Why are some buds on my blackcurrant swollen and round?
Big bud mite is a problem in its own right and also carries reversion, the virus-like disease that ends a blackcurrant's productive life — how to tell how far gone a bush is.
- Difficulty
- beginner
- Time
- 30 min
- Read
- 3 min
Short answer
Blackcurrant gall mite. Affected buds swell into round balls over winter instead of staying slim and pointed, then fail to open properly in spring. Pick off and destroy swollen buds in winter. If the bush also flowers oddly and yields collapse, it has reversion, which is incurable — remove the bush and plant certified stock elsewhere.
Two problems travel together here. The mite itself distorts and destroys buds, which costs you crop. More seriously, it carries reversion disease, which slowly makes a blackcurrant unproductive and cannot be cured. Distinguishing between a bush with a few galls and a bush that has gone over to reversion decides whether you pick buds off or dig the plant out.
Step by step
- Look at the buds in winter.Healthy blackcurrant buds are small, slim and pointed. Infested ones are round, swollen and noticeably larger, and they are easiest to spot between December and February when the bush is bare.
- Pick off and destroy every swollen bud.On a lightly affected bush this is effective and quick. Bin or burn them — do not drop them beneath the bush, because the mites migrate to new buds in spring.
- Cut out badly affected stems entirely.Where a whole stem is covered in galls, take it out at the base as part of the normal winter renewal pruning.
- Watch for reversion in summer.Reverted bushes have leaves with fewer main veins and fewer lobes than normal, flower buds that are bright magenta rather than dull grey-pink and hairless, and a yield that falls away year on year despite good pruning and feeding.
- Remove a reverted bush.There is no treatment. Dig it out, destroy it, and do not take cuttings from it or from anything near it.
- Do not replant blackcurrants in the same spot.Mites migrate short distances to new bushes. Plant certified virus-free stock in a different part of the garden.
- Buy certified stock.Certified blackcurrant plants are propagated free of gall mite and reversion. It costs a little more and is the single most effective preventive measure.
- Prune and feed to keep the bush vigorous.A well-renewed, well-fed bush copes with a light infestation. Take out a third of the oldest wood at the base each winter and feed with a general fertiliser plus potash.
- Consider resistant varieties when replanting.Ben Hope and Ben Gairn have useful resistance to gall mite and reversion respectively, and both are good garden varieties.
Tips
- Winter pruning is the natural time to do the bud inspection, because you are already looking at the bare stems.
- The mites move to new buds in spring as growth starts, which is why removal has to happen in winter to be worth anything.
- Big bud does not affect red or white currants or gooseberries in the same way — it is specific to blackcurrants.
- A bush with a handful of galls each year and a decent crop can be lived with. A bush with declining yields and magenta flower buds cannot.
Common mistakes
- Dropping picked galls under the bush — The mites simply move back up into new buds. Bag them and remove them.
- Taking cuttings from an affected bush — Both the mite and reversion travel with propagation material, so you spread the problem to a new part of the garden.
- Replanting a new blackcurrant in the same spot — Mites migrate to nearby bushes. Plant certified stock somewhere else in the garden.
- Persisting with a reverted bush — It will not recover, yields keep falling, and it acts as a source for any other blackcurrants nearby.
Questions people ask
What are the round swollen buds on my blackcurrant?
Blackcurrant gall mite, usually called big bud. The mites live inside the bud, which swells and then fails to open properly in spring.
Can I get rid of big bud?
Picking off and destroying every swollen bud in winter controls a light infestation. There is no effective garden spray, and a heavily affected bush is usually better replaced with certified stock.
What is reversion disease?
A virus-like disease spread by the same mite. Leaves have fewer veins and lobes, flower buds turn bright magenta and hairless, and cropping declines to nothing. It cannot be cured — remove the bush.
Is the fruit safe to eat?
Yes. Neither the mite nor reversion affects the safety of the fruit; they affect how much of it you get.