Black Spot on Roses: Prevention and Treatment
Black Spot on Roses
If you have roses or intend to grow them, you'll probably have to deal with the problem of black spot at some point.
Caused by the fungal pathogen known as Diplocarpon rosae, black spot is the leading fungal disease on roses in the world.
What does black spot look like?
Black spot first appears as feathery-edged black spots on the leaves (usually the lower leaves) of rose bushes. Sometimes, the damage looks as if someone took a cigarette to the leaves.
As the black spots grow larger, the leaves turn yellow and drop off. On stems, black spot appears as irregularly shaped blotches. These blotches, which may be deep purple, black or dark brown, eventually blister.
If black spot is left unchecked, it can work its way up the stems of rose bushes, defoliating the bush as it goes and ultimately killing it.
What conditions promote black spot?
Warmth, humidity and overnight wetness on rose leaves encourages Diplocarpon rosae to germinate and grow. Insects, people, pets, water, infected plant parts, dirty gardening implements—many things can spread the disease throughout the garden.
How to Prevent Black Spot on Roses
6 Easy Ways to Lessen the Likelihood of Black Spot
Choose the Right Location
Healthy plants are better able to resist disease than unhealthy ones. To promote the health of your rose bushes, make sure that they are planted in the right spot.
In order to thrive, roses need plenty of sunlight and lots of room for good air flow.
If your rose bushes are in a shady, crowded bed, draped with vines and/or surrounded by weeds, chances are they will develop black spot or some other fungal disease.
The fungus that causes black spot can germinate and infect your roses within the space of a day, with symptoms appearing in as little as 4 days, and new infections occurring every 10.
Increase Air Flow: Prudent Pruning
In addition to adequately spacing plants, pruning your rose bushes will assure good air flow. Using a by-pass pruner, snip out diseased and dead canes, leaves and flowers as well as crossed or rubbing canes, particularly those in the center of the bush.
Keep Your Garden Neat and Clean: Healthy Hygiene
Keep the area around your rose bushes free of weeds and debris (such as rose bush clippings). Weeds not only increase the humidity that encourages black spot, but they also provide food and shelter for pests.
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Rose bush waste may harbor Diplocarpon rosae, the fungus that causes black spot. After pruning, be sure to remove all buds, leaves, stems and branches from around the bush. Diplocarpon rosae spores can overwinter on discarded leaves and stems and, when conditions are right, germinate, reinfecting your rose bushes.
Also, because roses are prone to disease, avoid composting rose clippings. Doing so could spread black spot (as well as other diseases, such as mildew) throughout your garden.
After pruning infected rose bushes, clean your pruning sheers by wiping them down with rubbing alcohol to prevent the spread of black spot. Diplocarpon rosae fungus can live on garden tools (and in soil) for up to a month!
Keep Humidity Down: Wise Watering
Water rose bushes at their bases, not overhead, either by hand, through direct-drip irrigation or with a soaker hose.
Keeping leaves dry during watering will make them less susceptible to black spot and other fungal diseases (like powdery mildew) that require moisture in order to germinate.
Buy Black Spot Resistant Roses
One of the easiest ways to avoid black spot in the rose garden? Opt for rose bushes that are less likely to succumb to it. Although no rose bush is impervious to the disease, some are more resistant than others.
Some Black Spot-Resistant Varieties of Roses
GRANDIFLORAS and FLORIBUNDAS
Always a Lady
Blanc double de Coubert
Carefree Series of Roses
Knock Out Roses
Apply Fungicides If You Must
Although I do not use any in our garden, treating your rose bushes with fungicide will also lessen the likelihood of black spot. Those who intend to show their roses (and are unwilling to tolerate any black spot damage) are most likely to apply traditional or alternative fungicide treatments.
Begin fungicide treatments in early spring, when the first leaves emerge, and continue spraying into fall. Follow the directions on the label of the fungicide to the letter.
Many treatments for black spot must be applied every 7 to 14 days, and they can be costly over time.
If your roses are simply for the enjoyment of yourself, your family and your friends, I recommend skipping them and relying on good gardening practices instead.
Some Traditional Fungicides and Their Product Names
- Captan (Captan)
- Chlorothalonil (Ortho Garden Disease Control, Bonide Fungonil Multi-purpose Fungicide, Daconil, Bravo, Echo, Fungonil)
- Mancozeb (Mancozeb, Stature, Dithane M45)
- Myclobutanil (Spectracide Immunox Multipurpose Fungicide, Eagle)
- Propiconazole ( Banner Maxx, Bonide Infuse, Fertilome Systemic Fungicide)
- Thiophanate-methyl (Fertilome Halt Systemic, Fungicide Fungo Flo, Quali-Pro TM, Systec, Cleary’s 3336)
Some Alternative Fungicides and Their Product Names
(These can be just as harsh as the others. Again, follow the directions for use carefully.)
- Potassium bicarbonate (Bonide Remedy Fungicide, Armicarb 100 )
- Copper hydroxide (Hi-Yield Copper Fungicide, Kocide, Tenn-Cop, Basicop)
- Copper salts (Bonide Liquid Copper Fungicide)
- Lime sulfur (Lime Sulfur)
- Neem oil ( 70% Neem Oil)
- Sulfur (Sulfur Dust, Wettable Sulfur)
"Blackspot." Rose Magazine. Rose Magazine Inc, 2012. Web. 5 July 2012.
The Maryland Master Gardener Handbook. University of Maryland Extension Master Gardener Program. 2008. Print.
"Save Your Roses From Black Spot." Better Homes & Gardens. Meredith Corporation, 2012. Web. 5 July 2012.
Wallis, Chris, et. al. "Black Spot of Roses." Fact Sheet: Agriculture and Natural Resources. Ohio State University, 2008. Web. 5 July 2012.
Watt, Bruce A. "Black Spot of Rose." Cooperative Extension: Insect Pests & Plant Diseases. The University of Maine, 2010. Web. 5 July 2012.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2012 Jill Spencer