Black Spot

by Nanette Londeree, Master Rosarian

SYMPTOMS

  • Characteristic round black spots on upper surface of leaves or stems
  • Spots have feathery or fiber like margins; no powdery growth on the underside of leaves
  • May be yellowing around the spots that extend to the entire leaf
  • Small black fruiting bodies may be present in spots on upper sides of leaves
  • Spots are generally seen first on leaves close to the ground
  • Infected leaves may drop off; the entire plant may be defoliated
  • Miniature roses are generally more susceptible

CAUSE

  • Infection by the fungus Diplocarpon rosae
  • Airborne spores infect leaf tissue

OPTIMAL CONDITIONS

  • One of the most common diseases of roses and other plants
  • Prevalent during rainy weather
  • Temperatures of 65 – 75°F and at least 7 hours with free water on leaves for germination
  • Fungus can overwinter as dormant mycelium or resting spores on infected stems and leaves

TREATMENT

Prevention:

  • Buy and plant disease-free plants
  • Choose resistant varieties; glossy foliaged varieties generally have better resistance to most fungal diseases
  • Plant roses in areas with good soil drainage and ventilation; avoid shady spots and dense plantings
  • Avoid wounding plants during transplanting
  • Maintain good garden sanitation; remove and destroy infected leaves and canes during the season
  • Avoid overhead watering if time / temperature is insufficient to dry leaves within a few hours
  • Spray with baking soda and horticultural oil, anti-transpirants, botanicals such as Neem oil or systemics such as Funginex
  • Dormant spray with horticultural oil and copper or lime sulfate after pruning

Elimination:

Spray with baking soda and horticultural oil, Banner Maxx, Benlate, Compass, Daconil 2787, Fungi-gard, Funginex, Fore or Sentinal

GOOD GUY / BAD GUY?

A really bad guy—one of the most common diseases of roses

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