Q What are rust and gall mites?

A They are tiny, elongated creatures that feed by sucking sap from the leaf surface. They inject toxic materials into the leaves, producing russet-like effects on leaves and stems. They also live inside galls produced on the leaves or fruits. Most are not serious pests, but may cause consternation by covering foliage with rusting or galls.

Two more serious pests are pear leaf blister mite and blackcurrant big bud mite. 

Caption: Rust and gall mites may look bad but don't cause much damage

Q Can you tell me more about rust and gall mites?

A There are many types, but the ones you are most likely to come across include the following:

Blackberry mite (Eriophyes essegi) feeds in the fruits or 'drupes' of cultivated and especially wild blackberries and, to a lesser extent, loganberries. It prefers to feed near the base of the fruit. This results in distorted fruits, typically with uneven ripening. This effect is sometimes called redberry, as damaged fruit looks abnormally red.

In severe cases, the berries ripen prematurely and their shrivelled remains persist until the following spring. The tiny, white mites overwinter beneath the bud scales and old, mummified fruits dried on to the stems. They build up their numbers early in the season, moving on to the flowers when they open. The flowers are not harmed and the characteristic fruit damage is only visible a few weeks before the fruit is ready for gathering. The damage is caused by the mites injecting the fruits with their toxic saliva. Damage increases during the picking season, as the mites build up very high populations through many overlapping generations.

Lime nail gall (Eriophyes tiliae), produces small, pointed, cone-shaped red galls on the upper surface of lime-tree leaves in early summer. The effect can be spectacular, with the whole leaf encrusted with galls. Despite this, the long-term damage seems to be minimal, with healthy trees suffering no obvious consequences. Where a tree is in poor health or is very young there may be significant damage. In these cases, improving water supply and feeding may be helpful.

Broom gall mite (Eriophyes genistae) turns buds into large, impressivelooking galls.

Nut gall mite (Phytoptus avellanae) causes enlarged buds to form on hazel bushes, like those found in cases of blackcurrant big bud. Fortunately, no serious disease transmission is associated with this mite, unlike the big bud mite.

Yew gall mite (Cecidophyopsis psilaspis) produces a 'big bud'-like condition in yew trees.

Chrysanthemum leaf mite (Epitrimerus alinae) causes russeting of stems and foliage loss on chrysanthemums.

Walnut gall mite (Eriophyes erineus) Although this is very common, producing conspicuous blisters on walnut leaves, it does not seem to do much harm to the tree. However, young trees are not a pretty sight once attacked. There is little that can be done about it, although picking off and destroying infested leaves may limit damage in light attacks. This mite lurks beneath bud scales all winter. When the weather warms up, the tiny, 0.2mm-long mites feed beneath the leaves among the leaf hairs. Several generations occur during the summer, with new foliage being attacked as soon as it unfurls.

Apple, plum and pear rust mites feed beneath the leaves, yellowing the leaves and eventually making them turn silver. Sometimes they cause fruit russeting, as well. The long-term effects are insignificant and there is seldom any need to treat this pest in gardens.

Plum leaf gall mites. Many kinds of tiny, 0.2mm mites cause leaf and even fruit galls on plums and related plants. Although the affected parts look spectacular, it is seldom worth trying to do anything about these attacks, as the effect on the long-term health and cropping of the tree is slight.

Q What damage do rust and gall mites do?

A Except for the few members of this group that cause serious damage, such as blackcurrant big bud, pear leaf blister mite and blackberry mite, the damage is simply cosmetic, making plants unsightly.

Q What can I do about them?

A Picking off and destroying damaged foliage may help to reduce the scale of next year's attacks. Improving the feeding and watering of affected plants may make them more resistant to these pests in future.

In the case of severe blackberry-mite attacks, cut the bushes back to ground level as soon after an attack as possible. This will sacrifice next year's crop, but will go a long way towards eradicating the pest. Lesser infestations can be kept in check by removing and burning old stems after harvest.

There are no chemical controls available to gardeners to control any of these mites.