Bay Tree Problems: Bay Sucker, Scale Insects and Sooty Mold

Bay sucker: curled, thickened, yellow leaf margins

If the edges of your bay leaves have rolled under, gone thick and yellow, and later turned brown and brittle, that is bay sucker. It is a small sap-feeding insect that overwinters as an adult in the bark and cracks around the plant, and moves onto the new spring growth as soon as it opens. The nymphs feed under the rolled leaf edge, protected by a white waxy fluff, and the leaf grows deformed around them.

There are usually two generations a year, one on the spring flush and one on late summer growth, so a plant can look clean in early summer and be disfigured again by autumn. The damage is entirely cosmetic in the sense that the plant survives it and goes on growing, but the affected leaves never recover their shape and are not worth picking for the kitchen.

Control is mechanical rather than chemical. Pick off and bin every curled leaf as soon as you see it, which removes the nymphs before they mature, and clear fallen leaves from around the base where the adults shelter. On a small clipped plant this is realistic and effective. On a large tree, accept some damage and simply cut out the worst shoots during the annual trim.

1. Check the new growth weekly in spring The first curling appears on the softest leaf tips within a week or two of the spring flush opening. Weekly checks at that point catch the first generation before it spreads across the plant.
2. Remove and bin curled leaves Pick each affected leaf off at the stalk and put it in the bin, not the compost pile. The nymphs are inside the roll and go on developing in a warm heap.
3. Clear the base in autumn Rake up fallen leaves and any debris around the stem where the adults overwinter. Doing this in autumn cuts next spring's first generation noticeably.
4. Do not spray the whole plant The nymphs are protected inside the rolled edge and under wax, so contact sprays reach very few of them while killing the predators that do reach them.

Scale insects and the black film that follows

Turn a few leaves over. If you find flat brown or yellowish oval discs, up to about a fifth of an inch (5 mm) long, sitting motionless along the midrib and on the stems, that is scale. They look like part of the plant, which is why they are usually found long after they arrive.

Scale feeds on sap and excretes honeydew, a sugary liquid that drips onto the leaves below. Sooty mold then grows on the honeydew, and that is the black sticky film that people usually notice first. The mold is not itself attacking the plant, but it blocks light and makes the leaves useless in the kitchen. If your bay is sticky underneath and black on top, look for scale rather than for a disease.

Scale is far worse on plants brought indoors for winter, where warmth and no predators let it multiply all season. A bay that stands outdoors year round rarely gets more than a light population.

How to clear scale by hand

On a plant of a size you can reach round, hand removal genuinely works and needs no chemicals. It has to be repeated, because the eggs under the old scales hatch over several weeks and the mobile young crawlers are too small to see.

1. Wipe every leaf underside Use a cloth or a soft brush dipped in warm water with a drop of dish soap. Work along the midrib and into the leaf axils, where they cluster. Expect this to take twenty minutes on a clipped standard.
2. Do the stems and the branch forks Most people clean the leaves and miss the woody stems, where the largest, most productive scales sit. Run a thumbnail along every stem.
3. Repeat after two weeks, then twice more Three or four rounds a fortnight apart cover the whole hatching period. One thorough clean followed by nothing brings the population straight back.
4. Wash off the sooty mold last Once the insects are gone, the mold has nothing to feed on. Wipe it off with plain soapy water so the leaves can work; it will not come back.
5. Open the plant up Thin some inner growth so air moves through. Scale, sooty mold and leaf spot all build up fastest in a dense, still, shaded interior.

If you use a proprietary insecticide instead, apply it when the crawlers are active in early summer rather than against the mature scales, whose waxy shell repels most sprays. Do not pick leaves for cooking from a treated plant until the interval on the label has passed.

Damage that is not a pest at all

Brown, crisp leaf edges on an otherwise green leaf, appearing over winter or after a cold windy spell, are scorch, not disease. Frozen roots cannot replace the water the leaves lose to the wind, so the tissue furthest from the veins dies. The plant recovers and grows out of it, though the marked leaves stay marked.

Sudden yellowing and leaf fall in a pot in spring is almost always cold wet compost rather than an insect or a deficiency. Check the drainage before you reach for anything else. Whole branches dying back on an old plant, with no insects present, usually means the roots have been in standing water for a long period or the plant has been in the same exhausted compost for many years.

Knowing when to give up on a plant

Bay is very hard to kill outright and worth persisting with. A plant cut to the ground by a severe winter frequently reshoots from the base in late spring, so do not dig out a bare bay until midsummer has passed with no sign of life. Scratch the bark near the base with a thumbnail: green underneath means it is alive.

The case for replacing rather than nursing is a plant whose stems are ringed with scale, whose roots fill a pot as a solid mat, and which has produced no new growth for two seasons. At that point a hard renovation cut plus a full repot is the only thing worth trying, and if that produces nothing by the end of the following summer, start again with a new plant.

Record the date you first notice each problem every year. Bay sucker and scale both appear at a consistent point in your own garden's season, and after two years of notes you will know to start checking a fortnight before the damage would otherwise become visible. That single habit is worth more than any treatment.