves on roses

For roses, the most common site of an infestation is the leaves but the stems, roots and blossoms are susceptible too. In rose gardens, the occurrence of these problems easily spread among the plants and for horticulturists growing them as cut flowers, these results to poor flower quality.

A good gardener should know the difference between pests and pathogens because they may require their own set of treatments. Plant inspection should be done regularly to detect these organisms before they become a problem. Pests and pathogens create or leave signs and symptoms on the plant that will help gardeners identify them.

There are many commercially-available pesticides as well as nature-friendly organic means to control them. There are also gardening techniques and practices that can be done to prevent and avoid their occurrence. It is always important to keep the environment in mind when treating these pests and diseases so as to effectively control them and not cause any more disturbance in the ecosystem.

Black Spots on Rose Leaves

black spot rose leaves
Credit: Scot Nelson/Flickr

The black spot is the most common and important disease occurring on any rose plant (1). It is the reason why there is a major decline in rose growth and production in many parts of the world like the United States.

Black spot is caused by a kind of fungus called Diplocarpon rosae, usually attacking the plant foliage. Fungi like this exist in areas with excessive moisture, high relative humidity, and insufficient sunlight.

Fungal spores germinate in the spring and they scatter onto the plant parts by splashing water, insects, and cultivation of soil. The effect is the formation of rapidly spreading dark violet to black round and fringed spots on the upper leaf surface. The adjacent areas turn yellow and later on, the infected leaves fall (2).

Badly infected plants lose almost all their leaves rendering them incapable of photosynthesis which results in deterioration.

black spot on rose leaves

New varieties of roses today are almost resistant to black spot but this fungus is also developing new strains that are becoming more persistent. Smart plant treatment such as integrated pest management must be performed to keep the fungus at bay.

Infected rose plants cannot be cured of black spots but certain preventive measures can control the spread of disease. The initial step is to remove and destroy all infected leaves and prune adjacent stems. The young leaves should then be protected by spraying with fungicide at least once a week from spring to fall (1).

How Do You Treat Black Spot On Roses Naturally

treating rose black spot
Credit: Scot Nelson/Flickr

Baking soda and water mixture is a common home solution to mild black spot infestation but more effective fungicides are available in the market. These include tebuconazole and titriconazole or any low-impact pesticides with sulphur, calcium sulphide or citric acid as active ingredients (3).

Fungal spores need moisture to germinate so roses must be kept dry by avoiding overhead watering. Irrigate in the morning so the leaves will dry throughout the day.  The rose plant can also be moved to an area where it will receive more sunlight and better air circulation.

Otherwise, avoid planting highly susceptible rose varieties such as most yellow-colored ones. ‘Bonica’, ‘Grand Opera’, and ‘Simplicity’ are some of the preferable varieties that are highly resistant to black spot (4).

Common Rose Insects and How to Treat Them

Here are some common insects you need to be aware of when it comes to rose care.

Aphids

rose aphids
Credit: Bernard DUPONT / Flickr (CC BY-SA 2.0)

Aphids are tiny insects that suck on plant sap. They are usually found in clusters on new leaves, flower buds, and stems of the rose plant. Some infested leaves have black discoloration because aphids secrete a sticky substance where sooty mold forms. Aphids cause deformed leaves and dropping of flower buds, and they carry diseases as well (2).

Aphids can be controlled by natural predators like ladybugs and orange-and-black soldier beetles. They can also be treated by manually wiping them off and spraying with water or insecticidal soap. Severely infested parts will have to be pruned. To avoid rapid shoot growth during seasons when aphids are active, slow-release fertilizer application is recommended.

Rose Midge

Rose midges are small, white fly larvae often found in clusters at the base of the rose bud. They cause the buds to wither and die, rendering the plant flowerless. All the infested parts must be removed.

In order to prevent further infestation, the soil around the plant must be cultivated since midges live buried in the soil when not feeding on the rose buds. Insecticides can also be applied directly onto the soil to eradicate them (5).

Rose slugs and Sawfly Larvae

rose slug sawfly
Credit: Scot Nelson/Flickr

Rose slugs are tiny and light green with brown head, some with bodies covered with hair, others are shiny. Both chewing creatures feed on the underside of leaves leaving small, round holes. In severe cases, the entire leaf is eaten leaving only the hard veins.

They can be treated by manually removing them together with the infested leaves. Insecticide can also be sprayed, drenching the infected leaves and stems (5).

Thrips

rose thrips
Credit: Scot Nelson/Flickr

These tiny orange insects with elongated bodies attack the bases of rose buds, often penetrating the closed petals. The buds either do not open at all or when they bloom, the petals are stained and deformed.

It could be difficult to prevent infestation when thrips are buried within the rose buds so these buds should be removed and destroyed instead. Susceptible plants should be treated with insecticides like Malathion in the spring when buds haven’t developed yet (1).

Caterpillars

roses caterpillars
Credit: TiggerT/Flickr (CC BY-NC-ND 2.0)

Caterpillars are moth and butterfly larvae that chew on leaves and soft buds of the rose plant. Some signs of infestation include rolled or eaten leaves and damaged rose buds or flowers.

These pests can be removed by spraying with a powerful stream of water and by application of common insecticides or more effective pesticides containing Bacillus thuringiensis, an active ingredient that is fatal to caterpillars but harmless to the plant (5).

Cane Borers

Cane borers are usually white or yellow larvae of moths that enter the canes through mechanical wounds. The result is wilting of new shoots, leaves and stems as well as swelling of some parts of the cane where the borer is buried.

The affected parts should be destroyed. Cuts are made on the swelling parts to make sure that the borers are removed. Shellac or white glue should be applied on exposed tips and open wounds to prevent the entry of this pest (5).

Leafcutter Bees

leafcutter bees rose
Credit: bramblejungle – Flickr (CC BY-NC 2.0)

These insects are blue, purple or shiny black bees that use the leaves of roses, particularly tea rambler roses, to build their nests.

This causes small tiny holes on the margins of the leaves and may impede photosynthesis but is not necessarily detrimental to plants. The recommended treatment is to just remove the damaged foliage since it is more important to preserve bees (1).

Spider Mites

spider mites rose
Credit: Scot Nelson/Flickr

These microscopic arachnids infest leaves of roses and many other plants during summer as well as in areas where there is hot, dry weather. A mass infestation could be observed as a bronze sheen on a dry leaf, a fine webbing on the underside of leaves which can lead to premature leaf fall.

Mites easily develop resistance to miticides so the use of more than one type of pesticide should be used. They can also be removed by running a strong stream of water onto the infested leaves (5). Some beneficial insects like ladybugs also feed on this tiny pest.

Rose Curculio

rose curculio
Credit: Ingrid Taylar/Flickr (CC BY-NC 2.0)

Rose curculio is dark-reddish weevils that chew on rose buds as well. When rose buds bloom, they have small holes drilled by the pointy snouts of weevils. They are usually hand-picked and destroyed or a mild soap solution is directly applied onto the infested buds (5).

Other Rose Bush Diseases and Remedies

Powdery Mildew

powdery mildew on rose leaves
Credit: Scot Nelson/Flickr

Another fungal disease to which roses are highly susceptible is powdery mildew. It is associated with fungi under the phylum, Ascomycota and they often target new, emerging plant shoots. 

They exist in humid areas and are spread by wind. They appear as white or grey powder covering new leaves, shoots, and flower buds (5) and the effect on rose plants is stunting and curling of these parts which dry out later on.

Plant parts that are infected with this disease should be immediately removed and should never be used in composting. It is important to spray the whole plant with sulphur-based fungicides to prevent the spread of disease. Avoid activities that will enhance new shoot development such as heavy fertilization and pruning (1).

Many red-colored varieties are very vulnerable to this so as much as possible, install more resistant cultivar in areas where there is high humidity.

Rust

rose rust
Credit: Malcolm Manners – Flickr (CC BY 2.0)

Rusts (also known as Phragmidium) are reddish-brown or yellow spots on any growing part of a rose plant. They often infect the underside of leaves, rapidly spreading and can even cover an entire leaf. Infected leaves curl, wilt, and fall prematurely. Rusts are caused by fungi under the order, Uredinales, and they are referred to as obligate parasites or those that grow on living hosts only (1).

Despite this nature of rust-causing fungi, their spores can thrive in soil therefore, all infected parts must be removed and burnt. They prefer moist air and moderately cool temperatures so planting them in areas with such conditions must be avoided (5).

Botrytis Blight

Another fungal disease that prefers a damp environment is botrytis blight caused by Botrytis cinerea. Many hybrid tea roses are vulnerable to this disease which prevents the flowers from blooming.

Rose buds are covered with greyish brown mold hence the disease is otherwise known as “grey mold”. Some flowers that do open have yellow or brown stain and the outer petals are wilted. There is also discoloration on the upper part of the stem near the flower.

The fungus causing this disease is prevalent during the rainy season so cut off and dispose of any infected parts and make sure to disinfect the tools used. Aliette is an effective fungicide in preventing further spread of the disease.

In case of severe infection, application of low-impact pesticides containing Bacillus subtilis as active ingredient is a recommended biological control (6).

Spot Anthracnose

Somewhat similar to black spot in appearance, anthracnose is caused by a fungus called Elsinoe rosarum. All above-ground parts of a rose plant are susceptible to this disease. On the leaves, it starts as a purplish-brown spot with the center turning grey and then falls out of the leaf at maturity (1).

The fungus spreads in water so overhead watering is not advised. The same treatment as with black spots can be used for anthracnose (5).

Common Stem Canker

A defining characteristic of this disease is the formation of reddish-brown spots on the woody stem or cane of the rose plant. Roses are known for their thorny stems and mechanical injuries on the plant itself caused by this become the entry points of fungal infection.

It starts out yellowish and turns brown, becomes sunken, and crack over time. The infection can reach the leaves and if they already have black spots, the effects are intensified leading to wilting and death of the plant.

There is no known cure for this type of disease but careful planting techniques can be done to avoid its occurrence. Since the fungus causing this disease is soil-borne, it is necessary to plant the uninfected roses in pasteurized soil. If the plant is already showing symptoms of the disease, use a sharp pruning tool and properly cut the cane below the canker (5).

Crown Gall

Agrobacterium tumefaciens causes crown gall in many plants including roses. The galls or tumor-like swellings form at the crown, the plant part just above the soil. These galls are the result of the bacterium’s ability to transfer and integrate part of its DNA into the plant genome. The disease is not specifically harmful to roses aside from a slight change in plant metabolism and the reduced marketability of the plants (7).

Infected plants are usually destroyed but a biological control method can also be done by cutting out the galls and submerging the roots and lower stem of the rose plant to a streptomycin solution for 2 hours. It can then be replanted but never on the same contaminated soil (1).

Mosaic Virus

mosaic virus roses
Credit: Malcolm Manners/Flickr (CC BY 2.0)

Mosaic virus is a pathogen that causes a distinct yellow mottling or streaking on the leaves of the rose plant. It slows the metabolism and growth of the plant and if no action is taken, the plant can die.

There is no known cure or prevention for this disease. Many times, the symptoms just disappear but if they persist and the plant condition becomes severe, the infected plant should be removed immediately (5).

Heat and Winter Freeze Damage

Extreme weather conditions also cause stress to rose plants. Long exposure to hot weather results in wilting of leaves and then browning. This can easily be alleviated by providing an ample amount of water to the plant but avoid watering when the heat is up. Watering should be performed early in the morning or late in the afternoon when the temperature is lower to avoid shock or further stress.

The extreme cold of winter, on the other hand, causes the canes to turn brown inside out (5). New shoots may be observed but they soon curl and die since the stems are already deteriorating and only using the last reserves of the plant. Potted rose plants are advised to be kept in the greenhouse and damaged canes should be removed.

rose leaf problems

Pests and Diseases Treatment in Consideration of the Environment

To preserve the quality of rose plants, it is our goal to successfully cure and prevent the spread of all these pests and diseases. Effective chemicals are commercially available and easily accessible to aid us in this endeavour. But the ultimate goal should be to efficiently treat these garden problems without disrupting the ecosystem.

Pesticides come in chemical forms that can easily be applied and are proven effective when used as recommended. However, over time, residue build up in the soil and in water reservoirs which can have harmful effects on plants, animals, and humans. There are even strong chemicals that when applied, can kill all kinds of plants on the spot.

yellow rose bud

Modern times and environmental advocacies have helped reduce the use of harmful chemicals and pesticides to pursue quality production or just to enhance backyard gardening. There are now options for mild pesticides that are also effective and are generally safe to use.

Integrated pest management has been improved and is recommended for implementation in landscape maintenance, both large-scale and small-scale. The use of biological methods (beneficial insects) and botanical insecticides (neem oil and pyrethrin) to control pests are now preferred over synthetic materials.

There are many approaches to controlling rose pests effectively. Some people and companies prefer to use quick methods while others use practical and organic means and they still yield the most beautiful roses.

It still boils down to our initiative to think about the bigger picture, not just our roses in the garden but the entire ecosystem as well. These pests are really enemies in the garden but we all could definitely live with a few insects here and there.

Up next: When to fertilize roses

References

Reference List:

(1) Pirone, P. “Diseases and Pests of Ornamental Plants.” John Wiley and Sons. 1978. Pp. 584.

(2) Kingsley, R. “Roses and rose growing.” Richard Clay and Sons, Ltd. 1908. Pp. 138-140.

(3) “Rose black spot.” Royal Horticultural Society. Retrieved from https://www.rhs.org.uk/advice/profile?PID=270.

(4) “Black spot of rose.” Missouri Botanical Garden. Retrieved from https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/fungal-spots/black-spot.aspx.

(5) “Pest and diseases of roses.” The Northern Sydney Institute. Retrieved from https://nsfsakai.nthsydney.tafensw.edu.au/access/content/group/179a89f1-0f5a-4f18-90a1-ecbe32dd40d4/Floristry/PrepareAndCareForFloristryStockAndMerchandise/Loco/Pest%20an%20diseases%20of%20roses.pdf

(6) Mousavi, E. et al. “Biocontrol of gray mold on Rosa hybrid cv. Baccara with Bacillus subtilis”. Trakia Journal of Science. Vol. 15. No. 2. 2017. Pp. 168-173. link

(7) Collins, A. “Agrobacterium tumefaciens.” North Carolina State University. 2001. Retrieved from https://projects.ncsu.edu/cals/course/pp728/Agrobacterium/Alyssa_Collins_profile.htm

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