11 Rose Bush Diseases: Identification, Prevention, and Treatment
Wondering what’s plaguing your roses? Though hardy and tough, roses are susceptible to certain diseases. In this article, expert gardener and rose enthusiast Danielle Sherwood explains the most common rose diseases, along with how to identify, prevent and treat them.
Contents
Even the most experienced rose gardeners are likely to encounter diseases in their gardens at one point or another. While good gardening practices are key to prevention, sometimes diseases prevail despite our best efforts.
When your rose falls prey to a virus or fungus, you need to know what you’re dealing with to react appropriately. If foliage is deformed, discolored, or dropping, blooms are withered or failing to open, or canes are weak and spotted, your rose is likely struggling with disease. Correct identification gives you the best chance of treating the problem successfully and preventing its reoccurrence.
In this article, I’ll give an overview of the most common rose diseases and their symptoms, with tips on how to prevent and treat them. Let’s dig in!
Black Spot
Ah, the dreaded, inevitable black spot. This disease, spread by the fungus Diplocarpon rosae, thrives on wet foliage. It is so common that even expert growers encounter it when conditions are right. Thankfully, it isn’t life-threatening to your roses and can be managed and prevented with extra precautions.
Black spot causes irregular black blotches on leaves that are often surrounded by a yellow ring. You will most likely see it appear during the cool, moist spring and fall months.
Black spot is spread by fungal spores that travel easily in splashing water and via infected debris. If impacted leaves are left at the base of your roses, the disease will likely overwinter and strike again the following year.
It takes about 7 hours of moisture for black spot to develop. Proper watering practices and good airflow will go a long way toward prevention.
Identification
You’ll see foliage with circular black spots, sometimes surrounded by a yellow ring or halo. The disease begins at the bottom of the plant and works its way up. Leaves will eventually fall from the plant. Sometimes, canes are also affected and develop raised rust-colored blotches.
Prevention
Your first line of defense is choosing disease-resistant roses. Once planted, water in the mornings so plants have time during the day to dry off. Aim toward the base of the plant rather than overhead, so foliage remains dry. Prune plants in spring to maintain an open center with good airflow, and space roses at least 2-3 feet from one another. Sanitize your pruners with rubbing alcohol between plants. After pruning, compost or discard any debris.
Treatment
You can’t cure black spot, so prevent further spread by promptly removing any affected foliage. Consider spraying the healthy tissues of the plant with a fungicide to retard any further fungal spread. If the disease is severe and canes are also infected, prune the canes down to healthy green growth.
Powdery Mildew
Powdery mildew is another common fungal disease that affects roses and other garden plants. It is spread by the microscopic spores of the fungus Podosphaera pannosa. This disease is most likely to strike when warm days are followed by cool nights. It loves humidity.
Powdery mildew looks like a fuzzy white coating on the leaves and stems of the rose. Leaves may curl up, and the entire plant can look weakened and withered. Flower buds may fail to open.
Planting in full sun can help prevent powdery mildew. Similar to black spot, it’s helpful to water in the mornings and avoid wetting foliage to prevent the moist, humid conditions the fungus prefers.
Identification
A white fuzzy or powdery coating on stems and leaves, along with curled leaves and an overall withered appearance, are symptoms of powdery mildew.
Prevention
Plant roses in full sun. Prune and space to maintain good airflow. Water in the mornings at the base of the plant.
Treatment
Trim badly affected foliage and buds. During a hot and dry day, spray it off with a sharp hose blast. If this doesn’t get rid of the problem, consider applying a copper fungicide. Some sources suggest that a milk-solution spray may treat the fungus as well as prevent future infections. However, milk treatments for powdery mildew on roses have no scientific grounding.
Stem Canker
Stem cankers may develop when rose canes have been wounded via pruning, friction from rubbing canes, or winter and insect damage. Different fungi, like Leptosphaeria and Cryptosporella, colonize the wound and infect the cane.
Cankers look like wrinkled and cracked lesions that may be yellow, tan, or black with brown or purplish margins. The canker can spread and girdle the cane and, if allowed to reach the crown, may cause harm to the entire plant.
Cankers are common and generally are not cause for alarm, assuming they do not spread. Prompt removal and good sanitation can help keep cankers at bay. Weakened, damaged plants are more susceptible, so keep your roses healthy with good irrigation, spacing, and pruning practices.
Identification
Look for discolored (usually tan, yellow, or black) and oddly textured lesions on canes, especially near recently pruned areas. The lesion edges may be tinged with brown or purple.
Prevention
The fungal pathogens that cause cankers in damaged plant tissue can be spread via infected pruning tools and splashing rain. Always sanitize tools with rubbing alcohol between plants, and anytime they come into contact with infected plant material. When pruning, cut just above a leaf node, and remove any canes that rub each other. The friction can cause wounds where the fungus may enter.
Treatment
Trim diseased canes to about 5 inches below the canker. There is no chemical control for canker at this time.
Crown Gall
Crown gall is a bacterial disease spread by the soil-borne pathogen Agrobacterium tumefaciens. It begins when a wound near the soil level comes into contact with the bacteria.
Once exposed to the pathogen, the crown of the plant develops soft, white to light-green tumor-like swellings that eventually turn brown, black, and woody. If advanced, the crown may develop a cauliflower-like appearance.
Crown gall eventually kills the plant by inhibiting its ability to absorb nutrients and moisture. The process may take several years. In the meantime, you will notice a weakening and wilted plant.
Crown gall is highly infectious and easily spread via contaminated soil and tools. The bacteria can live in the soil for up to 3 years, making the site inhospitable for future rose plantings.
Identification
Look out for unusual swellings at the base of your rose that may start as small as a pea and gradually get larger, darker, and woody. Over time, your rose may wilt as a result of reduced water and nutrient intake.
Prevention
Don’t plant roses in soil that has previously hosted plants with gall. Examine the crown of any newly purchased roses for unusual growth. Be careful when planting new roses to avoid creating wounds in the crown. Disinfect your pruning tools between plants.
Treatment
Dig up infected plants and their immediately surrounding soil and dispose of them. Avoid planting any roses in the same spot for at least 3 years.
Rose Rust
Signs of rose rust first appear on the lower leaves of the plant. The tops of the leaves will have yellow spots, and the undersides will be dotted with orange to black circular pustules that house the spores.
The fungus Phragmidium causes rose rust. If you notice the infection while the spread is still isolated, you may be able to prune it out. If the entire plant is affected, you will need to remove it.
Phragmidium spores proliferate in wet, mild conditions. They may spread via wind, plant debris, splashing water, or unsanitized tools.
Identification
Look for yellow spotted leaves and inspect the undersides. If rose rust is present, you will see orange or black powdery dots (spore pustules). Canes can be infected as well. When infection advances, the leaves may turn entirely yellow and drop off.
Prevention
Water in the morning and aim at the soil level to prevent wet foliage (Rose rust requires 2+ hours of wet foliage to grow). Remove all pruning debris. Sanitize pruning tools thoroughly between plants.
Treatment
If damage is minimal, prune out affected foliage and canes and dispose of them promptly. Watch carefully for further spread. If the entire plant shows signs of disease, remove and dispose of it. Fungicides may prevent, but will not treat rose rust.
Rose Mosaic Virus
Rose Mosaic is a viral disease that causes unusual yellow patterns on foliage. Leaves may appear mottled, spotted, or have squiggly lines. They may even look attractively variegated. Leaves can look puckered or distorted.
The virus is spread via transmission by insects, as well as by infected cuttings, rootstock, and grafts. So far, no scientific evidence supports its spread via contaminated tools.
Fortunately, mosaic virus doesn’t kill the rose. In fact, infected roses can look healthy and show no symptoms at all. Severely impacted roses may have decreased blooms or distorted foliage. The virus is unlikely to spread to the other roses in your garden if properly spaced.
Identification
Look for irregular yellow patterns on foliage, which could be mottled, striped, spotted or squiggled.
Prevention
Examine roses carefully for signs of mosaic before purchasing. Even if the damage is pruned off, the disease persists in the rest of the plant.
Treatment
Rose mosaic cannot be cured. If the plant looks healthy aside from yellow leaf patterning, you may leave it in your garden. If the virus causes it to weaken or underperform, remove and dispose of it. If you have a plant with signs of rose mosaic among healthy plants, consider removal to prevent pests from acting as virus vectors.
Anthracnose Leaf Spot
Roses infected with anthracnose will have small reddish-tan to brown spots ringed in black. The disease looks similar to black spot, but treatment is the same, so there is no need to stress over which disease you’re dealing with!
Climbing roses, wild roses, and ramblers are the most susceptible, but any rose can get anthracnose. It’s spread by the fungus Sphaceloma rosarum, a different pathogen than the anthracnose that affects trees.
If untreated, anthracnose may cause complete defoliation. Similar to black spot, it prefers the mild, wet conditions of fall and spring. Damage is unsightly but, when caught early, will not greatly impact the health of the rose.
Identification
Look for smooth-edged brown, tan, or reddish spots outlined in black. Centers may become yellow and fall out (called the shot-hole effect). Over time, the plant may lose all of its leaves.
Prevention
Look for smooth-edged brown, tan, or reddish spots outlined in black. Centers may become yellow and fall out (called the shot-hole effect). Over time, the plant may lose all of its leaves.
Treatment
Prune and dispose of any damaged foliage as soon as you see it. The rest of the plant can remain healthy once the infection is removed. Fungicidal sprays can be used as a preventative measure after disposing of infected foliage to prevent reoccurrence.
Botrytis Blight
The fungus that causes botrytis blight, Botrytis cinerea, affects many ornamental plants such as roses. You will observe it primarily on blooms and canes. Infections will most likely appear during mild, rainy weather (around 60 ℉).
Affected flowers may have soft, tan-to-brown specks and blotches. Buds may not open at all and remain brown and mushy. Canes might die back or acquire a gray wooly coating. Botrytis blight can enter cane wounds, causing cankers that slowly spread.
Sanitation is key to preventing the spread. Prune out any infected matter and bag it up right away. Use rubbing alcohol to clean your pruning shears between every plant or between infected canes and those which currently show no signs of damage.
Identification
Look for soft brown patches on rose petals. Canes may have canker lesions where the fungus has entered a previous wound. When attacked plant material begins to decompose, it will be covered by a wooly gray coating.
Prevention
Discourage mechanical transfer by thoroughly sanitizing pruners. Avoid wet, humid conditions by planting in full sun, pruning for good airflow, and watering when plants will have the chance to dry out quickly.
Treatment
Monitor plants frequently during rainy weather. Prune off affected flowers, leaves, and canes to prevent spread. This is often enough to stop further infection. Botrytis blight is often resistant to fungicides.
Downy Mildew
Downy mildew is caused by the pathogen Peronospora sparsa. It shows up in yellow, brown, or purplish spots on leaves, sometimes with a greyish mass of sporulation on the underside of the leaf. Discolorations caused by downy mildew usually run along the veins of the foliage and are subsequently more angular than the similar-looking black spot and anthracnose.
Downy mildew is less prevalent than black spot but is an increasingly serious problem for industry growers. Spores can spread via wind, water, and tools. It also strikes cane berry plants like raspberries and blackberries.
It prefers mild, moist conditions with high humidity. If you’re noticing a pattern here, you’re right! Roses are much less susceptible to fungal disease when conditions are dry.
Identification
Look for discolored spots on foliage. If they are slightly angular due to growing along the straight leaf veins, you are most likely dealing with downy mildew.
Prevention
Prevention occurs when good gardening practices are maintained. Always clean and sanitize your pruning shears. Space plants 2-3 feet apart, and prune so that rose bushes have an open center for airflow. Water at the base of the plant rather than overhead.
Treatment
Remove badly infected plants. If caught early, prune out affected leaves and canes, then spray with fungicide. The disease develops resistance to repeated applications of the same fungicide, so alternate products every few weeks. If the bush doesn’t respond after a few months, it’s best to get rid of it.
Dieback
While cane dieback is sometimes thought to be a disease, it is actually a symptom of a variety of damage and pathogens, including botrytis blight. Dieback most often refers to cane tips that are damaged and black.
Dieback can sometimes be caused by frost that killed off new tender growth. Improper pruning may have caused a wound, allowing a fungus to enter and develop a canker. If left unpruned, damage may spread further down the cane.
The best practice when dealing with dieback is to prune the affected cane to where you see healthy green growth. Regardless of the cause, this will help prevent the spread to the rest of the plant.
Identification
Look for brown or black stem tips and discolored lesions on canes.
Prevention
Provide winter protection for tender roses. Don’t prune when frost is likely, leading to potential damage of new growth. Prevent fungal infections that may exacerbate dieback by keeping rose foliage dry when watering.
Treatment
Prune blackened canes and infected areas to where it is healthy and green, ideally above a leaf node. If infection rather than winter damage is suspected, identify and follow treatment protocols for the specific disease.
Rose Rosette
Rose rosette, a viral disease spread by a tiny mite, is a rapidly growing problem for rose gardeners. Also called witches’ broom, it produces red distorted new growth with thick and excessively thorny canes.
The “rosettes” refer to small, tightly clustered buds. If the flowers open at all, they may be distorted. Foliage may be stunted and deformed.
Rose rosette is severely contagious. The mites feed on their host plant (most commonly a multiflora rose) and can travel via wind and tools. Adjacent plants are quickly infected. If suspected, report the rose rosette to your local extension office. There are no treatments for the disease, so the rose must be completely removed, placed in a garbage bag, and thrown away.
Identification
Look for abnormally fast-growing, extra-thorny red canes with lots of red, deformed foliage at the ends. Buds will be tightly clustered together.
Prevention
Don’t plant susceptible varieties like multiflora and Knock Out roses. Since the mites overwinter in flower buds and hips, trim off any remaining in early spring before roses break dormancy. Don’t use leaf blowers around your roses, which can quickly spread the mites to new areas.
Treatment
There is no treatment for rose rosette. Once the disease is confirmed, dig up the rose and immediately surrounding soil, placing it all in a tightly tied garbage bag before disposal. Carefully monitor any nearby roses.
Final Thoughts
Knowing the signs of most common rose diseases will help you determine if a simple pruning or a more serious approach is needed. Fortunately, most pathogens with similar symptoms, like black spot and anthracnose, have similar treatments.
Monitor your roses often for leaf discoloration, damaged canes, and irregular growth. The more you observe, the more likely you are to detect diseases before they spread.
Most fungal diseases prefer wet foliage and confined spaces. Remember that the same good gardening practices that keep most plants healthy also apply to roses: water in the morning only when the soil is dry, and aim for the base of the plant. Provide good airflow. Always sanitize your tools. Only purchase plants in good condition. Keep these tips in mind, and enjoy your roses!
