Azalea Pests and Problems: Lace Bug, Chlorosis and Root Rot

Azalea lace bug: stippling on top, black specks below

Lace bug is the pest that disfigures more azaleas than anything else, and it is routinely diagnosed too late. The insects live on the underside of the leaf and feed by puncturing cells and draining them. From above, the leaf loses color in a fine even speckle, pale dots scattered across the blade, until a badly hit leaf looks silvery or bleached gray rather than green.

Turn the leaf over and the identification is certain. You will find shiny black tar-like specks of excrement stuck to the surface, often with cast skins beside them, and on a live infestation small flat insects with clear lacy wings held over their backs. The black specks stay stuck to the leaf long after the insects have gone, which is why finding them alone does not prove the population is still active.

Damage does not repair. A stippled leaf stays stippled until it falls, so judge whether you are winning by looking at the newest leaves rather than the old ones. Plants in full sun and plants that are drought-stressed are hit far harder than the same plant in dappled shade with a damp root run, and moving a badly infested plant into shade is a genuine long-term control.

1. Check the undersides in late spring Turn over leaves on the sunniest side of the plant as the first flush of growth hardens. Catching the first generation is worth more than any amount of spraying later in the season.
2. Wash the undersides with a strong jet A forceful spray of water aimed upward at the leaf undersides dislodges nymphs, which cannot easily climb back. Repeat every few days through the early part of an outbreak.
3. Treat with a contact spray, underneath only Insecticidal soap or horticultural oil kills on contact but only where it lands, so the whole leaf underside must be wetted. A spray applied to the top of the leaves does nothing at all.
4. Repeat on the generation interval There are several overlapping generations in a season. One application clears the insects present that day and nothing else, so plan on repeating every seven to ten days until new leaves come through clean.
5. Reduce the stress that favors them Shade over the hottest hours, a deep mulch and consistent watering all cut lace bug numbers measurably. A plant in the wrong spot will be reinfested every year no matter what you spray.

Telling the leaf problems apart

Three different things make azalea leaves look wrong, and they are treated in opposite ways, so it is worth being certain before acting. Insect damage is patterned: fine speckling from lace bug, notched leaf margins from vine weevil adults, distorted curled tips from aphids. Nutrient problems are smooth and even: yellow blade with sharp green veins on the newest leaves is iron shortage from alkaline soil.

Root problems announce themselves in the whole plant rather than in individual leaves. A shrub that wilts on a day when the soil is damp, whose leaves go dull olive and hang down, and which does not recover overnight, has a root or crown problem, most often rot from waterlogging or from being planted too deep.

Two disfiguring but harmless things also turn up. Thick, pale, swollen leaves are azalea gall; pick them off before they turn white and dusty. Sooty black film is a mold growing on honeydew dropped by aphids or scale, so controlling the insect clears it.

Vine weevil and root rot: the two that kill

Everything above is disfiguring. Two problems actually kill plants, and both work out of sight at the roots. Vine weevil grubs are creamy white, comma-shaped, with a brown head and no legs, and they eat the fine roots and then strip the bark from the stem base. Containers are far more at risk than open ground. The first above-ground sign is often a plant that suddenly collapses and lifts out of its pot with almost no roots attached.

Adult weevils give you earlier warning by chewing distinctive angular notches out of the leaf edges at night. Notched leaves mean adults are present and laying, so that is the moment to act on the compost rather than the foliage. Biological control with parasitic nematodes, watered into moist compost while it is warm, is the reliable treatment.

Root rot comes from water sitting around roots that need air. Heavy soil, a hole dug like a sump and filled with compost, a pot standing in a saucer of water, or a plant set an inch too deep will all do it. The plant wilts, does not recover with watering, and the roots when you look are dark, soft and smell sour instead of pale and fibrous. Prevention is the whole treatment: plant high, drain the site, and never bury the crown.

Record the date you first see stippling each year, and on which plants. Lace bug returns at a similar point each season, and after two years of notes you will know when to start checking undersides rather than waiting until the damage is obvious across the whole shrub.

When to treat, and when to give up on a plant

Not every problem is worth a spray. Stippled leaves are ugly but the plant survives them, and a shrub with a good root run in dappled shade will usually shrug off a light infestation without help. Reserve treatment for plants where the newest growth is being hit, where the damage covers most of the canopy, or where the same plant has been badly attacked two years running.

When you do treat, treat the cause alongside the insect. A plant baking in afternoon sun on dry soil will be reinfested every single year, and no spray program changes that. Moving it in autumn, when azaleas transplant easily because of their compact root mat, solves the problem permanently in a way that repeated spraying never does.

Give up on a plant when the roots have gone. A shrub that lifts out of the ground with almost no root system, or whose roots are dark, soft and sour-smelling, will not recover, and leaving it in place risks nothing but your time. Take the opportunity to find out what killed it before replanting, because waterlogging and buried crowns will do the same thing to the replacement.