Horticulture Hack: Caring for your Plum Trees
February 5, 2020
Nurtured in Norfolk’s gardening expert Martyn Davey answers all your questions and gives his top horticulture hack.
If you would like any horticulture query answered please do e-mail our head grower at email@example.com to add your thoughts to our horticulture hacks.
I am enclosing some leaves of my damson tree. The marks appeared this year mainly confined to the new growth. I am concerned that it will spread to my plum trees nearby.
Could you please tell me what it is and if I need to treat this and if so with what?
R. Gay, Beccles
The problem has been caused by one of the two aphids that affect plum trees the one affecting your damson is the plum leaf curling aphid which can cause problems on plums, damsons, greengages and sloe. The other is the mealy plum aphid. Plum aphids are sap-sucking insects that feed of the foliage of plums and other related fruit trees.
Plum leaf-curling aphid causes severe leaf distortion on the foliage of all types of plum during April to late May. Leaves that develop later are not affected, although the earlier damage often remains visible throughout the summer.
Mealy plum aphid is active in mid- to late summer, when dense colonies of pale greenish white aphids cluster on the underside of leaves and shoot tips. This aphid does not cause leaf distortion but it excretes honeydew that makes the foliage and fruit sticky and allows the growth of sooty mould.
Both mealy plum and plum leaf-curling aphids overwinter as eggs that are laid in autumn in bark crevices and around the buds. Eggs of plum leaf-curling aphid hatch at bud burst but those of mealy plum aphid hatch later in April.
Aphids feed by sucking sap from the foliage. Leaf-curling plum aphid secretes chemicals into the foliage that cause the young leaves to develop in a crumpled and distorted fashion. In this species, winged forms of the aphid develop during May and the winged adults then fly away to various herbaceous plant where they spend the summer. The infestation on plums dies out in late May-early June and after that time the tree produces normal foliage.
Where possible tolerate infestations of plum aphid. Whilst infestations of leaf-curling plum aphid can be unsightly the effect on yield is limited. Aphids are eaten by a wide range of predators and parasites but these are not usually present in sufficient numbers, particularly early in the growing season, to prevent plum aphid infestations developing.
Research has indicated that earwigs on fruit trees can reduce aphid numbers and in fruit trees they do not cause damage. Providing shelters such as flower pots loosely stuffed with hay in trees can help increase earwig numbers.
On plants small enough to be sprayed thoroughly, overwintering eggs can be destroyed by using a plant oil winter wash (organic e.g. Vitax Winter Tree Wash) when the buds are fully dormant in November-early February on a dry frost-free day. Plant oil winter washes are likely to be least detrimental to natural enemy populations and also can mean that spring sprays are unnecessary. This should reduce or eliminate any need for spraying during the growing season.
Insecticidal sprays are less likely to be successful if leaf distortion has occurred. Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of aphids that are not hidden within distorted leaves. These pesticides have a very short persistence and so may require reapplication to keep aphid numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults.
More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer). As with organic types these contact insecticides are less likely to control aphids in distorted leaves
Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval. As a guide the minimum interval that needs to be left between treatment and picking the fruit is 14 days for acetamiprid, seven days for deltamethrin and lambda-cyhalothrin and one day for pyrethrum. Do not spray during plants in flower due to the danger to pollinating insects.
Jobs for this week
Horticulture Hack 1: Now is the time to prune any shrubs that flower on new wood in summer, BuddleJa davidii and Perovskia are good examples of suitable shrubs as are roses.
Horticulture Hack 2: Cut back old canes from autumn fruiting raspberries to ground level. New canes will bear fruit later in the year.
Horticulture Hack 3: Hedge cutting and pruning should be completed by the end of the month before the nesting season begins.
Martyn Davey – Head Grower