plum leaf curl
One of our plum trees - satsuma + santa rosa grafted on - developed plum leaf curl on several branches this spring. See attached photo, taken today 7/8/19. Aphids aplenty were obvious on the underside of the affected leaves up to two weeks ago. Aphids are no longer apparent. Any suggestions for preventive treatment before this tree leaves out next spring? Greg
The leaf curl plum aphid is often found inside curled leaves. It is shiny and varies considerably in color from green to brownish green or brownish yellow. This aphid overwinters in the egg stage near the base of buds. In spring it rapidly builds populations on new foliage, causing affected spurs to develop tightly curled leaves. In May, the aphids migrate from the orchard to summer host plants in the family Asteraceae.
Colonies of this pest cause leaves to curl tightly. Often only one limb or a portion of a limb is infested early in the year. Large amounts of honeydew are secreted by this aphid. Tree growth and fruit sugar content can both be reduced by populations of this aphid.
Several natural enemies are important in the control of aphids in the orchard, but aphid populations often require treatment. The best indicator of populations is orchard history. The best time to treat is during the dormant or delayed dormant period. If aphids are a chronic problem in the orchard, apply a treatment early in dormancy; otherwise, sample during dormancy to determine the need to treat as described below. Spring treatments may also be made. After harvest, a zinc sulfate application will provide zinc to the trees as well as hasten leaf fall. Without the leaves on the tree, the aphid life cycle is disrupted. Zinc sulfate (36%) applied at 10-20 lb/acre can be applied in early to mid-October to help in this process.
There are many natural enemies that feed on leaf curl plum aphid; however, fruit size may still be reduced and curled leaves will not uncurl after aphids are suppressed. The recent introductions of Aphidius colemani has led to substantial levels of parasitism of this aphid. Important predators include: lady beetles, green lacewings, brown lacewings, syrphid flies, and soldier beetles.
Organically Acceptable Methods
Biological control and sprays of narrow range oil or neem oil are organically acceptable methods of controlling this pest.
Monitoring and Treatment Decisions
If aphids are a chronic problem, a treatment in late fall/early dormancy (November 1) is a very effective way to manage these pests and is less likely to create water quality problems caused by pesticide runoff than treatments applied during the rainier season in January and February. If leaves are still on trees at this time, aphids and parasites can be present. Oil treatments are not recommended at this time because they are very damaging to parasite populations and not effective for aphid control.
Spring monitoring. If aphids have been a problem in the past or if a dormant or delayed dormant application was not applied, monitor leaf curl plum aphid in spring along with mealy plum aphid.
Hope this helps!
Just to reinforce for next year. Be sure to do your preventive spraying next winter with a dormant oil and rake up and dispose of all leaves in the fall. Monitor for aphids early in the spring so you can stay on top of the infestation.