Big Bud Mite on Blackcurrants

Round, fat, swollen buds on your blackcurrants are the classic sign of big bud mite. The mites live and breed inside the buds, blowing them up like tiny balls. The real problem isn't the swelling itself — it's that these mites spread blackcurrant reversion virus, which slowly stops your bushes cropping altogether.

So if you spot it, act over winter. Here's how to recognise it, why it matters, and what to do.

How to recognise it

The tell-tale sign shows up in winter, once the leaves have dropped and the bare stems are easy to inspect.

• Healthy buds are slim, pointed and tight against the stem.
• Infested buds are round, fat and swollen — like miniature Brussels sprouts or peppercorns stuck to the wood.

The mites themselves are far too small to see (about 0.2mm), but the swollen buds give them away every time. Each affected bud can hold thousands of them, all sheltering inside through the cold months.

In spring, infested buds often fail to open properly or simply die. You'll notice gaps along the stem where buds should have burst into leaf or flower.

It's mainly a blackcurrant problem. Redcurrants and whitecurrants are far less prone, though related mites can affect them. If your trouble is on those, check the currants guide for the full picture.

Why it matters

A few swollen buds won't ruin a harvest on their own. The danger is what the mites carry.

As they move from bud to bud — and from bush to bush on the wind, on tools or on your sleeves — they spread reversion virus. There's no cure for it. An infected bush gradually "reverts": leaves become narrower with fewer main veins, flowers turn a brighter pinky-purple, and crucially the bush sets fewer and fewer berries each year until it barely fruits at all.

By the time you notice the cropping fall off, the virus is well established. That's why big bud mite is worth treating early — you're really trying to stop the virus before it takes hold.

What to do

There's no chemical fix at home, so the approach is practical and physical.

1. Pick off and burn swollen buds. Through winter, go over each bush and pinch out every fat, round bud you find. Don't compost them — the mites will survive — so burn them or send them out with the household waste. This won't eradicate the mite, but it knocks the numbers right back and slows the spread.

2. Prune hard if a stem is riddled. If a whole shoot is covered in swollen buds, cut it out at the base and burn it. Winter is the right time to prune blackcurrants anyway — see pruning currants for how to do it without losing next year's fruit.

3. Replace badly affected or reverted bushes. If a bush has lots of big bud, or is showing the reversion signs above and cropping poorly, it isn't worth nursing along — it's a reservoir of virus for your healthy plants. Dig it up, burn it, and start again somewhere else in the garden.

4. Always replant with certified stock. When you buy a new blackcurrant, choose certified virus-free plants from a reputable supplier. They start clean, which gives you years before any mite finds them.

5. Choose resistant varieties. The modern Ben series from the James Hutton Institute in Scotland was bred with UK growing in mind. 'Ben Hope' has good resistance to big bud mite, and 'Ben Gairn' carries resistance to reversion virus itself — both excellent first choices for a beginner replacing an old bush.

How to prevent it

Once your bushes are clean, keep them that way:

• Buy clean, plant clean. Certified, virus-free plants are the single best defence — never accept "free" cuttings from a friend's bush unless you know it's healthy, as that's a common way the mite arrives.
• Inspect every winter. A quick walk along the stems each year catches swollen buds while there are only a few to pick off.
• Don't propagate from infected plants. If you take cuttings to make new bushes, only ever use clean, vigorous, well-cropping parents.
• Keep bushes growing strongly. Good soil and a sensible feed help a bush shrug off knocks, even if it does pick up a little mite.

Caught early and managed each winter, big bud mite needn't cost you your blackcurrants. The key is to look, pick, and — when a bush is past saving — replace it with certified, resistant stock rather than letting the virus quietly spread down the row.

PS — found a fat bud in February? Pinch it off now, then put a winter bush-check in your diary for next year. Five minutes of looking saves a whole crop.