Gadenin日本語Get the appStart by ruling out the non-pests
Most colocasia trouble is not a pest at all. The leaves are huge, thin and short-lived, and a plant growing fast will always have one leaf on the way out at the bottom while it pushes a new one at the top. Before you reach for a spray, check the three cultural causes, because treating a wind tear with insecticide wastes your time and the plant carries on being damaged.
The quick triage is this: torn or split blades are wind, blackened collapsed leaves after a cold night are frost or chill, crisp brown margins on an otherwise healthy plant are dry air or a hard wilt it has already recovered from, and a single old leaf yellowing evenly from the tip while everything above it looks well is simply that leaf being retired.
| Symptom | Most likely cause | Not a pest because |
|---|---|---|
| Clean straight splits from the margin inward | Wind | The tears follow the veins and appear after a windy day |
| Whole leaf blackened and limp overnight | Frost or cold below 50F (10C) | It happens to every leaf at once |
| Dry crisp edge all round an otherwise green leaf | Dry air or a past hard wilt | New leaves come out clean if conditions improve |
| One bottom leaf yellowing evenly | Normal leaf turnover | New growth continues unaffected |
| Pale, washed-out green across all leaves | Hunger, water leaching the feed away | Color returns within two weeks of feeding |
Spider mites: the one that actually kills
Spider mites are far and away the most damaging pest on this plant, especially indoors, under cover, or outdoors in a hot dry spell. They live on the undersides of the leaves, are barely visible without a lens, and multiply astonishingly fast in warm dry air. Because colocasia leaf area is so large, an infestation that would take months to matter on a small plant can strip a colocasia in three weeks.
The first sign is a fine pale stippling on the upper surface, like someone dusted the leaf with pinpricks of light. Turn the leaf over and you will find the mites and, once it is advanced, fine webbing where the stalk meets the blade. By the time you see obvious webbing you are already late.
- Check the undersides weekly Tilt a leaf up to the light once a week and look along the main veins. Thirty seconds a week catches this pest at a stage where a rinse solves it.
- Hose the plant down hard Take it outside or into a shower and blast every leaf underside with water. Mites do badly in wet conditions and a thorough wash physically removes most of the population.
- Treat with soap or oil, repeatedly Spray insecticidal soap or a light horticultural oil, covering the undersides completely until they drip. Repeat every five to seven days for at least three rounds, because eggs are not killed and hatch in between.
- Raise humidity and lower heat Mites thrive in hot dry air. Moving the plant out of a heated dry room, grouping it with other plants or running a humidifier makes the environment far less favorable to them.
- Sacrifice the worst leaves A leaf that is more than half stippled is finished and is only a mite nursery. Cut it off at the base and dispose of it away from your other plants.
Check any plant coming indoors in autumn before it joins the rest of your collection. Mites brought in on a colocasia will spread to everything in a warm dry room over winter.
The other pests, and what to do about them
Nothing else on this list is normally fatal, but all of them are easier to deal with early. The large soft leaves and thick stalks are attractive to chewing pests outdoors and to sap-sucking ones indoors, and the constantly wet compost creates conditions of its own.
| Pest | What you see | Treatment |
|---|---|---|
| Aphids | Clusters on new stalks, sticky honeydew, sooty mold | Wash off with water, repeat, then soap spray if they return |
| Slugs and snails | Ragged holes eaten from the leaf edge, slime trails | Clear hiding places, check the pot rim at night, use barriers |
| Fungus gnats | Small flies rising from the wet compost surface | Empty the tray for a week, let the top dry, use sticky traps |
| Mealybugs | White fluff where stalk meets corm and in leaf folds | Dab with alcohol on a cotton bud, then treat with soap |
| Caterpillars | Large sections chewed out, dark droppings on lower leaves | Pick them off by hand, they are large and easy to find |
| Thrips | Silvery streaks with tiny black specks, distorted new leaves | Blue sticky traps, soap or oil spray, remove worst leaves |
Rot: the disease that really matters
This plant will tolerate standing water for months in summer and rot in a fortnight in autumn, and the difference is temperature and whether it is in active leaf. Cold plus wet plus no growth equals a soft corm. Warm plus wet plus leaves equals the best growth you will ever get from it. Almost every corm lost over winter is lost because the summer watering regime carried on too long.
The symptoms are a plant that wilts and stays wilted after watering, stalks that go soft and translucent at the base and pull away with a squeak, a sour smell from the compost, and a corm that gives when you press it. Act immediately, because rot spreads through a corm faster than you expect. Unpot it, wash the corm, cut every soft part back to hard white or cream tissue, let the cuts dry for a day or two, and repot into barely damp fresh compost in a small pot, kept warm and almost dry until new growth shows.
| Condition | Rot risk | Why |
|---|---|---|
| Warm, in full leaf, standing in water | Very low | Roots are working and the corm is warm |
| Cool autumn, still standing in water | Very high | Growth has stopped but the compost stays saturated |
| Dormant corm in damp compost in a cold room | Very high | Nothing is drinking and the corm stays cold and wet |
| Dormant corm stored dry and frost free | Very low | The intended winter condition |
| Freshly cut corm potted into wet mix | High | The open wound is the entry point |
A corm that has lost half its tissue to rot can still make a plant if what remains is hard and has a bud. Do not throw it away because it looks butchered. Do throw it away if there is no firm tissue left at all, because it will not recover and it can infect the compost.
Leaf spots, and keeping a record
In wet, still, humid conditions you may see small water-soaked spots that enlarge into tan or dark patches with a yellow halo, sometimes running together until the leaf collapses. It spreads by splashing water and by leaves touching. Improve the air movement, stop wetting the foliage in the evening, cut off badly affected leaves and take them away rather than leaving them on the compost surface.
The single most useful habit is a short dated note whenever you see something. Write down the date you first found mites, the date of each spray, the date the last frost blackened the leaves, and the date any corm went soft. Within two seasons that record shows you your own recurring pattern: perhaps mites always arrive within a fortnight of the plant coming indoors, or rot always follows a specific week of cold nights. That is far more actionable than a general warning, because it tells you which week of your year to act in.
Every part of this plant is toxic if eaten, and the sap irritates skin and burns the mouth and eyes because of needle-shaped crystals in the tissue. Wear gloves when cutting rotten corms or removing leaves, and keep children and pets away from cut material.