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Basil plants: disease and insect control

Identify and treat the most common diseases and insects like downy Mildew, fungal disease and thrips

Although basil can repel certain insects, it is vulnerable to other insects and disease. The heat and humidity it requires to thrive also favours the growth of fungi and attracts harmful insects. Careful monitoring is a must. Below are the main afflictions affecting basil plants.





Impatiens Necrotic Spot Virus (INSV)

Virus spread by western flower thrips.

Symptoms: a patchwork of brown necrotic spots; stems are blotched, spotted or mottled.

Early detection is important.

Destroy infested plants. You can try a soap-based anti-aphids treatment or apply an insecticide.

Fusarium wilt

Fungal disease spread by contaminated seeds or soil.

Symptoms: brown streaks on the stems;

stunted and wilted plants with yellowish leaves.

Aeration is important to promote dry leaves. Do not water the leaves. Apply a bio-fungicide as prevention.

No treatment.

Remove and destroy infected plants.

Basil Sunburn

Caused by too much sun and too little water on plants not fully 'hardened.'


Yellow leaves with some scorching.

Ensure plants are hardened off before transplanting.

Place affected plants in partial shade or bring the shade to them, particularly during a heat wave.

Pinch off yellow or scorched leaves.

Downy Mildew

Fungal disease,

likely spread through contaminated seeds, infected leaves at the market, or wind-swept spores. Green-leafed basil plants are particularly susceptible.

Symptoms: initially, leaf yellowing, followed by leaf browning. On the underside of leaves, a grey-purple fuzzy material will develop, which looks like a fine layer of dirt.

Aeration is extremely important to promote dry leaves.

Take care to water the base of plants and not the leaves.

Plant in a sunny location with sufficient space between plants.

Apply a bio-fungicide as prevention.

Treat with a fungicide.

Remove and destroy affected plants.

Gray Mold

Fungal disease.

Symptoms: brown to gray fungal growth on leaves and stems. Poor air conditions, high humidity and cold temperatures are favourable to this disease. Easily spread by handling, splashing water or touching diseased and near-by healthy plants.

Aeration is very important to facilitate dry leaves.

Avoid watering the leaves.

Apply bio-fungicide as prevention.

No treatment.

Remove and destroy affected plants.


Tiny insects active at night. They damage plants by sucking out juices and by transmitting viruses (such as INSV).

Symptoms: irregular patches of silvery tissue on leaves. Leaves may turn pale and splotchy before dying.

Reduce the places where thrips might breed: clean up plant debris on the ground and clear around the garden (dry mulch will not attract thrips but green mulch will).

Introduce beneficial insects; earwigs, ladybugs, and pirate bugs are efficient predators.

Destroy affected plants.

Try a soap-based anti-aphids treatment or treat with a natural contact insecticide.

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.