Pieris Problems: Lace Bug, Leaf Scorch, Chlorosis and Toxicity

Lace bug, the one pest that really matters

Lace bug is the pest that disfigures pieris more than anything else, and it is easy to miss until the damage is done. The insects feed on the underside of the leaf, puncturing cells and sucking out the contents. What you see from above is a fine pale stippling, as though someone had dusted the leaf with silver-gray. It starts as scattered flecks and spreads until whole leaves look bleached and metallic.

Turn a leaf over and the diagnosis is immediate. The undersides carry small dark tar-like specks of excrement, along with the insects themselves: flat, roughly an eighth of an inch (3 mm) long, with clear intricately veined wings that give them their name, plus dark spiny nymphs. Nothing else on this plant leaves that combination of silver above and black specks below.

Position drives the problem. Lace bug builds up in hot, dry, sunny spots and is far less of an issue on a plant in cool dappled shade with steady moisture. A pieris in full afternoon sun against a warm wall is the classic case. There are usually two or more generations in a season, so numbers climb through summer and the damage accumulates rather than fading.

Damaged leaves never green up again. Pieris holds its leaves for two or three years, so a bad infestation stays visible for a long time after the insects are dealt with. That is the argument for catching it early: check the undersides of leaves from late spring onward, particularly on the sunniest side of the plant, rather than waiting until the whole shrub looks gray.

Any spray only works if it lands on the underside of the leaf, because that is where the insects live. Treating the top surface achieves nothing at all, whatever the product. Aim upward from beneath the canopy and expect to use more liquid than you think.

Scorched new growth: frost and wind

The bright red or bronze spring flush is soft, unwaxed tissue and it burns easily. A late frost blackens it within hours, usually worst on the outer tips where it is most exposed. Cold drying wind produces a different pattern: brown crisping that begins at the leaf margins and works inward, concentrated on the windward side of the plant.

Both look alarming and neither kills an established shrub. The plant either pushes a second, smaller flush later in spring or simply carries on with the previous year's leaves, which it keeps for two or three years anyway. Do not cut the damaged shoots off in a panic; wait until you can see where the new growth is coming from, then trim back to it during the post-flowering window.

The cure is position rather than treatment. Shelter from the prevailing wind and an aspect out of early morning sun are what prevent it. Frozen tissue thawing slowly in shade very often recovers unmarked, while the same tissue struck by low dawn sun thaws so fast that the cell walls rupture. If your plant is scorched in the same place every year, the site is telling you something that no amount of feeding will change.

For a small shrub, row cover over the plant on a clear still night during the flush, removed the next morning, is worth the trouble. For a large one, plant or build a windbreak on the exposed side and accept the occasional bad spring.

Yellow leaves, root problems and dieback

A yellow leaf with sharply green veins, showing on the newest growth first, is chlorosis from soil that is not acid enough. Test the pH four inches (10 cm) down. Above 6.0 and the iron in the soil is locked into forms the roots cannot use. Chelated iron watered onto the root zone greens the plant within two to three weeks as a rescue; elemental sulfur applied while the soil is warm, plus an acidic bark mulch renewed yearly, is the longer-term correction.

Even yellowing across the whole leaf, veins included, beginning on the oldest leaves low down, is a different fault. It means nitrogen shortage or, more often, roots sitting in wet ground. Pieris will not tolerate waterlogging. Check that the plant is not sitting in a hollow and that it was not planted too deep, which is the commonest slow killer of this shrub.

Sudden wilting of a whole branch, with the leaves hanging on rather than dropping, points to root or stem rot in soil that stays wet, or occasionally to dieback entering through a pruning wound. Cut the affected branch back into clean healthy wood and look at the cut surface: dark staining in the wood means the trouble is traveling in the stem. Improve drainage before replanting anything in the same spot.

Scale insects sometimes appear as small brown limpet-like bumps on stems and leaf undersides, with sticky honeydew and black sooty mold below them. They are a minor problem on a healthy plant and usually indicate a shrub already stressed by drought or poor soil.

The whole plant is poisonous

Every part of a pieris is toxic if eaten: leaves, stems, flowers and nectar. It contains grayanotoxins, the same class of compound found in rhododendrons, which act on nerve and muscle cells. Ingestion causes salivation, vomiting, abdominal pain, weakness and, in serious cases, cardiac and neurological effects. Livestock, particularly sheep and goats, are poisoned by browsing it, and dogs and cats can be affected by chewing leaves or twigs.

Treat it with the same practical caution you would give any toxic garden shrub. Do not plant it where grazing animals can reach it, keep prunings out of paddocks and off compost destined for animal bedding, and do not burn large quantities in an enclosed space. Wash your hands after handling and pruning. If a person or animal has eaten any part of the plant, seek medical or veterinary advice promptly and take a piece of the shrub with you for identification.

The toxin carries into honey made from the nectar, which is worth knowing if you keep bees near a large planting, though a single garden shrub is very unlikely to produce enough for it to matter.

None of this is a reason to avoid growing it. It is a reason to know what it is, to site it sensibly, and to tell anyone who looks after the garden or the animals what the plant is.

Keep a note of what you treated, when, and what the weather was doing. A record of the first date you saw lace bug stippling each year, and which side of the plant it appeared on, is the single most useful thing you can write down, because that pest returns to the same warm dry aspect every season.