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The Dark Side of Black Spot

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Donna W. Moramarco

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Late summer rose
Photo Credit: Donna Moramarco
Protect your roses from the evil ways of black spot.
I’m an amateur rose grower. I’ve been dabbling with them in my garden for years, and every year I swear (sometimes at them) that it will be my last for growing roses. One thing’s for certain: When the weather’s not in your favor, it can be tough growing these beautiful plants. For me, the challenge is all about facing the notorious leaf disease called black spot.
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Black spot
Photo Credit: Donna Moramarco
Ideal conditions for black spot include periods of high humidity, prolonged wet foliage and air temperatures between 75 to 85 degrees F.
It sounds as menacing as it is: Black spot is an ugly fungus that attacks a wide range of roses. Ideal conditions for this garden troublemaker include periods of high humidity, prolonged wet foliage and air temperatures between 75 to 85 degrees F. (Sounds like your typical summer to me!)

How can you be prepared for battle against this rose enemy? An integrated approach is your best defense.

First and foremost, think about how you water: Do you use an overhead sprinkler that wets the leaves? If your answer is “yes,” and black spot has been a problem in your garden, you’re setting yourself up for continual disaster. Splashing water spreads the spores of the fungus. It’s best to water the soil, not the foliage. Invest in (and use) a soaker hose to keep those leaves dry.

Warnings
  • Certain chemical and organic fungicides can be used to control black spot. (Read and follow all label directions!)
Facts
  • Black spot is known to plant pathologists as Diplocarpon rosae.
Tips
  • Remember: An integrated approach is your best solution to black spot problems.
  • Keep leaves dry when watering; direct water to the soil.
Definitions
  • Plant pathology: The formal study and science of diagnosing and managing plant pathogens.
    Translation: “Disease” is a catchall term used to discuss plant problems. A disease can be further broken down as to whether it is fungal, bacterial, viral and so on. Fungi (plural for fungus) are the largest group of plant diseases. Examples of fungi: black spot, powdery mildew and wilt.
 
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Articles
  • Disease Basics: The Disease Triangle
    Understand the three conditions that must be present for a plant to fall victim to disease: the plant itself (host), the disease (pathogen) and the right weather conditions (environment).
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