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Roses: Diseases and pests

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The Japanese beetle (Popillia japonica) is an insect pest for roses
Photo: Insectarium de Montréal (André Sarrazin)

Diseases

Powdery mildews
(Erysiphe sp., Microsphaera sp., Oidium sp., etc.)

Signs and symptoms

  • Powdery white spots on leaves and flower buds.
  • Infected parts dry out

Control methods

  • Remove and dispose of infected parts; disinfect pruning tools regularly with a 70% isopropyl (rubbing) alcohol solution.
  • In hot, dry weather: spray the foliage with a hose to slow the growth of the fungus; avoid wetting the foliage late in the day, so as not to encourage other fungal diseases.
  • Never compost infected plant material.
  • In case of a serious infection in the previous year or years, as a preventive measure, apply a low-impact pesticide with sulphur, calcium sulphide or calcium polysulphide (lime sulphur)Bacillus subtilis (strain QST 713),  citric acid or lactic acid as the active ingredient. Read the product label carefully and follow the manufacturer’s recommendations.

Botrytis blight or grey mold
(Botrytis cinerea)

Signs and symptoms

  • Greyish down on leaves and flower buds
  • Infected parts dry out or rot

Control methods

  • Cut off and dispose of affected parts; destroy severely affected plants.
  • Regularly disinfect tools with a 70% isopropyl (rubbing) alcohol solution.
  • Never compost infected plant litter.
  • In case of a serious infection in the previous year or years, as a preventive measure, apply a low-impact pesticide with Bacillus subtilis (strain QST 713) as the active ingredient. Read the product label carefully and follow the manufacturer's recommendations.

Rust
(Phragmidium spp.)

Signs and symptoms

  • Yellow spots turning orange on upper leaf surfaces
  • Orange pustules on leaf undersides
  • Fruit infected

Control methods

  • Remove and destroy all infected leaves.
  • If your roses were severely infected in the previous year(s), use a low-impact pesticide in which the active ingredient is sulphur, calcium sulphide or calcium polysulphide (lime sulphur). Read the product label carefully and follow the manufacturer’s recommendations.
  • A baking soda solution may also be used.

Black spot
(Diplocarpon rosae)

Signs and symptoms

  • Fringed black spots on upper leaf surfaces
  • Leaves turn yellow and drop

Control methods

Crown or stem gall
(Agrobacterium spp.)

Signs and symptoms

  • Growths on stems or crown
  • Bacterial disease

Control methods

  • Avoid mechanical injury.
  • Destroy any infected plants.
  • Do not replant roses in the same spot for at least three years.

Pests

Rose midges
(Dasineura rhodophaga)

Signs and symptoms

  • Buds wither and die back
  • Upper stems wither
  • Whitish or reddish maggots in flower and leaf buds

Control methods

  • Remove all affected parts.
  • Cultivate the soil in the fall or early spring to expose the larvae to predators and severe weather.

Caterpillars (butterfly larvae) and sawfly larvae
(Allantus cinctus, Cladius difformis, Endolomyia aethiops, Choristoneura rosaceana, etc.)

Signs and symptoms

  • Leaves rolled or eaten (holes, skeletonized or devoured entirely)
  • Flowers or flower buds damaged 
  • Signs of egg laying on stems: scars accompanied by tissue browning
  • Presence of caterpillars or sawfly larvae. The latter often resemble caterpillars, but are not the same thing. True caterpillars are butterfly larvae, and can be distinguished from sawfly larvae by the number of false abdominal legs: caterpillars normally have five pair, whereas sawfly larvae have at least six pair. In addition, adult sawflies are not butterflies but hymenoptera that look something like wasps or flies. The name “sawfly” stems from the fact that they have a serrated ovipositor.

Control methods

  • Regularly inspect the foliage (not forgetting the undersides of leaves), and manually destroy larvae; crush them or drown them in soapy water.
  • Dislodge larvae by spraying with a powerful stream of water.
  • Remove all affected parts. Female sawflies may make incisions in rose stems to deposit their eggs; cut off and throw away stems that show signs of egg laying.
  • As a last resort, use a low-impact pesticide in which the active ingredient is insecticidal soap. Low-impact pesticides whose active ingredient is Btk (Bacillus thuringiensis subsp. kurstaki) can be used against caterpillars (butterfly larvae). Btk is ineffective, however, against sawfly larvae.. Read the product label carefully and follow the manufacturer’s recommendations.

Cane borers
(Oberea affinis)

Signs and symptoms

  • Swollen bands on canes
  • Evidence of egg-laying between swellings
  • Shoots wilt and die
  • Larvae under bark

Control methods

  • Cut back to 5 cm below the lowest swelling.
  • Make sure to cut low enough to remove all parts containing insects.

Rose aphids
(Macrosiphum rosae and other species)

Signs and symptoms

  • Thick clusters of insects on stems and under the leaves of young shoots
  • Leaves wither, yellow, curl or drop
  • Deformed leaves or flowers
  • Flower buds drop off
  • Presence of honeydew, which attracts ants
  • Development of a black fungus (sooty mould)

Control methods

  • Dislodge aphids by spraying them with a hose; repeat this treatment as necessary.
  • Remove and dispose of all heavily infested stems and leaves; crush groups of insects on a plant by hand.
  • Since aphids are attracted to the colour yellow, install sticky yellow traps near your plants; place yellow containers full of soapy water or water mixed with yellow food colouring at the base of infested or susceptible plants. As a last resort, use a low-impact pesticide with insecticidal soap as the active ingredient. Read the product label carefully and follow the manufacturer’s recommendations.

Japanese beetles (Popillia japonica) and rose chafers (Macrodactylus subspinosus)

Signs and symptoms

  • Skeletonized leaves
  • Flowers and buds eaten

Control methods

  • Pick off adults in early morning.
  • Rake the soil in spring or fall to expose the larvae.

Spider mites
(Panonychus ulmi, Tetranychus urticae, etc.)

Signs and symptoms

  • Tiny white dots on leaves
  • Leaves turn greyish, brownish or coppery and drop
  • Presence of fine webs

Control methods

  • Remove and destroy heavily infested shoots.
  • Spray the foliage and branches with a strong, steady stream of water to dislodge the mites and increase humidity levels.
  • During dry spells, spray the leaves of susceptible plants regularly to maintain humidity levels.
  • Water early in the morning, to allow the foliage to dry out during the day; avoid overwatering, however, so as not to encourage fungal infections.
  • In fall, remove all plant litter, to reduce the overwintering population, which is the first source of infestation in the spring; do not compost infested plant litter.
  • As a last resort, use a low-impact pesticide with insecticidal soap or mineral oil as the active ingredient. Read the product label carefully and follow the manufacturer’s recommendations.

Leafcutting bees
(Megachile spp.)

Signs and symptoms

  • Circular or semicircular holes on leaf edges

Control methods

Gardeners often worry when they find rounded holes on the leaf edges of roses or other plants. Such minor damage by leafcutting bees does not merit control measures, however, as these bees are considered beneficial. They are valuable pollinators in the wild and on farms and in urban environments. They also serve as food for other animals, both predators (birds) and parasites (beetles and wasps).

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