18 ways to treat pests and diseases on shrubs

Properly positioned, shrubs growing in good soil seldom develop any serious pest or disease problems. But here are 18 ways to identify potential problems and control them before they get out of hand.

1. Shrub is disfigured. Pests may be visible: Aphids, lace bugs, leafhoppers, whiteflies, or mites. Use dormant oil in late winter. Spray with insecticidal soap or permethrin
2. Leaves distorted, pale, yellowed, or pinkish, with silvery dots: Mites. Frequent sprays with water (forcefully), insecticidal soap at 5-day intervals, especially underside
3. Leaves covered with sooty mould, pests may be visible: Honeydew released by aphids, mealybugs, whiteflies. Control insects as above, wash off mould with soap and water spray
4. Leaves have holes or ragged edges: Caterpillars, inchworms, or beetles, usually visible. Spray caterpillars with Bacillus thuringiensis, inchworms and beetles with insecticidal soap or neem
5. Leaves have semi-transparent blotches or lines: Leaf miners. Spray with neem
6. Leaves rolled or tied together: Leaf rollers. Spray with insecticidal soap or Bacillus thuringiensis
7. Leaves or leaf stalks have swellings or pimples: Gall insects or mites. Seldom serious. In fall, rake up and dispose of fallen foliage
8. Leaves wilt, stems chewed at base, or roots eaten: Root weevils. Difficult, pest is resistant to most controls. A beneficial nematode can be used if the soil remains moist. It is available commercially
9. Branches and stems have rough, corky areas: Scale insects. Prune out and destroy infected branches. Spray with dormant oil in winter, insecticidal soap or summer oil in May
10. Leaves, buds, or flowers are covered with a gray mould: Botrytis (fungus). Keep shrub growing well, spray with summer oil
11. Leaves have brown blotches and fall early, shoots die: Anthracnose (fungus). Spray with dormant oil in late winter, with Bordeaux mixture in spring, starting as leaves unfurl. Repeat 3 times at 10-day intervals
12. Leaves have circular spots that may run together: Leafspot (several different fungi). Spray with a copper-based fungicide or wettable sulphur
13. Small brown spots on petals expand rapidly, flowers wilt and fall: Mostly on rhododendrons and azaleas. Petal blight (fungus). Pick off and remove infected flowers as soon as seen. If problem repeats, spray with Bordeaux mixture in spring
14. Leaves, flowers, or buds have a white powdery coating and turn black eventually: Powdery mildew (fungus). Good sanitation. Dispose of leaves as they fall. Spray with potassium bicarbonate fungicide
15. Leaves have rusty to orange spots; on conifers, irregular galls turn orange in spring: Rust (fungus). On conifers, prune out as soon as seen. On other plants, spray with a fungicidal soap
16. Leaves and young fruits have sunken areas that turn corky: Scab (fungus). Spray with lime sulphur in late winter, with dilute lime sulphur several times in spring
17. Leaves and flowers turn black, wilt, but don't fall: Fire blight (bacterial). Prune off and remove infected branches. Spray with lime sulphur in spring
18. Spongy swellings develop on branches, stems, and roots: Crown gall (bacterial). Prune out, disinfect pruners between cuts. If galls are on main stems, remove and destroy plant

You may never experience any issues with your shrubs, but if you do, this handy list of 18 things to watch for will help you return your plants to their former glory.