Gadenin日本語Get the appScale and mealybug in the cane sheaths
Every dendrobium cane is wrapped in papery sheaths that dry to a pale brown and stay in place for years, and those sheaths are the best hiding place a sap-sucking insect could ask for. Scale and mealybug breed under them in the dark, undisturbed, for months before anything shows on the visible surface. By the time the leaves are sticky and blackened with sooty mold, the population under the sheaths is large.
Look for them deliberately rather than waiting. Peel back a sheath on an older cane and inspect with good light. Scale shows as small flat oval brown discs pressed hard against the cane, or clusters of white slightly fluffy specks which are the males and the crawlers. Mealybug shows as white cottony tufts, especially at the joints and at the base of new growths, and sometimes down in the mix on the roots.
Treatment starts physically. Strip every loose dry sheath from the canes and bin it, which removes the insects and their shelter at a stroke and makes future inspection far easier. Scrub the exposed canes and both sides of the leaves with a soft brush and soapy water, then treat with horticultural oil or insecticidal soap, covering every joint and crevice. Repeat every seven to ten days for at least three rounds, because eggs sheltering under dead females survive the first application.
Isolate an infested plant while you work through the cycle. Crawlers walk between touching leaves, and a bench of orchids pushed together is one continuous plant as far as they are concerned.
The rest of the pest list
The other pests sort themselves by season. Summer outdoors brings the chewers and the sap suckers that arrive from the garden; winter indoors brings the ones that thrive in dry, warm, still air. Knowing which season you are in tells you what to look for first, and nearly everything on this list is trivial at a dozen individuals and a serious job at a thousand.
| Pest | First sign | Where to look | What to do |
|---|---|---|---|
| Red spider mite | Fine silver stippling and a dull gray cast to the leaves | Leaf undersides, fine webbing in the leaf joints | Raise humidity, hose the foliage, spray oil or soap three times at five day intervals |
| Scale | Sticky leaves and sooty black mold | Under the dry cane sheaths and along leaf midribs | Strip the sheaths, scrub, then oil weekly for three rounds |
| Mealybug | White cottony tufts at the joints | Cane joints, base of new growths, roots in the mix | Dab with alcohol on a cotton bud, repeat weekly, check roots at the next repot |
| Aphids | Distorted buds and a sticky flower stalk | Tips of developing flower stalks and unopened buds | Wash off with water or a soap spray before the buds open |
| Slugs and snails | Buds and new growth tips chewed off overnight | Under the pot rim, in the drainage holes, in the mix | Stand pots on feet, check by torchlight after dark, use a barrier or bait |
| Thrips | Silver streaks and brown flecking on the petals | Inside open flowers, in tight buds | Remove damaged flowers, spray while in bud, keep weeds down outdoors |
Slugs do the most damage per night of anything here, because they eat the soft tip of a new growth or a swelling bud and remove a whole season in one visit. From the moment buds appear, check the pot rim and drainage holes by torchlight every few nights.
Rot at the base, and spots on the leaves
Base rot is the one that kills a dendrobium, and it almost always starts in a mix that has decomposed into a fine wet material that no longer drains. The roots go hollow and brown, the rot travels up into the base of the canes, and a cane that was firm last month goes soft and blackens from the bottom. Unpot immediately, cut away every soft cane and dead root with a sterile blade, and repot the sound remainder into fresh coarse bark in a small pot.
Bacterial soft rot is the fast version and can appear on a healthy plant. It shows as a translucent water-soaked patch on a leaf or a new growth, turns brown or black within a day or two, and smells foul. It spreads through water splash and through water standing in the joints between leaf and cane. Cut it out at once with a margin of clean tissue, keep the plant dry and airy for a week, and stop wetting the foliage in the evening.
Fungal leaf spots are the slow ones: small round brown to black spots, often with a yellow halo, appearing on older leaves and enlarging over weeks. They are unsightly rather than dangerous. Remove the worst leaves, improve air movement, and accept that existing marks will not heal.
On flowers, small brown pinprick spots spreading in cool, damp, still air are botrytis. That is why you never mist a dendrobium in flower. Move the plant somewhere with gentle air movement, keep water off the blooms, and remove spotted flowers before the spores build up.
Problems that look like pests and are not
A great deal of worry about dendrobiums turns out to be normal behavior or an environment problem. Leaves yellowing and falling from a jointed cane in autumn is not disease; on a deciduous plant it is the start of the rest period and it is exactly what should happen. Light lengthwise wrinkling of bare canes through a dry winter is also normal. Deep shriveling that leaves a cane soft and empty is not, and means either too warm a rest or dead roots.
Buds that yellow at the stalk and drop before opening are bud blast, and the cause is nearly always the air rather than an insect. Warm dry heat from a radiator, a sudden move to a different room, a pot allowed to go bone dry while the buds are filling, or ethylene from ripening fruit or an unflued gas heater will all do it. Settle a budded plant in its final position, keep the temperature steady, and stop moving it.
Plantlets appearing along the canes instead of flowers are not a disorder either. They mean a deciduous plant was watered and kept warm through the winter, so the dormant nodes went the vegetative route. Nothing is wrong with the plant; the correction is next winter's cold dry rest.
| Symptom | Usual cause | What to do |
|---|---|---|
| Leaves yellow and fall in autumn | Normal dormancy on a deciduous type | Stop watering, move somewhere cold and bright |
| Bare canes lightly wrinkled in winter | Normal water loss during the rest | Nothing, resume water when buds break |
| Canes deeply shriveled and soft | Rest too warm, or roots dead | Check the roots, shorten the rest next year |
| Buds yellow and drop before opening | Dry heat, a move, drought or ethylene | Steady temperature, keep it still, away from fruit |
| Plantlets along the canes, no flowers | Watered and kept warm through the rest | Grow the plantlets on, fix next winter |
| Brown dry leaf tips | Fertilizer salts or a spell of drought | Flush the pot with plain water, correct the interval |
Catching problems early, and keeping a record
Two habits prevent most of this. Inspect on a fixed rhythm rather than when something looks wrong: once a week in summer, once a fortnight in winter, always turning a leaf over and always peeling back one sheath. And quarantine every new plant for a month before it joins the others, because most infestations arrive rather than appear.
The two moves between outdoors and indoors are the riskiest moments of the year. Going out in spring exposes clean foliage to garden aphids and slugs. Coming in for autumn takes a plant carrying a few outdoor mites into a dry room where they multiply unchecked. Hose the foliage thoroughly top and underside, check the sheaths, and look at the pot base before the plant comes back in.
Write down what you found, where on the plant, and what you sprayed, with dates. Dendrobium pest problems are seasonal and repeat, so a note saying mite appeared six weeks after the plant came indoors last autumn tells you exactly when to start looking this year. The same record shows whether three rounds of spray actually cleared an infestation or only quietened it for a month, which is the difference between a solved problem and one that comes back in spring.