Houseplant Handbook. David Squire

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      Remove dead leaves.

      Houseplant Problems

      Few plants can escape from pests and diseases or cultural problems (often known as physiological disorders), such as leaves and flower buds falling off. It is much easier to prevent attack by pests and diseases than to eliminate them from badly affected plants. Here are some prevention tips.

      The chemicals permitted by legislation vary from country to country and state to state, so if prevention fails, check with your local nursery or garden center about the most suitable insecticide to use for your specific problem.

      Pests

      Aphids: also known as greenfly, aphis, and aphides, they are the main pests of houseplants. These small, usually green, sap-sucking insects infest flowers, shoot tips, and soft leaves, sucking sap and causing mottling and distortion. They also excrete honeydew, which attracts ants and encourages the presence of a fungal disease called sooty mold.

      Control: spray with a suitable insecticide as soon as aphids are seen. Repeat every 10–14 days throughout summer.

      Cyclamen mites: these infest a wide range of plants, including cyclamen, pelargoniums, saintpaulias (African Violets) and impatiens (Busy Lizzies). They are minute, eight-legged, spider-like creatures that cluster on the undersides of leaves. They suck sap, causing leaves to crinkle and darken. Flowering is shortened and buds become distorted and may fall off.

      Control: remove and burn seriously infested leaves and flowers. Spray with an acaricide. Destroy badly infected plants to prevent the mites from spreading.

      Mealy bugs: white, waxy, woodlice-like creatures that live in groups and especially infest ferns, palms, azaleas, and hippeastrums. They suck sap, causing distortion, loss of vigor, and yellowing of leaves. They excrete honeydew, which encourages the presence of ants and sooty mold.

      Control: wipe off light infestations with cotton swabs dipped into methylated spirits (rubbing alcohol). Spray with a suitable insecticide or burn seriously infected plants.

      Red spider mites: also known as greenhouse red spider mites, they are spider-like, minute, usually red, and have eight legs. They suck leaves, causing mottling and, if the infestation is severe, webs. These are unsightly, reduce air circulation around the plant, and make eradication difficult.

      Control: mist-spray daily with clean water (but avoid spraying flowers or soft leaves) and spray with a suitable acaricide.

      Scale insects: swollen, waxy-brown discs, usually static, under which female scale insects produce their young.

      Control: young scale insects can be wiped away with a cotton swab dipped in methylated spirits (rubbing alcohol). Established colonies are difficult to eradicate, and badly contaminated plants are best burned.

      Thrips: these tiny, dark brown, fly-like insects jump from one plant to another. They pierce leaves and flowers, sucking sap and causing silvery mottling and streaking. Undersides of leaves develop small globules of a red liquid that eventually turns black.

      Control: spray several times with a suitable insecticide or burn seriously infected plants. Keep the compost moist, because dryness intensifies the damage caused by these insects.

      Vine weevils: serious pests in the adult form, when young, and as larvae. Adults are beetle-like, with a short snout. They chew all parts of plants. The larvae—fat, legless, and creamy white, with brown heads—inhabit compost and chew roots.

      Control: immediately water the compost with suitable insecticide and spray the leaves. Destroy badly infected plants.

      Whitefly: small, moth-like white insects that flutter from one plant to another when disturbed. They have a mealy or powder-like covering and are mostly found on the undersides of leaves, sucking sap, causing distortion, and excreting honeydew, which encourages the presence of ants and sooty mold.

      Control: eradication is not easy; several sprayings at five-day intervals with a suitable insecticide are essential.

      Diseases

      Black leg: a disease mainly of cuttings, especially of pelargoniums; the bases become black and soft.

      Control: this is encouraged by cold, wet, airless compost, so ensure compost is well aerated when taking cuttings. If cuttings are infected, they are best removed and destroyed; for rare or unusual varieties, remove cuttings, cut off the blackened area, and reinsert the base in fresh compost.

      Botrytis: also known as gray mold, it attacks soft parts of plants, such as flowers, shoots, and young leaves, covering them with a gray, furry mold.

      Control: cut off and destroy infected parts. Spray infected plants with a suitable fungicide. Remove dead flowers to discourage spreading. Avoid damp, still air and excessive water.

      Damping off: attacks seedlings soon after they germinate, causing them to turn black and collapse.

      Control: sow seeds thinly in well-drained compost and place in a warm and well-aerated location. Avoid excessively watering the compost.

      Powdery mildew: fungal disease that produces a white, powdery coating. It usually appears in spring and summer, infecting leaves, flowers, and stems.

      Control: remove badly infected leaves, stems, and flowers; increase ventilation and keep the atmosphere dry.

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