Gadenin日本語Get the appRule out the non-pest causes first
Before you reach for a spray, check the two things that account for most reported problems. Turn the plant out of its pot: dark, soft, smelly roots mean the compost has been staying wet and no insecticide will help. Then ask when the plant last moved rooms, came in from outside, or arrived from a shop - leaf drop in the weeks after any of those is a light adjustment, not a disease.
| Symptom | Likely cause | First move |
|---|---|---|
| Whole leaves yellow, soft, drop over weeks | Compost too wet | Stop watering, check drainage, tip out and inspect roots |
| Sudden general leaf drop after a move | Light adjustment | Brightest window, water sparingly, wait 6 to 10 weeks |
| Fine pale speckling, faint webbing at shoot tips | Spider mites | Rinse the whole plant, then treat weekly |
| Sticky leaves, sooty black film, brown bumps on stems | Scale insects | Scrape off, then treat every 10 to 14 days |
| Crisp brown leaflet tips only | Dry air or dry compost | Move off the heating vent, water more evenly |
| Pale washed-out new leaves, long thin shoots | Too little light | Move to bright glass or add a grow light |
Scale insects: the commonest indoor pest
Scale shows up as small brown or fawn bumps sitting along the stems and the undersides of the leaf midribs. They do not move, which is why they get missed for months. Run a thumbnail down a suspect stem; a scale pops off and leaves a wet mark, a bud does not. The giveaway before you spot the insects is stickiness - honeydew dripping onto leaves and the floor below, then going black with sooty mold.
- Isolate the plant Move it away from every other houseplant before you do anything else. Scale crawlers travel to neighbors on touching leaves and on your hands.
- Scrape off what you can see Work over every stem and leaf underside with a cotton bud dipped in rubbing alcohol, or your thumbnail. This removes the adults, which are the ones producing more.
- Treat the whole plant Spray to run-off with horticultural soap or a light horticultural oil, covering stem undersides and every leaf underside. Repeat every 10 to 14 days for at least three rounds to catch new crawlers.
- Wash off the sooty mold Once the insects are gone, wipe the black film off with a damp cloth. It is not a disease - it grows on the honeydew and blocks light until you remove it.
Spider mites and dry air
Spider mites explode on plants sitting in warm dry air, which describes most heated rooms in winter. The first sign is a fine pale stippling across the leaf surface, as though someone has dusted it lightly; hold a leaf up to the light to see it. Fine webbing at the shoot tips and in leaf joints means the infestation is well along.
Start by putting the whole plant in a shower and rinsing every leaf surface, particularly the undersides - a hard rinse removes most of the population on its own. Follow with a soap or oil spray weekly for three or four weeks, since the eggs hatch in a rolling wave. Moving the plant off the direct blast of a heating vent removes the conditions they need.
Never spray oil or soap on a plant standing in direct sun or on one that is bone dry at the root. Water it first, treat it out of the sun, and let it dry in shade.
Other trouble worth naming
Mealybugs appear as white cottony blobs in the leaf joints and are treated exactly like scale - alcohol on a cotton bud, then repeated soap or oil sprays. Powdery mildew, a white dust on leaf surfaces, follows still, crowded air; thin the canopy and get some air movement rather than reaching for a fungicide first.
Outdoors the tree is largely trouble-free, which is part of why it grows so fast. The exception is dieback of shoot tips after a cold snap: cut back to live green wood once the weather has settled and it will regrow from below the damage.
Keeping a problem log
When you find a pest, note the date, which pest, and which part of the plant it started on. Scale that appears every winter on the same plant is usually re-infesting from a neighbor you never isolated, and only a written record makes that pattern visible.
Record what you sprayed and on which dates, too. The single biggest reason scale treatments fail is stopping after one application; a list of dates makes the ten-to-fourteen-day cycle something you follow rather than something you intend to follow.