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Pests and diseases

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Opuntia lindheimeri var. lindheimeri
Photo: Jardin botanique de Montréal (Gilles Murray)

In general, succulent plants have few enemies except for a few insect pests and certain diseases caused by poor growing conditions. The most common insect pests are mealy bugs, scale insects, spider mites and root mealy bugs. The most common diseases are root rot, soft rot, dry rot, powdery mildew and leaf spot. The best way to avoid such problems is to inspect your plants regularly, act quickly if you do spot a problem and keep a close eye on them afterwards.

Pests

Mealy bugs

Signs and symptoms

  • Woolly white mass on leaf tips, in cracks or in leaf axils;
  • Discolouration and loss of plant vigour.

Solutions

  • Swab the plant with insecticidal soap or a 5% alcohol solution.

Scale insects

Signs and symptoms

  • Small, hard, brown moundlike insects on stems and leaves;
  • Discolouration and loss of plant vigour.

Solutions

  • Swab the plant with insecticidal soap or a 5% alcohol solution.

Root mealy bugs

Signs and symptoms

  • Woolly whitish mass on the root collar, roots or inside edge of the pot;
  • Discolouration and loss of plant vigour.

Solutions

  • Clean the roots with tepid water or a 5% alcohol solution;
  • Prune damaged roots and repot in well-drained soil.

Spider mites

Signs and symptoms

  • Yellowish mottled spots on leaves and at growing tips;
  • Webbing;
  • Pests barely visible to the naked eye (0.25 mm).

Solutions

  • Raise relative humidity and lower temperature;
  • Clean affected parts with a strong spray of water or insecticidal soap.

Diseases

Root rot

Signs and symptoms

  • Plant stops growing, leaves and stems turn yellow or discoloured;
  • Roots turn soft and brown.

Solutions

  • Prune infected roots, repot in well-drained soil and water less frequently.

Soft rot

Signs and symptoms

  • Translucent dark or blackish spots, usually appearing at the base of the plant and moving upward;
  • Plant turns limp and weak;
  • Sometimes gives off an unpleasant odour.

Solutions

  • Water less frequently, lower relative humidity, increase air circulation;
  • Cut off and root sound plant parts;
  • Repot using a soil mixture with well-decomposed organic material.

Dry rot

Signs and symptoms

  • Small dark spots on stems;
  • Body of the plant turns pale and soft;
  • Plant stops growing.

Solutions

  • Repot using a fresh soil mixture;
  • Place in sunlight and water less frequently.

Powdery mildew

Signs and symptoms

  • Downy whitish substance on leaves and stems, turning darker and causing affected parts to decline.

Solutions

  • Increase air circulation and lower relative humidity;
  • Treat affected parts with a baking-soda solution.

Leaf spot

Signs and symptoms

  • More or less circular, brownish or reddish spots with a darker ring;
  • Centre of the spot often dried out.

Solutions

  • Lower relative humidity and raise temperature;
  • Remove affected parts;
  • Sprinkle any affected parts that cannot be removed with charcoal powder.

5% alcohol solution: Mix 7 ml of rubbing alcohol (70%) with 93 ml of water.

Insecticidal soap: Use a commercial insecticidal soap (follow the instructions on the label) or 5 ml of liquid dish soap diluted in 1 litre of water. Note that using liquid dish soap instead of insecticidal soap specially formulated for plants increases the risk of phytotoxicity.

Baking soda solution: Dissolve 5 g (1 tsp.) of baking soda in 1 litre of water and add 5 ml of insecticidal soap.

CAUTION. Avoid using horticultural oil, because some species are particularly sensitive to it. It is also a good idea to use insecticidal soap sparingly, because it can burn plant tissues. No matter what product you use, start by applying it to a small area on the plant and wait a few days to see whether the plant is sensitive to it before treating the entire plant. The risk of phytotoxicity is greater when plants are stressed (too dry, for instance) or in high-temperature conditions.

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