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Tomato: fight against diseases and insects

How to prevent diseases like powdery mildew, downy mildew and canker on tomato plants and keep insects from destroying your crop.

Tomato plants are also susceptible to attacks from insects and disease. The heat and humidity tomatoes need to thrive also produce the conditions that lead to fungi and attract insect pests. Careful monitoring is essential. Below are the most common diseases that affect tomato plants.

NameDescriptionPreventionTreatment
1 - Blight

Caused by a fungus.

Symptoms: black spots on lower leaves and stems.
  • Aeration is essential so leaves remain dry. Water the soil, not the plants, to prevent splashing.
  • Stake the plant to keep leaves and fruit from touching the ground.
  • Apply small doses of copper-based fungicide (Bordeaux mixture).
  • There is no treatment.
  • Remove and destroy affected parts of the plant. Sterilize pruning shears after each use.
2 - Powdery Mildew

Caused by a fungus.

Symptoms: white down under the leaves.
  • Aeration is essential so leaves remain dry. Since wet leaves allow fungi to spread, water the soil, not the plants, to prevent splashing.
  • Stake the plant to keep leaves and fruit from touching the ground. Apply small doses of copper-based fungicide (Bordeaux mixture).
  • There is no treatment.
  • Remove and destroy affected parts of the plant. Sterilize pruning shears after each use.
3 - Aphids

Tiny insects that feed on plant juices.

Symptoms: leaves curl inwards.		Companion planting with marigolds and lavender that naturally repel aphids, or plants that act as 'traps,'attracting aphids away from tomatoes, such as nasturtiums.Spray a soapy solution (1 teaspoon of dishwashing liquid for 1 litre water) or a natural plant purin (a blend of nettles, ferns, or rhubarb leaves).

NameDescriptionPreventionTreatment
4 - Blossum-End Rot

Caused by a calcium imbalance.

Symptoms: water-soaked spot at the blossom end of the tomato. Spots turn brown and leathery.
  • Maintain consistent levels of moisture in the soil. Water regularly, particularly during heat waves.
  • Put straw around your plants to keep the soil moist and minimize evaporation.
5 - Whiteflies

Tiny insects (cousins of aphids) with white wings cluster under leaves, sucking out sap.

Symptoms: leaves become sticky with honeydew secreted by whiteflies, then turn yellow and die.

Introduce natural predators, such as ladybugs and dragonflies. Create a habitat favourable to hummingbirds, another predator.

Companion planting with nasturtiums, which whiteflies detest.

Try an anti-whitefly treatment or apply an insecticide every 3 days for 2 to 3 weeks.

Note: some commercial insecticides will not only kill whiteflies, but beneficial insects as well.
6 - Tomato Leafminer

The larval stage of a moth that hollows out tunnels on leaves, stems and tomatoes.

Symptoms: puncture marks, exit holes and rot.		 Destroy infested plants.

Applying a fungicide helps protect tomato plants from most disease and insect pests, but it must be used in moderation.

Do not throw plants or parts of plants infested with fungi or harmful insects into the compost; you will ruin the compost for future use. Instead, destroy plants by burning them.