donnan.com

  

Black Spot on Roses

How to treat the worst rose disease

By: Sandy Feather 2006
Penn State Extension


Q. I have a 'Blaze' rose that had a severe case of black spot. Even though we treated it, it has lost most of its leaves. Also, only the top third of it bloomed well. Is there a reason for this? If so, can I do something about it, or should I get rid of it? I like the climbing rose and would like to plant another in the same place. What do you suggest?

A. Black spot is the most common and severe disease that rose growers have to face. Its development is favored by rain or overhead irrigation (leaf wetness), high humidity and moderately warm temperatures for several consecutive days. Black spot is characterized by round black leaf spots that have feathery margins. Infected leaves turn yellow and drop prematurely. Roses can be completely defoliated by severe infestations.

 

Flowers, fruits and canes are also affected. Symptoms on canes appear as purplish, irregularly shaped blotches. The causal fungus, Diplocarpon rosae, overwinters on fallen leaves and infected stems. The initial infection in spring happens when rain splashes on fallen leaves or canes that are infected, releasing spores that land on new growth and start the cycle over again.


PROTECTIVE FUNGICIDE APPLICATIONS ON ROSES

Black spot has many cycles of infection through the growing season, as long as weather conditions favor disease development. Consequently, protective fungicide applications must be made through the growing season. A single treatment in the spring will not keep black spot at bay for the entire growing season.

beautiful rose blossom

It is likely that the severe infection kept your rose from blooming as much as it should. Repeated defoliation weakens plants because they cannot undergo photosynthesis without leaves. Without photosynthesis, they cannot produce the carbohydrate reserves necessary for proper growth and flowering.


RESISTANT ROSE VARIETIES

A multipronged approach to controlling black spot works best. You could choose to replace your susceptible 'Blaze' (red) rose with a climber that is more resistant such as 'New Dawn' (pink). Remember that resistance is not immunity. Even resistant roses can become infected if infected roses surround them, and weather conditions are favorable for black spot development.

Good garden sanitation is also important. Clean up fallen leaves promptly and prune out infected canes in early spring to reduce the amount of fungal fruiting bodies present. Use drip irrigation, soaker hoses or a watering wand to water the soil while keeping the foliage as dry as possible.


ROSE FUNGICIDE APPLICATIONS

Finally, repeated fungicide applications are very effective at keeping roses free of black spot. Daconil (chlorothalonil), Dithane (mancozeb), Systhane (myclobutanil), Bayer Advanced Disease Control (tebuconazole), Cleary's 3336 (thiophanate methyl), and Funginex (triforine) are labeled to control black spot on roses.

Organic growers can use sulfur-based fungicides such as Sulfur Guard from Gardens Alive. Repeated fungicide applications should be made as long as weather conditions favor disease development. Follow label directions regarding how often the fungicide you are using should be applied, typically 10- to 14-day intervals. You can stop making fungicide applications during hot, dry weather.

MORE

Photos of Roses

Palace of Gold rose garden

Watering new landscape plants

 

   


home | terms of use | contact | search | site map
Copyright 2017  DONNAN.COM  All rights reserved.