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Nearly every alocasia problem can be sorted into one of four causes, and you can tell them apart in about a minute. Turn two or three leaves over and look at the undersides against the light, then put a finger into the compost. Those two checks separate pests from rot from position.
| What you see | Most likely cause | Where to look next |
|---|---|---|
| Fine pale stippling, faint webbing at the leaf join | Spider mites | Undersides of leaves and stalk joints |
| Sticky leaves or a sticky floor below | Scale or mealybugs | Stalk joints and the underside of the midrib |
| Several leaves yellowing and drooping at once | Corm or root rot | Unpot it and squeeze the corm |
| Dry brown crisp margins only | Dry air or a heater draft | Where the plant is standing |
| Small flies over the compost | Fungus gnats | The top inch of compost, which is too wet |
Wear gloves for anything that involves cutting. The sap contains calcium oxalate and burns broken skin, and it is genuinely dangerous in an eye.
Spider mites are the pest that matters
An alocasia is a magnet for spider mites, and a warm dry heated room in winter is exactly the condition they multiply in. They are barely visible, about the size of a full stop, and they live on the leaf undersides where you do not look. By the time you notice fine webbing where the leaf meets the stalk, the population is already large.
The first sign is a fine pale speckling across the leaf, as though it has been dusted with something. Hold the leaf up to a window and it looks pin pricked with light. Left alone the leaf goes dull, then bronze, then dries and drops, and the plant loses leaves faster than it can replace them.
- Isolate the plant first Move it away from every other houseplant the moment you find mites. They spread across touching leaves and on your hands, and treating one plant in the middle of a group is wasted effort.
- Wash the leaves properly Take it to a shower or sink and rinse both surfaces of every leaf with tepid water, aiming upward at the undersides. Support each leaf with your hand. This alone removes the large majority of the population.
- Treat, then treat again Spray the undersides thoroughly with insecticidal soap or a horticultural oil until they are dripping. Repeat every five to seven days for at least three applications. A single spray never works, because eggs hatch after you have sprayed.
- Raise the humidity to stop them returning Mites struggle in humid air. A pebble tray, a humidifier, or grouping plants together makes the plant much less attractive to them and is the only lasting prevention in a heated room.
- Remove the worst leaves A leaf that is more than half bronzed will not recover and is a reservoir of mites. Cut it off at the base of the stalk and bin it rather than composting it.
Rot is what actually kills the plant
Every other problem on this page damages leaves. Rot destroys the corm, and once the corm has gone there is no recovery. It follows from cold compost that stays wet, and it is far more common in autumn and winter than in summer for exactly that reason.
The warning signs are several leaves yellowing and drooping within a week rather than one at a time, a base of the stem that feels soft when you press it, and a sour smell from the compost. Do not wait to see whether it improves. Unpot it the same day.
- Unpot and wash the corm Take it out, wash the compost off under a tap and look at the corm and roots directly. Firm and pale means sound. Brown, soft or hollow means rot.
- Cut back to clean tissue With a clean sharp blade, cut every soft area away until the cut surface is uniformly firm and pale. Rot spreads through the corm, so leaving any is leaving all of it.
- Dry the cuts before repotting Dust the cut faces with cinnamon or a fungicide and leave the corm in the air for several hours until the surfaces are dry to the touch. Repot into fresh dry open mix and do not water for two or three days.
- Cut the top back to match If you removed a third of the corm, remove roughly a third of the leaves too. What is left cannot support the top it had before.
Even a corm reduced to a small firm fragment will often sprout again if it is kept warm and barely damp. Do not throw away anything that is still hard.
Losing every leaf in winter is usually not death
This is the single most misread event in growing an alocasia. As the days shorten and the room cools, the plant retires its leaves one after another until there is nothing above the compost at all. The pot looks like a pot of soil. Almost every one of these plants is then thrown away, and almost every one of them is alive.
It is dormancy, and it is what the plant does in the cold half of the year. The corm is intact underground, waiting for warmth. Tip the pot out and squeeze the corm before you do anything else. If it is firm, put it back and read the dormancy advice in the seasonal calendar rather than binning it.
| Test | Dormant, keep it | Dead, discard it |
|---|---|---|
| Squeeze the corm | Hard, no give | Soft, hollow or mushy |
| Cut surface if you nick it | Pale and moist inside | Brown or black throughout |
| Smell of the compost | Earthy or neutral | Sour or foul |
| Weight of the corm | Solid for its size | Light and papery |
Keep the pot somewhere above 59F (15C), barely damp, and check it monthly. New leaves usually appear as the room warms in spring, six to twelve weeks after the last leaf fell.
The lesser pests
None of these will kill a plant quickly, but all of them will drag it down over a season if you leave them, and all three are easier to clear early.
| Pest | How to recognize it | What to do |
|---|---|---|
| Mealybugs | White cottony tufts in the stalk joints | Dab each one with alcohol on a cotton bud, repeat weekly |
| Scale | Small brown lumps on stalks that do not brush off | Scrape off by hand, then treat with horticultural oil |
| Thrips | Silvery streaks and tiny black specks on the leaf | Wash the leaves and spray, isolate the plant, repeat three times |
| Fungus gnats | Small flies rising from the compost when disturbed | Let the top inch dry between waterings, use yellow sticky traps |
Fungus gnats are not really a pest problem, they are a watering problem. They breed in permanently wet surface compost, so they are usually a warning that the corm is at risk too.
Keeping a record of what went wrong
Pest problems on an alocasia are seasonal and repetitive, which makes them worth writing down. Note the date you first saw mites, what the room conditions were, which treatment you used and how many applications it took before the stippling stopped spreading. Note the date the last leaf fell in autumn and the date the first new one appeared in spring.
After one year that record turns guesswork into a schedule. If mites appeared six weeks after the heating went on last winter, you know to raise the humidity and start checking the undersides in the fifth week this time, which is far easier than clearing an established infestation.