Scouting for Diseases

Diseases vary in prevalence and severity each year depending on the weather, cultural practices, variety of crop grown, and the presence or absence of disease-causing agents. Efficient disease management requires that growers anticipate disease problems well in advance. For example, planting disease-resistant varieties will minimize disease problems. Proper crop rotations will also reduce the likelihood of some diseases occurring in the future.

Scouting provides the information on diseases present, their severity, and potential crop loss if untreated. Scouting of fields for disease should be undertaken weekly from crop emergence until maturity. The information gathered can be used to justify the use of fungicide application for disease control. It will also prevent the unnecessary use and expense of fungicide application or ensure that a fungicide application is made when it will provide the greatest economic return. Growers should have an organized method of scouting fields and should take the time to learn how to identify disease problems correctly. Fields should be monitored throughout the season to detect any developing problems. Learning about weather conditions that favour diseases will also help growers know when to scout fields or to apply fungicides.

To justify disease control measures, identify any diseases present and determine the severity of each. Your agricultural representative can provide further information on disease identification, scouting techniques, and a list of resistant varieties suited for your production area.

Take care to identify the diseases correctly so that the proper fungicide can be selected.

To scout fields for disease:    

  1. Review the field history. Identify any fields that have had chronic disease problems. Ensure that you check for these diseases when scouting.
  2. Scout weekly. Check the most disease-prone locations first. Foliar diseases tend to be worse in thick growth. Row ends, where seeding and fertilizer rates are inadvertently high, often have the heaviest disease pressure.
  3. Scouting requires examination of 5-10 randomly selected sites that represent the character of plants within the field. Closely examine the leaves of each of the plants selected. The use of a magnifying glass or hand lens may be particularly useful, especially when first beginning to scout.
  4. Identify all diseases present as soon as possible. Some diseases increase quickly (e.g. rust) and must be dealt with promptly. Multiple diseases sometimes require different control measures.
  5. Map the areas where diseased plants are found to determine where treatments should be applied, to monitor any disease increase, and to assist in planning future crop rotations.

If you are not sure you have properly identified a disease, collect 1 or 2 plants (roots, plant, and soil) and take them to your provincial agricultural representative or an agronomist. Map any area of the field where the disease was found and make a point to check it again on your next visit.

Keep good records. About one week before harvest, a disease map should be drawn. Draw an outline of the field and circle the trouble spots you have identified. Be sure to label what disease was found in that spot and percent infestation found. Do not try to identify each specific spot you observed but the general area of the field in which you found the disease. This map will help if the same crop is grown in this field again.

The common crop diseases are described on the following pages and are grouped by crop affected.

For further information, contact your GO representative.