Clip 'N Keep:
Weeding out the causes of plant problems

By Marilyn Dykstra, Pest Diagnostic Clinic

All plants, from trees and shrubs to perennials and annuals, are valuable components of our landscapes, contributing to the beauty and value of the area. So when problems arise, whether in the production phase or in the landscape, it is critical that the cause be quickly determined and corrective measures be taken immediately. However, sorting through the many potentially harmful environmental stresses, diseases and pests, which can adversely affect plants, to get to the root of the problem, can be challenging.

     Having a broad a background in the plant world is a great asset in diagnosis. A good diagnostician draws from his or her experience and uses common sense to solve plant problems. Using well-honed detective skills and powers of observation, the horticulturist must collect all relative information prior to making a diagnosis. The following are some of the major points to consider when faced with diagnosing a plant problem.
  1. Know the plant.
    Be familiar with the growth habit, growing requirements and common disease and pest problems of the plant with which you are dealing. We must know how a healthy plant would normally look and grow before we can recognize that a problem exists.

  2. Identify the affected plant(s).
    Examine the area where the plants are showing problems. Is there more than one type of variety affected? This is important because different plant species are susceptible to different diseases. If two very different plant species are affected, there is a strong possibility that the problem is due to environmental/physiological factors. Plants that are very different rarely develop the same disease. Given the variability in common names from region to region, be sure to use scientific names whenever possible.

  3. Evaluate the symptoms.
    Symptoms are the abnormal growth of plants that tell you there is a problem. They include abnormalities such as leaf spots, wilt, scorch, discolouration, dieback, poor vigour and many others. Are the symptoms becoming worse with time? Short, temporary stresses such as a night frost cause immediate injury, which does not develop further. Disease or pest infestations progress, causing symptoms to worsen, eventually even killing the plant.

  4. What parts of the plant are affected?
    Disease and pest problems can affect any part of a plant, whether it is flowers, fruit, branches, stems, roots or vascular tissues. Be sure to check the entire plant for symptoms. Root and vascular problems can cause symptoms at a distance from the tissues affected directly.

  5. Look for common denominators.
    What do the affected plants have in common? Have they been treated the same? Are they in a low or high area, along a fence row or road, in close proximity on a greenhouse bench, or are all plants generally affected? What is different about the care, location and management of affected and unaffected plants?

  6. Look for physical evidence of pests and diseases.
    Many pests and diseases leave physical evidence of their presence. Insects may leave cast skins, excrement, and webbing or may be visible themselves. Although for the most part microscopic, plant pathogens may produce mycelium (e.g. powdery mildew) or fruiting bodies (spores producing structures), which "give away" their presence. Check at the point where affected and unaffected tissues met for evidence of pathogens or pests. Note that if signs of insects or fungi are found, it is important to determine whether the organisms detected are actually causing the problem or are secondary invaders of weakened, dead or dying plant tissues. Often weak pathogens such as canker fungi gain foothold only after a plant has been weakened by other factors. Gall forming insects, defoliators and other minor insects may be present only by coincidence. Bark beetles and borers attack only weakened trees.

  7. Review cultural practices.
    Have the affected plants been cared for properly? Have the correct management practices been followed? Had the plants been sprayed, pruned, fertilized or watered immediately before symptom development? Be sure to check rates and methods of all pesticide and fertilizer applications to ensure all were done properly.

  8. Review environmental conditions.
    What type of weather was experienced both at the time of symptom development and over the past year(s). Remember that trees, shrubs and perennial plants suffer the effects of stresses over many growing seasons.

  9. Review the site characteristics.
    Evaluate site factors such as soil type, drainage, degree of sun and shade and proximity to other landscape features such as buildings, roadways, bodies of water, etc., which might affect plant growth and vigour.

  10. Diagnose the problem.
    Once all of the relevant information has been collected, weigh all information and diagnose the problem. If you are unsure, don't guess. Consult other individuals and resources. Be familiar with resources available to you. Resources include specialists within the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), the Pest Diagnostic Clinic, your peers, books and extension publications, etc. All of these can be consulted for assistance in diagnosis. Remedial practices should be implemented as soon as the cause of the problem has definitely been determined.

Diagnostic services at the Pest Diagnostic Clinic, Laboratory Services Division
Following the above steps should enable you to diagnose or at least narrow down the probable causes of the problem. If assistance is required, however, plant samples can be submitted for diagnosis to the Pest Diagnostic Clinic at Laboratory Services Division, University of Guelph. The Clinic provides diagnostic and identification services for nurseries, landscape maintenance firms, garden centres, greenhouse growers, farmers and commercial growers who need assistance with the diagnosis of plant problems. The Clinic specializes in the identification of plant diseases caused by fungi, bacteria, viruses and nematodes, and also provides insect and weed identification services.

How to submit samples
  1. Plants for disease diagnosis;
    Choose plants or plant parts showing typical symptoms. Samples may consist of entire plants or branch samples. Completely dead material is inadequate for disease diagnosis.

  2. Turf samples;
    Submit a 10 to 15 cm2 piece of turf from the affected area. Include the thatch layer and at least a 5 cm depth of root zone. If symptoms are general, sample from an area with symptoms of intermediate severity. If the symptoms are in a ring or patch, sample the edge of the affected area and include affected, moderately affected and healthy grass. A collection of small (loony size) pieces of turf or dead material is inadequate for disease diagnosis.

  3. Plants for identification;
    Submit as much of the plant as is possible, or several plants at different growth stages, including flowers, seeds, fruit, lateral buds, leaves, stems and root systems.

  4. Insect identification;
    Submit dead, hard bodied insects cushioned in a sturdy container to ensure they arrive as intact as possible. Insects sent loose in an envelope are usually destroyed during shipment. Do not tape insects to paper as this makes identification features difficult to see. Soft bodied insects (i.e. caterpillars) should be preserved in alcohol. Do not send insects in water. Live insects should have sufficient food to survive until arrival. Clearly label the package with "live insects."

  5. Soil and roots for nematode counts;
    Soils should be sampled whenever the ground is not frozen. Nematode populations are generally highest in May-June and September-October. A sample should be about one litre in volume and should be comprised of at least 10 sub-samples to ensure it is representative. Roots of large plants can also be sampled for nematode extraction. Never let samples dry out or be exposed to extreme heat or be frozen, as this will kill the nematodes. For additional information on sampling for nematodes, please contact the Pest Diagnostic Clinic.

  6. Submission form;
    A diagnostic submission form should accompany all samples. Submission forms are available from the PDC or on our web site. Be sure to fill out the form as completely as possible and include your mailing address, telephone and fax numbers, the history and severity of the problem and any questions you wish to have specifically answered.

  7. Diagnostic fees;
    The basic fee for diagnosis is $25. Additional fees are charged for specialized diagnostic techniques. Fees can be paid at time of sample submission or can be invoiced later. Cheques should be made payable to the University of Guelph.
         Tips for packaging samples - Do not add moisture when packing plants, plant parts or soil for nematodes. Wrap plant samples in newspaper (tying off roots and soil separately to reduce contamination) and place in a plastic container. Avoid shipping over the weekend.

  8. Send samples to:
    Pest Diagnostic Clinic
    Laboratory Services Division
    University of Guelph
    95 Stone Rd. W.,
    Guelph, ON N1H 8J7
    Tel: (519) 767-6256
    Fax: (519) 767-6240
For additional information on Clinic services, please contact the Pest Diagnostic Clinic or consult our web site at