Disease Control

As heat and humidity can arrive throughout the growing season, perfect conditions may exist for various diseases to spread into your garden. Plants have a risk of suffering from disease related issues depending on the climatic conditions in your region. Although the weather is favourable for us, it may not be for your gardens. There are however, successful ways to prevent, treat and control plant diseases.


The first defense in treatment is to identify the cause. If disease issues happen in your garden, awareness is the key to success. Listed below are some of the more common issues that present themselves from year to year all across the country.

Once proper diagnosis has been made, then treatment can begin. In some situations—as a preventative measure—treatment will have to be applied before possible onset of disease. An example of this would be late blight on potatoes or tomatoes. Once certain diseases have set in, then all that can be done at this point is to prevent further spreading. Certain diseases are incurable at this stage and can only be treated. Listed and pictured below are some of the more commonly known diseases that could potentially cause complete destruction of plants, trees or shrubs.

Tar Spot

Pictured above is a common issue on many different cultivars of Maples, in particular Norway Maple. The familiar black spot on this leaf is known as Tar Spot (Rhytisma acerinum).

Late Blight

Late blight affects mostly potatoes and tomatoes. The pictures below show beginning and later stages of the onset of late blight on a tomato plant. Late Blight will ultimately destroy the whole tomato crop if no prevention is practiced. The best time to start your prevention regime on both tomato and potato plants is before humidity, wet or hot weather set in. Daily scouting this time of the year will help alert you to any changes. Use of this practice will aid in scouting other diseases as well.

There are some products that are recommended to use on these plants if you choose to practice prevention. Other planting practices that will benefit your plants in ensuring disease risk at its minimum is to plant in an area where they can receive ideal air circulation followed by proper spacing and timely pruning. If watering is needed, be sure to water plants preferably at ground level in the morning to ensure leaves will dry during the day. Watering prior to night time will not allow foliage enough time to properly dry. This may increase vulnerability as well as contribute to perfect breeding grounds for any types of blights, molds or mildews to set in.

The most reliable way of ensuring successful control of late blight is to choose late blight resistant varieties. These varieties have shown great resistance to late blight spores even when planted next to other varieties that are infected with the disease.

Early Blight

Early blight (alternaria) is another disease common to potatoes and tomatoes. This disease should properly be treated, however, it is not as fatal as late blight (phytophthora). Pictured below are examples of both types of blight and their differences to help you make a proper diagnosis.

There are reports of various types of blights and another common one that has been mentioned in the more recent years is fire blight, Erwinia amylovora. This blight is classed as bacterial mostly found on fruit trees as well as some Ornamentals in the rose family. Stem tips are affected first and will later show a wilted look almost making foliage appear as if it had been scorched.

Downy Mildew

Another very common disease among both flowers and vegetables is Downy mildew (Plasmopara viticola). Reports of this disease have been made in the more recent years due to the impact it has on the popular annual plant known as Impatiens. It has affected the walleriana impatiens but not the hawkerii type also known as New Guinea. Side effects of this disease are diminished if the weather is dry and humidity levels are low. Following cultural practices such as watering in the day light hours as well as at ground level are some ways to prevent disease spread.

There are different types of mildew may cause confusion in making proper diagnosis as well. The leaf that is pictured above, shows the beginning signs of Downy mildew when viewing the underside of the leaf. It tends to take on a more furry gray appearance, whereas another type of common mildew known as Powdery mildew will look as if plants were covered with a dusting of icing sugar affecting mostly leaf surface.

Powdery Mildew

Powdery mildew (Podosphaera leucotricha), as mentioned above generally affects vine crops, such as squash, cucumbers and pumpkins. It also can be commonly found on some perennials such as phlox. This disease is caused and spread by a fungus and presents itself with an infamous dusting of white to gray powder. It coats the whole leaf surface leaving it to eventually wilt, brown and resulting in death. Most harvest is done before this is likely to terminate the plant and there as great disease resistant varieties available.

Leaf Spots/Black Spot

Other types of diseases that are quite commonly mentioned are leaf spots/black spot (especially on roses), Anthracnose, Rusts, Wilts, Root and stem rots as well as cankers, just to name a few. With this list in mind, preventative measures or even simple maintenance such as leaf picking will help solve these issues. Diseases may come with a vengeance but once they serve their time and control is gained it eventually won't pose as much damage year after year.

Other issues that mimic disease

It is quite possible that plants, trees and shrubs may not even be infected with disease. Some physiological issues can mimic symptoms of what a specific disease may look like. Physiological issues that are quite commonly confused as a disease are deficiencies of a particular micro-nutrient such as magnesium. This deficiency will display yellowing or odd brown patches on the leaves of your plants as pictured above. This may cause confusion and doubt, so before a treatment regime begins, use available resources to assist in making a proper diagnosis.

Inspect the area for insects, as they too, can be confused with making leaves appear as if the damage was caused by disease. This of course would require a completely different type of treatment. Often times, when plants become compromised with disease, it will make them more vulnerable to pests.

Your Best Defense

The best defense against any disease is to buy plants or seeds from reputable companies, plant in areas where there will be ideal air circulation, provide regular applications of fertilizer, removal of aged blooms and foliage, trimming broken stems or branches as well as making clean cuts and water early in the day at ground level taking care and not splash on leaves. Following these cultural practices will certainly help ensure disease risk is minimized.