How to recognise it
The first symptom is a swollen and less pointed appearance in buds. The buds may then fail to open or produce distorted leaves if they do open. The problem is most obvious in winter and early spring.
The mites themselves are minute in size (about 0.25mm).
The mites spend most of their year living inside the buds, where they damage the meristem and induce the bud to produce many scale leaves (which gives the bud its swollen appearance).
They also breed within the buds from January to April and breed again from June to September. They then overwinter within the buds.
In May the mites finally emerge from the buds and produce silk threads to climb along to spread to other, healthy buds and plants. They can also be carried on the bodies of aphids. They then enter healthy, dormant buds over the summer ready for the buds to develop the following season.
Why it’s a problem
Infected buds fail to open or produce distorted leaves. More critically, the mites can also carry the virus responsible for blackcurrant reversion disease, which stunts the plant and reduces fruit production.
Where you are likely to find it
Primarily in blackcurrant plants, but it can also affect other Ribes species.
How to deter it
Ensure that any new stock is certified free of infection and from a reliable source.
How to get rid of it
Prune out any stems where big bud mites are present as soon as it is noticed and destroy (not on your compost heap). Where plants are badly infested they should be dug up and destroyed. This is best done in the winter when the mites are inactive to limit the spread of the infestation.
Spraying with a sulphur formulation in May and June (when the mites emerge from the buds and migrate) can be effective. There are no chemical means of attacking the mite while it is in the bud.
If the plant is infected by blackcurrant reversion as a result of the big bud mite infestation then it cannot be cured.
Is it good for anything?!
Other useful information
While the nut gall mite (aka the hazel big bud mite) Phytocoptella avellanae is similar in terms of symptoms, the two pests cannot be transferred between hazels and blackcurrants respectively.