Plant Pests and Diseases

Plant Pests and Diseases

Plant diseases are often symptoms of other environmental or cultural problems. When the plant is in a weakened state it is more likely to succumb to the ravages of a disease that it can usually resist. The first control for plant disease is to make sure the plant is well fertilized, growing in appropriate light and water conditions, and a suitable environment. Even under good conditions, however, disease can occur. Here are a few of the most common and troublesome types.


Damping Off

Everyone who has ever started plants from seed will be familiar with this problem. Damping off is caused by several common soil fungi including Phytophthora spp., Pythium spp. and Rhizoctonia spp. Normally, the plant can resist these fungi, but often, in the bedding plant trays, there is too much moisture at the soil surface, the temperature is too warm or too cold, and the plants are weakened from insufficient light. Most commonly the soil is too wet.

There are chemical fungicides to control damping off, however, cultural practices are more practical and less toxic and expensive.

Powdery Mildew

This is a common problem later in the summer when the air is dry. Powdery mildew is caused by a fungus, Erisyphe spp. which prefers dry conditions. It appears as a white powdery deposit on the leaves, often mistaken for white paint spots. It is not usually of economic importance, unless the fungus completely covers the leaves, preventing light absorption. It is common on Delphinium and shade loving plants, as well as currants, and roses. It can cause leaf curling and deformation of fruit and flowers as well as being unsightly on leaves.

One home remedy suggests a mixture of 1 tbsp dishwashing liquid with 2 – 3 tablespoons of baking soda in a gallon of water. Cultural controls could include moving the plants to a more hospitable site, and increasing the moisture level around the plants by hosing them down frequently. The disease overwinters in the soil so it will reappear every year unless some control measures are put in place.



This fungus attacks stems and flower heads of many plants including sunflowers. It produces a soft, rotted area in the stem that is easy to break. It can also invade the heads of sunflowers. If the plant doesn’t fall right over, and the fungus has the opportunity to mature it will produce hard, black sclerotia, like small stones inside the plant tissues. These sclerotia drop to the ground and produce the spores for next year’s infection. They can remain in the soil for several years.
Aster Yellows

Aster Yellows

This has been a serious problem for growers of flowers in recent years. It is caused by a mycoplasma, a virus-like organism that can be spread by mechanical damage (tools that break the leaves can carry infectious particles), or by insects, usually sucking or chewing insects that transmit the disease through their mouth parts. The mycoplasma must enter the plant through a wound, but it can be a very small wound. It is extremely easy to transmit and spreads rapidly. It has a very wide host range and will infect most common, non-graminaceous, plants like canola, asters, marigolds, mustards, statice, strawflowers, delphinium, and many more.

It can overwinter in the soil, in plant debris and in infected plants. It is especially problematic on perennial plants as there is no treatment for the disease. The symptoms include a yellowing of the leaves and stems, extreme vertical, upright growth of stems that are usually more curved, and stunted stems and leaves.

The only control is to remove affected plants and burn them or dispose of them away from the garden. Do not add these to your compost pile. Wash tools, hands and boots after working with these diseased plants. Early detection is important as is swift action.


Stem Rot, Root Rot

Many plants are susceptible to a variety of fungal pathogens that attack at the soil level or just below the soil line. Often, environmental factors are important, the soil is too wet, too dry, too compacted, or there is a heavy infestation of fungal spores already in the soil from last year. Basil is especially susceptible to stem rot organisms, and almost always will show symptoms as a withering and darkening of the stem just above the soil line. The fungus attacks the stem vessels and impairs the plant’s ability to transport water and nutrients up to the leaves and down to the roots, eventually strangling the plant.

Try this home remedy for root and stem rots including damping off. Horsetails (Equisetum) are said to contain a natural fungicide. Try making a tea from a handful of equisetum stems in a gallon of water. Soak overnight and use this to water your plants. You could also mix dried, crushed horsetail stems into your potting soil for the bedding plants. You could also allow the horsetails to grow in your basil patch.


Other Problems

There are many nutrient deficiencies common in plants.

  •  Low nitrogen give a yellow, weak plant
  •  Low phosphorus causes brown blotches on the leaves
  •  Low potassium can cause problems in tomato fruits and bud and fruit set in many plants
  • Micro-nutrient deficiencies are even more varied but easily remedied by an application of complete fertilizer.

Plant Pests

One of the many challenges of gardening is the wide variety of insect and animal pests that can attack your favorite plants. Sometimes the damage is minimal and the plants survive with a few tattered leaves. Other times, however, the damage is more severe. Here are a few of the most common pests that affect the growth and development of flower and herb plants in Alberta.



Aphids can be identified by their color, usually the same green as the plants they are chewing on, but also found in black, orange and yellow. They pierce the plant’s epidermis and suck out the nutritious plant sap. Well fed aphids will excrete the excess nutrients and sugars as a sticky sap, honeydew, which ants will collect. It is commonly believed that the ants protect the aphids from insect predators and so perpetuate the infestation on our plants. If you can remove the ants, you may also solve the aphid problem.

Try spraying with soapy water. The soap disrupts their thin skin membranes and they burst. There are various chemicals registered for control of aphids, but very few of them are recommended for use on our ornamental gardens. Spray the soil around the plants at the same time as the leaves. The aphids leap off the plants to save themselves and crawl back up the stems later when you’re not looking.

Tarnished Plant Bugs

These are burnished brown, shield shaped bugs, that actually chew on leaves and young growing tips of plants. They cause problems, just because of the physical damage they do, but they can also spread diseases from plant to plant on their mouth parts. The bugs over winter in the soil in an immature form so you can get an increase in the population from year to year if you don’t control it. This may be one insect that requires a chemical spot control.


There are several types of caterpillars that eat leaves, stems, growing tips and various other parts of our plants. The best control for caterpillars is a toxin produced by a common bacteria, Bacillus thuringensis. The bacteria is now cultured in huge vats and the toxin isolated and purified so that it can be used in many formulations. Once sprayed on the leaves, the toxin is ingested by the caterpillars and it prevents them from digesting their food normally. The little devils starve to death within a few days. One application at the right time can remove most of the pests, but for a serious infestation try a repeat application after a week. The B.t. toxin is not harmful to humans, only caterpillars. It is available at garden centres under various trade names. Read the labels on the containers to find one with B. thuringensis toxin.