Shikuwasa Pests and Problems: Scale, Mites, Cold Damage and Split Fruit

Scale insects, and the sooty mold that gives them away

Scale is the commonest serious pest on home citrus and shikuwasa is more prone than most, because its naturally dense twiggy canopy gives the insects exactly the still, shaded, humid interior they want. They look like small brown or gray bumps stuck along the undersides of leaves and on the young stems, and because they do not move, people take them for part of the bark for months at a time.

You usually notice the honeydew before the insect itself. The leaves and whatever is under the tree turn sticky, then black sooty mold grows on that film until the canopy looks dirty. By that point the population is well established. Ants running steadily up and down the trunk are the other early sign, because they farm scale for the honeydew and defend it from predators.

Deal with it mechanically first, which is realistic on a tree this size. Wipe the stems and leaf undersides with a cloth and soapy water or scrape the bumps off with a thumbnail, then repeat 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. Then fix the cause: prune the interior open so air moves through it, because a congested shikuwasa will be reinfested every year no matter how diligently you wipe.

Leafminer and mites on the new growth

Citrus leafminer shows as silvery winding tunnels inside the leaf, usually with the leaf edge curled and puckered around them. It attacks only soft new growth, so it appears on each flush and is worst on any late-summer flush. On an established tree it looks far more alarming than it is: there are plenty of older leaves and the tree grows through it. On a young tree with only one flush of leaves to its name, it is worth acting on.

The practical control is to reduce how much soft new growth is available late in the season. Stop feeding at the end of summer, avoid heavy summer pruning that forces a flush, and pick off the worst affected leaves. Sprays are largely useless because the larva is inside the leaf where nothing reaches it.

Spider mites give a different signature: no tunnels, but fine pale stippling across the leaf surface that makes the whole canopy look dull and grayish, sometimes with faint webbing in the leaf axils. They thrive in warm dry air, which is exactly the condition of the indoor space a tender citrus spends winter in. Check leaf undersides with a hand lens every couple of weeks on an overwintering tree, raise the humidity, and spray the undersides with plain water regularly. An unnoticed mite population on a tree that is indoors from autumn to spring can strip it before you put it back out.

Cold damage, and how to read it in spring

Because this citrus has so little frost tolerance, a proportion of what looks like disease on a shikuwasa is simply cold. Learning to recognize it saves you from treating the wrong thing.

1. Sudden leaf drop a few days after a cold night The classic signature. The tree sheds leaves that looked fine at the time, often most of them. Do not feed and do not water heavily in response. Move it somewhere frost free, keep the compost only just moist, and wait.
2. Brown, crisp shoot tips on the outermost growth Frost takes the youngest and most exposed wood first. Do not cut it in winter; the dead tissue insulates what is behind it and you cannot yet tell how far the damage runs.
3. Wait for the spring flush before cutting anything Shoots that look brown and finished in late winter frequently break bud. Let the tree tell you what is alive, then cut back to green tissue once the new growth shows you the boundary.
4. Fruit damaged while the wood looks fine Hanging fruit is more frost-sensitive than the branches, so a light frost can ruin a holding crop without marking the tree. Pick anything affected immediately, because frosted fruit rots quickly.
5. Whole tree collapses after a wet cold spell Cold plus waterlogged roots kills far more container citrus than cold air alone. Check whether the pot has been standing in water, because that is usually the real cause.

Yellow leaves: work out which yellow you have

Yellowing is the symptom reported most often and it has several causes that call for opposite responses. Reading where on the tree, and where on the individual leaf, the yellow appears tells you which one you are looking at.

Old leaves yellowing evenly and dropping from the bottom is nitrogen shortage, or the natural shedding of the oldest leaves in spring; if it spreads upward, feed with a citrus fertilizer. New leaves yellow with green veins mean iron or manganese is locked up, usually because the compost has gone alkaline from hard water or is too cold and wet; correct the watering first, then apply a chelated trace element feed. Broad yellow blotching on mature leaves in late summer is often magnesium, drawn down by a heavy crop. A whole tree dull yellow over wet compost with leaves dropping is root rot: stop watering, check the drainage holes, and if the rootball smells sour, repot into fresh free-draining mix and cut away the black roots.

Do not respond to yellow leaves by feeding harder as a default. On a container citrus the single most common cause is waterlogged roots that cannot take up what is already there, and adding more fertilizer to a sour rootball makes it worse rather than better.

Fruit drop, split fruit and rubbed rind

A tree shedding a proportion of its small fruit a few weeks after flowering is behaving exactly as it should. Shikuwasa sets far more than it could ever 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 already beginning to swell, is a supply problem and in almost every case the answer is water. Either the tree dried out hard, or its roots have been sitting wet and are starting to fail. Check the rootball rather than the leaves. A pot so full of roots that it dries out within a few hours is the other frequent cause, and the fix there is repotting rather than watering more often.

Split fruit close to harvest is always about swings, and thin rind makes this tree more vulnerable than most. A dry spell followed by heavy watering or rain makes the flesh expand faster than the rind can stretch, and it gives way. It cannot be repaired: pick split fruit at once and fix the watering rhythm. Rubbed and scarred rind is the related problem, caused by fruit pressed against fruit or a twig in wind, and it is prevented at thinning time and by shelter, not at harvest.

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, including from deep inside the canopy where you would rather not reach, and look along 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. Check 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 consistent point in the season in any given garden, and after two years of notes you will start looking a fortnight before the date rather than finding the problem by accident. Note which side of the tree it started on too: a recurring hotspot usually points at an airflow or shelter problem you can fix permanently.