The Importance of Late-Season Disease Scouting and Record Keeping

It may be too late to control the plant disease in your crop this year, but it is not too late to learn from them. The best way to manage field crop diseases is through an integrated approach of cultural control strategies (such as crop rotation), host plant resistance and fungicide application when required. To develop an optimal integrated disease management plan, you need knowledge of the disease risk and field history. You can evaluate disease risk during the growing season by monitoring environmental conditions and scouting throughout the growing season to look for initial symptoms of the disease or signs of the pathogen. You can acquire knowledge of the field’s history through end-of-season disease scouting and accurate record keeping.

Many plant diseases are strongly influenced by crop rotation. Short rotations between susceptible crops increase pathogen levels within the field, as well as the potential for yield and quality loss due to disease. When you document the disease history of the field, you can use crop rotation and other disease management strategies to manage pathogen levels and reduce the occurrence of disease epidemics and substantial yield loss. In addition, you may also find early signs of fungicide-resistant pathogen populations or a breakdown in host plant resistance. For example, high levels of blackleg at the end of a season may indicate that the resistant gene in the canola variety is no longer effective against the blackleg pathogen population present in the field.  As a result, late season scouting and disease severity data will be an extremely valuable piece of information to use to guide blackleg management strategies the next time canola is grown. 

By monitoring the diseases present in your crop you will be able to monitor the differences in disease incidence from year-to-year, but will also be aware of new diseases as they first become present.  This is important for clubroot, a soil borne disease of canola which is best managed through a proactive approach to keep spore levels low or to prevent the introduction of the pathogen. Early detection of clubroot is important to enable proactive and informed disease management decisions and to prevent yield losses.  When scouting for clubroot, it is important to examine the roots for the presence of galls which are characteristic symptoms of clubroot.  In addition to scouting for visible symptoms the presence of the clubroot pathogen in the field can be confirmed through a DNA-based test on soil collected from the field. This method of testing for the clubroot pathogen is advantageous as it allows for the detection of the pathogen at levels lower than those required to cause disease symptoms in the field.  In Saskatchewan, the Ministry of Agriculture conducts a clubroot specific survey that involves soil tested and DNA-based testing for early detection of the clubroot pathogen.  If you would like to have your canola field tested as part of this survey please email pestsurveys@gov.sk.ca

The decision to apply a fungicide for disease control is often difficult. Scouting for disease levels at the end of the season can be a very good way to evaluate your fungicide application decisions; the results can also guide your decisions in subsequent years. This is particularly true if a fungicide-free check-strip has been left in a field. Leaving a check-strip makes it possible to compare the fungicide-treated area to a non-treated area and can be a good indication of whether or not the fungicide application was successful in reducing yield losses. 

When scouting, it is important to look at more than one location within a field. A good rule of thumb is to scout in a “W” pattern and look at multiple plants from a minimum of five sites in fields less than 100 acres and a minimum of 10 sites in fields greater than 100 acres. If patches of the field are demonstrating symptoms of premature ripening, it is important to examine the cause of those symptoms even if these patches fall outside of the sites located in the “W” pattern previously described. 

Pull multiple plants from each site and examine the entire plant for symptoms of the disease, including the roots. In canola, it is important to cut each stem to look for discolouration of the vascular system, a characteristic symptom of blackleg.  The amount of discolouration can be used to determine the blackleg disease severity.  Record what diseases were present, what percentage of plants examined had each disease, how severe the infection was and what plant parts were affected. 

Your scouting kit should include:

  • A magnifying glass;
  • A record-keeping sheet;
  • A digital camera;
  • A small digging trowel;
  • Paper or plastic bags to collect samples in case you encounter a disease or other plant injury symptom that you cannot identify;
  • Plant disease fact sheets or other publications;
  • Disposable plastic boot covers; and
  • A sanitation solution.

For more information on disease scouting, please check out the Saskatchewan Ministry of Agriculture’s Plant Disease Scouting 101 factsheet.