Gadenin日本語Get the appWinter under cover is where the problems start
A citron outdoors in summer, in moving air, with predators around it, rarely has a serious pest problem. The same tree in a warm, still, dry room for four or five months is a different plant. Sap-sucking insects breed without interruption, natural enemies are absent, and dry air suits mites perfectly. Nearly every infestation people find in spring started quietly in midwinter.
That gives you a simple discipline. Inspect the tree properly, with a torch, once a fortnight from the day it goes under cover, and pay attention to the undersides of leaves and the junctions where a leaf stalk meets a stem. Two minutes with a torch now beats an afternoon with a sponge by the end of the indoor spell.
The four you will actually meet
Nearly all citron problems reduce to a short list. Learning to name them fast matters because the responses are different and the wrong response wastes weeks.
| Problem | First sign | Where to look | What to do |
|---|---|---|---|
| Soft and armoured scale | Sticky leaves, then black sooty mold | Undersides of leaves, along midribs, on young stems | Scrape off with a thumbnail; wipe stems with dilute horticultural soap |
| Mealybug | White cottony fluff in tight angles | Leaf axils, under the rim of the pot, on the crown of the roots | Dab each colony with a cotton bud dipped in alcohol; repeat weekly |
| Red spider mite | Fine pale stippling, dull leaves, faint webbing | Undersides of the oldest leaves, on hot dry indoor plants | Raise humidity, hose the foliage, spray soap on the undersides |
| Citrus leafminer | Silver winding trails, curled distorted new leaves | The newest flush only, outdoors in warm weather | Nothing; cut nothing off. It disfigures leaves but does not reduce the crop |
Sooty mold is not a disease of the tree. It is a fungus growing on the sugary honeydew that scale and mealybug excrete. Kill the insect and the black film weathers off, or wipes off with a damp cloth. Spraying fungicide at it achieves nothing.
Leaf drop, and how to read it
Citron drops leaves for several unrelated reasons and the tree looks the same in each case, so diagnose by what changed rather than by the leaves. A tree that has just come indoors or just gone back out drops leaves for two to three weeks as it adjusts to a step change in light; that is normal, needs no action, and is best prevented by moving it in stages over a week or two.
Leaves that yellow evenly and drop from the bottom of the tree upward, on wet mix, are drowning roots. Stop watering, check the drainage holes, and if the mix smells sour repot into a grittier mix. Leaves that go dull, curl inward and drop with the mix bone dry are simple drought, and the tree will also shed fruitlets. Leaves that yellow between green veins on the newest growth are a micronutrient lockout caused by cold or waterlogged roots, not by a lack of fertilizer.
A whole limb that browns and dies back while the rest of the tree is fine is usually cold damage from a night that got away from you, or a split at the crotch from carrying heavy fruit. Cut back to live green tissue and treat it as a lesson about propping.
Blemishes on the rind, which matter more here than on other citrus
On a fruit grown for its peel, marks on the skin are the whole problem. Three causes account for most of them, and all three are preventable.
- Thorn scars A curved brown line or a puncture, always on the side of the fruit facing into the tree. It is caused by a neighboring thorny branch moving in wind. Find the offending branch and shorten or remove it as soon as you see a fruitlet sitting against it.
- Rub marks from bending branches A flattened, dull patch where the fruit has rested on the pot rim, a stake or the soil. Prop the branch before the fruit reaches half size; once the mark is there it will not heal.
- Wind rub and salt burn A general roughening or bronzing over an exposed shoulder of the fruit. Move the pot into shelter and out of any drying wind. Trees standing on a windy balcony rarely produce clean rind.
- Cold pitting Small sunken brown pits over the whole surface, appearing after a cold snap or after storage below about 45F (7C). There is no cure. Process the fruit at once rather than trying to keep it.
- Sunburn on exposed fruit A pale, papery patch on the side facing the strongest afternoon sun, usually after the tree was moved abruptly from shade into full sun. Reintroduce the tree to outdoor light over a week, not in one afternoon.
When to cut your losses, and what to record
Two situations justify giving up on part of the tree rather than treating it. The first is a heavy armoured scale infestation on old wood that keeps returning after three rounds of treatment: cut that limb out entirely and remove it from the area. The second is a rootball that smells sour and shows black, mushy roots, which means the rot has passed the point where drying out will save it. Take healthy shoot tips as cuttings if you want to keep the plant, and start again.
Keep a short pest log alongside your flowering and picking dates. Record what you found, the date, and where on the tree. The value shows up in the second year, because these problems recur on a schedule tied to when you move the tree under cover and back out, and a log tells you the week to start checking rather than leaving you to discover it after the damage is done.