Lime Tree Problems: Pests, Leaf Damage and What to Do About Them

Look under the leaves, weekly

Nearly every pest problem on a lime is easy while it is small and miserable once it is established, and the difference is a weekly look at the underside of the new growth. That is where scale, mites and aphids all start, and where you will never see them if you only look at the tree from a distance.

Take one minute per week. Turn over half a dozen of the newest leaves, run a thumb along a young shoot, and look at the tips. Sticky leaves, black sooty film, ants marching up the trunk, or silvered stippling are all secondary signs that something has been feeding for a while already.

Scale and the ants that farm it

Scale insects look like small brown or white bumps stuck to stems and leaf undersides, and they do not move, so people mistake them for part of the plant. They feed on sap and excrete honeydew, which coats the leaves below and grows a black sooty mold. The mold itself is harmless but it blocks light and tells you the infestation has been running for weeks.

Wipe or scrape the visible scale off with a cloth or an old toothbrush, then treat with a horticultural oil at label rate, covering leaf undersides thoroughly. Repeat after two weeks to catch the crawlers that hatched after the first pass. Do not spray oil in full sun or above about 90F (32C), which scorches the leaves.

Deal with the ants at the same time. Ants protect scale from its natural predators because they want the honeydew, and while ants are patrolling the trunk no ladybird or parasitic wasp will get near. A sticky band on the trunk, or moving the pot away from anything the ants can bridge across from, breaks the arrangement.

Leafminer and why feeding makes it worse

Citrus leafminer lays into the softest new growth and the larva tunnels between the leaf surfaces, leaving silvery winding trails and leaving the leaf curled and distorted. It looks alarming and on a mature tree it is largely cosmetic. On a young tree that is trying to build a canopy it genuinely sets growth back.

There is no useful spray, because the larva is inside the leaf where contact treatments cannot reach it. The practical control is to give it less to attack. Stop pushing soft growth with high nitrogen feed during the warm months when the moth is active, remove the worst affected leaves and bin them, and let the tree harden its growth. Hardened leaves are not attacked.

Do not strip every marked leaf. A damaged leaf still photosynthesises, and a young tree defoliated in the name of tidiness is worse off than one carrying scarred leaves.

Timing your flushes matters more than any product here. If you can hold back feed so the main flush of soft growth comes outside the moth's most active spell, the damage drops sharply without spraying anything.

Cold damage, root rot and the problems that are not pests

A lime is the least cold hardy of the common citrus, and cold injury is regularly misdiagnosed as disease. Leaves that go limp and then brown and drop a few days after a cold night, with no insects present and no smell from the roots, is frost, not fungus. Do not cut the damaged wood immediately. Wait until warm weather returns and growth restarts, then cut back to where the wood is green inside, because the extent of the kill is not visible for weeks.

Root rot from an airless wet mix produces yellowing that starts on the oldest leaves and works upward, with the mix staying wet for days and often a sour smell at the drainage hole. Stop watering, tip the tree out, and look. Healthy citrus roots are firm and pale; rotted ones are dark, soft and pull apart between the fingers. Cut the dead roots off, repot into a much grittier mix in the same size pot or smaller, and reduce the canopy to match the roots you have left.

Yellowing between green veins on the newest leaves, with older leaves still dark, is nutrient deficiency rather than illness. It is common on limes in hard water areas. Use a chelated citrus feed carrying iron, magnesium and zinc, and flush the pot with clear water to clear accumulated salts.

The hazard that is not to the tree

The most likely injury around a lime is to you. Juice or peel oil on skin, followed by sunlight, causes phytophotodermatitis: a genuine chemical burn where furocoumarins in the fruit react with ultraviolet light. It appears as redness and blistering hours later, then leaves dark staining that can take months to fade. It is not an allergy and does not require sensitive skin.

Wash hands and forearms with soap and water after picking, juicing or pruning before you go back into the sun, and keep any splashed skin covered for the rest of the day. Add the thorns to your risk list too: lime thorns are stiff enough to go through a thin glove, so use gauntlets when reaching into the canopy.

Records that pay for themselves

Log the date you first see each problem each year, along with what the weather had been doing. Pests are seasonal and remarkably punctual, and after two years your own dates beat any general advice, because they let you inspect and treat a fortnight before the population builds.

Also record what you applied, at what strength, and whether it worked. The most common mistake in the second season is repeating a treatment that did nothing the first time, simply because nobody wrote down that it failed.