Gadenin日本語Get the appScale insects and the sooty mold that follows
Scale are the commonest serious citrus pest and the easiest to miss, because they do not look like insects. They appear as small firm bumps, brown, gray or white, stuck along the stems, on the leaf midribs and on the fruit itself. They do not move. Most people find them by noticing something else first: a black sooty film over the leaves, or a sticky varnish on whatever is parked under the tree.
That black film is sooty mold. It is a harmless fungus growing on the sugary honeydew that scale, aphids and mealybugs excrete. The mold itself does not attack the tree, but a thick coating blocks light from the leaf and slows the whole plant down. Sooty mold is a symptom with an arrow pointing at it: find the sap-sucker producing the honeydew and the mold stops appearing.
Light infestations rub off with a cloth or a gloved thumb, and it is worth doing this every time you notice a few. For a heavier attack, a horticultural oil spray applied thoroughly to both leaf surfaces and along the stems will smother them, repeated after two weeks. Spray in the cool of the morning or evening rather than in strong sun, and never onto a tree in flower.
Watch for ants running up the trunk. Ants farm scale and aphids for honeydew and actively drive off the ladybirds and parasitic wasps that would otherwise control them. A sticky band around the trunk breaks the arrangement and often lets natural predators clear an infestation without any spraying at all.
| Problem | What you see | First action |
|---|---|---|
| Scale | Firm immobile bumps on stems, leaves and fruit | Rub off by hand; horticultural oil for heavy attacks |
| Sooty mold | Black film on leaves; sticky surfaces below | Find and treat the sap-sucker producing the honeydew |
| Aphids | Clusters on soft new shoots; curled tips | Strong jet of water; soap spray if it persists |
| Leaf miner | Silvery winding tunnels in new leaves | Leave the leaf; do not feed nitrogen in late season |
| Ants on the trunk | A steady two-way column | Sticky band on the trunk to let predators work |
| Split fruit | Rind cracked, usually at the base | Even watering; the split fruit itself cannot be saved |
| Yellow leaves | Even paling or green-veined patterning | Check soil moisture six inches (15 cm) down first |
Leaf miner and aphids in the soft flush
Citrus leaf miner leaves silvery winding tunnels inside the new leaves, and badly attacked leaves curl and distort. It looks far worse than it is. The miner only attacks soft young flush and cannot touch a leaf that has hardened off, so an established tree loses a fraction of one flush and carries on. Do not pick off the damaged leaves; a tunneled leaf still photosynthesises, and stripping them costs the tree more than the pest did.
The practical control is timing rather than spraying. Late nitrogen feeding pushes a soft autumn flush at exactly the season when miner numbers are highest, so stop feeding nitrogen well before the end of the growing season. That single change usually reduces miner damage more than any treatment, and it also avoids soft growth going into the cold, which matters especially on a pomelo.
Aphids cluster on the same soft new shoots, cause curled distorted tips and produce the same honeydew and sooty mold as scale. A firm jet of water knocks them off and most gardens have enough ladybirds and hoverflies to finish the job if you leave them to it. If an infestation persists on a young tree, an insecticidal soap works, applied to the undersides of leaves where the aphids actually sit.
Do not spray anything, including oil and soap, onto a citrus in open flower. You will kill the bees doing the pollinating and lose more crop than the pest would have taken. Wait until petal fall, and choose the cool of the day rather than midday, when oil on foliage in strong sun scorches leaves.
Split fruit, dropped fruit and cracked rind
Split fruit is a watering fault, not a disease. It happens when a tree that has been dry gets a sudden soaking during the final swell: the flesh takes up water faster than the rind can stretch, and the rind gives way, usually at the base. A pomelo's thick rind resists this better than a thin-skinned orange, but a big enough swing will crack it, and a split fruit cannot be repaired.
The answer is even watering rather than heavier watering. Mulch a wide ring under the canopy, keeping it clear of the trunk, and water long and slowly on a schedule set by the soil rather than by the weather forecast. Push a screwdriver in six inches (15 cm) at the canopy edge; if it comes out damp, wait.
Fruit drop has its own separate causes and most of it is normal. The wave right after flowering is unpollinated flowers letting go. The wave weeks later, at pea to walnut size, is the tree matching its load to its resources, and on a pomelo that adjustment is severe because the finished fruit is so heavy. Panic-feeding through a natural drop makes it worse, not better.
The drop that should worry you is a late one, when fruit that had been swelling well comes off in quantity. That points at drought, at root damage or standing water, or at a cold snap. Dig a small hole at the canopy edge and look at the soil before doing anything else; more late drops trace back to water at the roots than to any pest.
Yellow leaves: read the pattern before you feed
Yellow leaves on a citrus are a symptom with at least four different causes, and reaching for fertilizer without identifying which one you have is how a sick tree gets sicker. Before anything else, check the soil six inches (15 cm) down at the canopy edge. If it is wet, you have a drainage problem and feeding a waterlogged root system will only add salt to the injury.
Read the pattern on the leaf. Evenly pale leaves, oldest first, are nitrogen hunger and respond to a light balanced feed. Yellow between the veins with the veins staying distinctly green is a trace element shortage, usually iron, zinc or manganese, and it shows on the newest leaves first. A single leaf yellowing and dropping here and there is normal ageing and needs nothing at all.
Trace element yellowing often has nothing to do with the soil being short of the element. In alkaline soil the iron is present but chemically locked away where roots cannot reach it. Check the soil pH before adding more iron; a foliar feed sprayed onto the new flush will green a tree up in a fortnight when a soil application does nothing.
Keep a short note of what you saw and when: the date, which leaves were affected, whether the soil was wet or dry, and what you did about it. Yellowing is slow and easy to misremember, and next season's diagnosis is far quicker when you can see that the same tree yellowed at the same point in the year and turned out to be sitting wet.