Insect & Disease Problems

Updated: Sep 21

Improve your knowledge about insects and diseases that may

affect tree health and learn proper steps for diagnosis.Insects and diseases can threaten tree health.


Plant health requires sufficient water and light and a proper balance of nutrients. Too much or too little of any of these environmental elements may cause plant stress.

Trees deal with environmental stresses, such as shading and competition for water and nutrients, by adjusting their growth and development patterns to reflect the availability of the resources. Although trees are adapted to living in stressful conditions in nature, many times the stresses they experience in the landscape are more severe than they can handle and may make them more susceptible to certain insects and diseases.


Correct diagnosis of plant health problems requires a careful examination of the situation.

1. Accurately identify the plant. Many insects and diseases are plant-specific. This helps limit the list of suspected diseases and disorders.

2. Look for a pattern of abnormality. Compare the affected plant with other plants on the site, especially those of the same species. Non-uniform damage patterns may indicate insects or diseases. Uniform damage over a large area (perhaps across several plant species) usually indicates disorders caused by such factors as physical injury, poor drainage, chemical damage, or weather.

3. Carefully examine the landscape. The history of the property and adjacent land may reveal many problems. Most living pathogens take a relatively long time to spread throughout an area, so if a large percentage of plants becomes affected virtually overnight, a pathogen or insect is probably not involved.

4. Examine the roots. Brown- or black-coloured roots may signal problems. Brown roots often indicate dry soil conditions or the presence of toxic chemicals. Black roots usually reflect overly wet soil or the presence of root-rotting organisms.

5. Check the trunk and branches. Wounds caused by weather, fire, mechanical damage, or animals can provide entrances for pathogens and wood-rotting organisms. Large defects may indicate a potentially hazardous tree.

6. Note the position and appearance of affected leaves. Dead leaves at the top of the tree are often the result of environmental or mechanical root stress.

Twisted or curled leaves may indicate viral infection, insect feeding, or exposure to herbicides. The size and colour of the foliage may tell a great deal about the plant’s condition.