Gadenin日本語Get the appScale insects, and the sooty mold that announces them
Scale is the commonest serious pest on any home citrus and sudachi is no exception, particularly on a container tree that has spent winter in a still, warm indoor space. The insects look like small brown or gray bumps stuck along the undersides of leaves and on the young stems, and they do not move, which is why people take them for part of the bark for months.
You usually notice the honeydew before the insect. Leaves and the surface below the tree go sticky, then black sooty mold grows on the sticky film and the whole canopy looks dirty. By that point the population is well established. Ants running up and down the trunk are the other early tell, because they farm the scale and will actively defend it.
Deal with it mechanically first on a small tree, which sudachi conveniently is. Wipe the stems and leaf undersides with a cloth and soapy water, or scrape the bumps off with a thumbnail, then follow up weekly until you find none. Horticultural oil applied thoroughly, covering the undersides, smothers the crawler stage. Do not use oil in strong sun or on a dry, stressed tree. Prune out any heavily encrusted branch rather than trying to clean it, and open the canopy so air moves through it, because still air inside a congested tree is the condition scale needs.
| Problem | First sign | Where to look | First response |
|---|---|---|---|
| Scale insects | Sticky leaves, ants on the trunk | Leaf undersides, young stems | Wipe off, then horticultural oil weekly |
| Sooty mold | Black film on leaves | Anywhere below a scale colony | Treat the scale; the mold then washes off |
| Citrus leafminer | Silvery winding tunnels | New soft leaves only | Remove worst leaves, stop late feeding |
| Spider mites | Fine stippling, dull leaves | Leaf undersides, warm dry spots | Raise humidity, spray undersides with water |
| Aphids | Curled shoot tips | Tips of the new flush | Squash or wash off, watch for ants |
| Root rot | Yellow leaves, wet compost | Base of the pot, drainage holes | Stop watering, check drainage, repot if needed |
Leafminer and mites: damage to new growth
Citrus leafminer shows as silvery, winding tunnels inside the leaf, usually with the leaf edge curled and distorted. It attacks only soft new growth, so it appears on each new flush and is worst on the late-summer flush. On an established tree it looks alarming and matters far less than it appears: the tree carries plenty of older leaves and outgrows the damage. On a young tree with only one flush of leaves, it is worth acting on.
The practical control is to reduce the amount of soft new growth available late in the season. Stop feeding by the end of summer, avoid heavy summer pruning that forces a flush, and pick off the worst affected leaves. Sprays are largely ineffective because the larva is inside the leaf.
Spider mites are a different signature: no tunnels, but a fine pale stippling across the leaf that makes the whole canopy look dull and gray, sometimes with faint webbing in the leaf axils. They thrive in warm dry air, which is precisely the condition of an indoor winter space. Check the undersides of leaves with a lens on a tree that has come in for winter, raise the humidity, and spray the leaf undersides with plain water regularly. A mite population on an overwintering tree can strip it by spring if it is left alone.
Yellow leaves: work out which yellow it is
Yellowing is the symptom people report most and it has several different causes that need opposite responses. Reading where on the tree and where on the leaf the yellow appears tells you which one you have.
- Old leaves, evenly yellow, dropping from the bottom Nitrogen shortage, or simply the natural shedding of the oldest leaves in spring as the new flush comes. If it is spreading upward through the tree, feed with a citrus fertilizer.
- New leaves yellow with green veins Iron or manganese is present but locked up, usually because the compost has gone alkaline from hard water or is too wet and cold. Correct the watering first, then apply a chelated trace element feed.
- Broad yellow blotches on mature leaves in late summer Often magnesium, and common on a heavily cropping tree because the fruit draws it down. A magnesium supplement corrects it, and thinning the crop earlier reduces it next year.
- Whole tree dull yellow, compost wet, leaves dropping Root rot from waterlogging. Stop watering, check that the drainage holes are clear and the pot is not standing in water, and if the rootball smells sour, repot into fresh free-draining mix and cut away black roots.
- Sudden yellowing and drop after a cold spell Cold damage to the roots. Do not feed and do not water heavily in response; move the tree somewhere frost free and let it recover, and expect a new flush in spring.
Fruit drop and split fruit
A tree shedding a proportion of its small fruit a few weeks after flowering is behaving normally. Sudachi sets far more than it can carry and corrects itself. Do not react to that first wave at all, and do not thin until it has finished.
Drop that continues past that point, or that takes fruit which has already begun to swell, is a supply problem. In almost every case it is water: a tree that dried out hard, or one whose roots have been sitting wet and have started to fail. Check the rootball rather than the leaves. A pot that has filled with roots to the point that it dries out within hours is the other frequent cause, and the answer is repotting rather than more water.
Split fruit near harvest is always about swings. A dry spell followed by heavy watering or heavy rain makes the flesh expand faster than the rind can stretch, and the rind gives way. It cannot be repaired, so pick split fruit immediately and use it, and address the watering rhythm rather than the fruit.
Any fruit with a puncture, a split or a soft brown patch should come off the tree and out of the store immediately. One rotting fruit in a shallow box will take the ones touching it within a few days, and the whole batch shortly after.
Look on a schedule and write down the first date
Once a week through the growing season, turn over a dozen leaves from different parts of the tree and look at the young stems. That single habit catches scale and mites while they are still a handful of individuals, which is the only stage at which wiping them off actually works. Add a check of the trunk for ants at the same time.
Record the date you first see each problem each year, and what you did about it. Pests arrive at a remarkably consistent point in the season in any given garden, and after two years of notes you will know to start looking a fortnight before the date rather than discovering the problem again by accident. Note which tree or which side of the tree it started on too, because a recurring hotspot usually points at a shelter or airflow problem you can fix permanently.