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Dealing with Common Garden Pests and Diseases in Ontario: Prevention and Treatment

So, you have your garden perfectly planned and planted; you have a regular watering schedule; you’re all set for the season – right? Unfortunately, there is always the risk of garden pests and diseases that can be detrimental to your plants. The good news is, there are preventative measures and treatments for them! In this blog, we’ll dive into five of Ontario’s most common garden pests and diseases, ways to prevent them, and how to treat them!


Cankers are fungal diseases that affect the bark of trees and shrubs. In plants that have been affected by canker, patches of bark may split or peel away, and there is often a change in colour along with orange or black ooze from the affected area.

Prevention: Cankers are more likely to occur when plants are not at optimum health, so keeping them properly planted, mulched, watered, and pruned will help to prevent disease. Additionally, plants that are well-adapted to the area are less likely to be vulnerable to such diseases. Check out our previous blog for more about the benefits of native plants. 

Treatment: The best treatment is to prune affected branches to remove the disease. Use caution when doing so, as many pathogen’s main entry is through injuries. In severe cases, the entire tree or shrub may need to be removed. 

Leaf Blotches, Blight, and Anthracnose

There are several variations of these bacterial diseases that affect leaves of trees and shrubs. The most obvious symptoms are tan, brown, or black blotches on the leaves. This is common in spring, as they are most likely to spread in the damp weather. 

Prevention: Once again, the best prevention method is good sanitation practices to keep trees healthy. Proper irrigation to prevent excess water build-up can also help avoid these diseases. 

Treatment: Most trees are able to withstand this type of infection. The best treatment method is to discard the fallen affected leaves to prevent the spread of these diseases or reinfection.


Rusts also result in spots or blotches on leaves, but in this case, they are yellow, orange, or brown and are caused by fungi.  

Prevention: The best prevention in this case is to plant trees and shrubs that are resistant to rust diseases

Treatment: Control can be difficult with this type of disease. Treatment methods include pruning the affected branches and applying protective fungicides to help minimize infection.


Aphids are one of the most common insect pests, attacking ornamental and vegetable plants. The insects may be green, yellow, or black. They are quite small but usually appear in clusters. They also secrete fluids that may attract ants and cause a black fungus to grow, multiplying the damage and annoyance.  

Prevention: There are a few ways to try to prevent these insects, including removing weeds from your plants which can be hiding grounds for aphids, avoiding overfertilization with nitrogenous fertilizer which can attract aphids, and netting your plants with insect nets to protect them while developing. You can also introduce insects which are aphid predators to your garden to help prevent them. 

Treatment: The best treatment methods are chemical pesticides or insecticidal soap to get rid of these pests.


Finally, scale insects are also commonly found on many ornamentals in Ontario. They have a waxy whitish or brown coating and attach themselves to plant stems and branches like barnacles. 

Prevention: As usual, a healthy plant is the first step to preventing pests and disease. Additionally, be sure to remove any scale-damaged branches, twigs, or leaves as soon as they appear. Scale only becomes a real problem when they start to multiply. 

Treatment: In addition to pruning infested branches, horticultural oils or insecticidal soap can be sprayed on plants in dormancy. Insecticides should be a last-ditch effort with this type of pest.

With the proper knowledge and prevention, your garden can thrive! We hope this information helps you, and be sure to come to Bloom Green Garden Centre with any questions or inquiries! Good luck, gardeners!

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