Galls and gall mites

What you are looking at

A gall is plant tissue, grown by the plant. Something small has settled on a bud, a leaf or a stem and released a chemical that hijacks the plant’s own growth signals, and the plant obediently builds a structure around it. The animal or fungus inside gets shelter and food; the plant spends a little energy and carries on.

That is why galls are so precise. A red pimple standing on a lime leaf, a woody sphere on an oak twig and a pineapple at the base of a spruce shoot are all repeated to the same pattern, because the pattern is written into the instructions the causer sends. Damage from chewing or sucking is ragged and irregular; galls are architecture.

The causers are a mixed lot. Microscopic eriophyid mites, tiny gall wasps, gall midges, aphids and adelgids, and in a few cases fungi and bacteria. What they have in common is the method rather than any relationship to one another.

Most of them do not matter

This is the useful thing to know, and it runs against the instinct that anything abnormal on a plant needs treating.

Nail galls on lime leaves, spangle galls on the underside of oak leaves, oak marble galls, robin’s pincushion on wild roses, adelgid pineapples on spruce, maple bead galls and the blisters that pear leaf blister mite raises all take a trivial amount from the plant. A lime tree can carry tens of thousands of nail galls and put on a normal year’s growth. The leaf works around them.

Removing them costs more than leaving them. Picking off galled leaves takes away functioning foliage in the middle of the growing season, and spraying a tree for a mite living inside plant tissue would fail even if there were a product for it.

Knopper gall is the one people ask about most. It deforms English oak acorns into sticky ridged lumps and in a heavy year takes most of the crop. It arrived in Britain in the 1960s, it needs Turkey oak nearby to complete its cycle, and it does nothing at all to the tree. The loss falls on the jays and squirrels that would have eaten the acorns.

The exception, and why it is different

Blackcurrant big bud mite earns its own paragraph because it breaks the pattern.

Through winter the mites live inside blackcurrant buds and feed there, and the bud responds by swelling into a fat round ball two or three times the size of a healthy one. Those buds never open. On its own that is a nuisance, costing a proportion of next year’s fruiting wood.

What makes it serious is that the mite carries blackcurrant reversion virus. Once a bush is infected the virus is in it permanently, and there is no treatment. The bush gradually stops setting fruit, and the tell is in the flowers, which turn bright pink and lose their fine hairs where a healthy blackcurrant flower is dull grey and downy. The leaves change too, with fewer main veins and a narrower outline.

So the advice splits. A bush with a few big buds and normal flowers is worth picking over in winter and keeping. A bush with pink, hairless flowers and a poor set is finished, and the sooner it comes out the less chance it has of infecting the next one.

Hazel gets its own big bud mite, which swells the buds the same way and carries nothing. It is a nuisance on a nut crop and nothing more.

Crown gall, which is neither a mite nor a worry

Rough woody warts at the base of a stem, on a root, or along a graft union, are usually crown gall, caused by a soil bacterium that inserts a piece of its own DNA into the plant’s cells and makes them multiply. Roses, fruit trees, willows and many others get it.

On an established plant it does very little. The galls look alarming, they can be large and knobbly, and the plant usually grows away perfectly well with them. It is worth avoiding damage at the base when strimming or hoeing, since the bacterium enters through wounds, and worth rejecting a new plant that arrives with a gall on it. Cutting galls off an established plant achieves nothing.

What to do about it

Look at what plant it is on before doing anything at all.

On a blackcurrant, go over the bush between December and March, pick off every swollen round bud, and burn them. Check the flowers in spring for the pink, hairless look that means reversion, and grub out any bush that shows it.

Buy certified blackcurrant stock, choose a resistant cultivar such as ‘Ben Hope’ where the problem is about, and never take cuttings from an affected bush.

On a young or wall-trained pear, pick off the worst blistered leaves in early summer and rake up the leaf fall in autumn.

On a small camellia or azalea, pick the thick pale galls off before they turn chalky white.

On limes, oaks, maples, spruces and wild roses, leave them entirely alone.

What it gets mistaken for

Woolly aphid is white fluff sitting on the outside of the bark, which smears when rubbed and hides the insects rather than being made by the plant.

Peach leaf curl thickens and reddens whole leaves into puckered blistered masses, on peaches, nectarines and almonds.

Capsid bug damage is ragged and irregular, with small brown-edged holes in puckered young leaves.

Root-knot nematodes bead the roots underground with many small swellings.

Clubroot fuses brassica roots into a single distorted mass that rots and smells.

Leafy outgrowths on a conifer where the growth has gone soft and bunched are usually a witches’ broom, which is caused by a rust fungus or a mutation and is harmless enough that some are propagated deliberately as dwarf conifers.