Bonsai Pests and Diseases

This post shows the more common pests and diseases that might attack your bonsai. Fortunately, most bonsai trees and shrubs are hardy species that are not notably susceptible to pests and diseases, but, if a problem should arise, you should recognize it quickly and “nip it in the bud”. A useful preventative measure is to spray the bonsai monthly, from early spring and through the summer, with a systemic insecticide and fungicide. This should be ninety per cent effective, penetrating the sap and helping the plant to counter-attack any enemy over the following two or three weeks. Check that the brands of fungicide and insecticide are compatible, and follow the manufacturer's instructions. Only spray leaves that are in bud or fully open; unfolding ones may be damaged.

Pests

Blackfly
Aphids such as black-fly are common in many climates and suck sap from stems, leaves, and fruit of garden plants, and also carry viruses. Once spotted, they must be eradicated. They are easy to see; you may notice leaves and new shoots curling up.

Greenfly
Like most of the aphids, greenfly were named after their color. Aphids attack thin-leaved, deciduous trees in particular. They lurk on undersides of leaves and on young shoots, and secrete a sticky “honeydew”. Prune out any damaged shoots.

Mites
These red or black “pinhead” mites, found in most climates, can be seen under branches or in bark crevices. They feed on algae on the bark, but do no damage to the tree's growth. They are unsightly, but can be brushed away with a toothbrush.

Spittle Bugs
Larvae of this insect, found in many climates, inhabit the spit-like masses on leaves and shoots. They thrive on plant sap. The damage is similar to that done by aphids. Remove the “spit” by wiping with a damp cloth or spraying with clean water.

Scale Insects
These are a group of tiny, sap-sucking insects which look like hard white, yellow, red or brown blisters on leaves and stems. Signs of damage are a sooty mold and a sticky “honeydew” coating the leaves. A systemic insecticide is effective.

Caterpillars
As soon as they hatch in the spring, caterpillars will begin feeding voraciously on young foliage and stems. It is best to pick them off by hand or to use a contact spray. Remove any damaged leaves to improve the appearance of the tree.

Slugs and Snails
These attack roots, shoots, and leaves, making large holes in the latter. Liquid slug-killers, which must be watered in, are not very suitable for bonsai, because there is not much soil in the pot. Instead, leave a cream or pellet type of slug bait near the pot.

Vine Weevils
The adult beetle nibbles foliage, leaving notched edges; the larvae feed off roots. Getting rid of larvae in regions where they occur can be a problem as they can live in the soil for up to six months and may not be noticed until repotting. Soak pots in a solution of gamma-HCH.

Leaf Miners
If you notice brown or white markings on leaves, this is probably caused by leaf miners which feed on the soft, interior tissues. These pests are the larvae which hatch from eggs laid on the undersides of the leaves by moths and flies. They have a blister-like appearance. On bonsai, remove spoilt leaves and spray regularly with a systemic insecticide.

Diseases

Peach Leaf Curl
This problem, which causes leaf damage and so weakens the tree, occurs in many climates but is limited to a few Prunus species. Reddish blisters form on leaves, grow bigger, and turn white. Leaves distort, discolor, and drop. The fungus can survive over winter. Destroy affected leaves and spray with a copper fungicide.

Powdery Mildew
These fungal spores, which thrive in damp conditions, appear as white, powdery patches on leaves and stems, of oak, hawthorn, and crab apple in particular. They leach sap, causing the tree to lose vigor. Remove the affected shoots on which the disease overwinters. Routine preventive treatment is important.