By Barb Ziesman, AAg, Provincial Specialist, Plant Disease, Regina
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.
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. Pull multiple plants from each site and examine the entire plant for symptoms of the disease, including the roots. 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.
This is the best time to scout for clubroot in your canola fields. Clubroot is a disease that is best managed when detected early, making scouting and monitoring for disease development a critical step in a clubroot monitoring program. When scouting for clubroot, it is important to examine areas where the disease is most likely to occur. These areas include the field entrance, low spots and suspicious patches with above-ground symptoms such as yellowing, wilting and premature ripening. These above-ground symptoms indicate that something is wrong. To determine if clubroot is the cause, it is important to uproot plants and examine the roots for clubroot galls. It is important to also examine the roots of healthy plants at the field entrance, since the roots of the plant can be infected even though there are no above-ground symptoms.
When you uproot plants, examine the roots for swollen tissue (clubroot galls). When intact, the clubroot galls will appear white and fleshy. Later in the season, the galls will start to decompose, releasing the pathogen’s resting spores back to the soil. At this stage, the clubroot galls will appear rotten and spongy. In some cases, the root tissue may be completely decomposed, leaving no intact tissue. When this occurs, dig around the area looking for sponging root tissue or pull other plants in the area to look for intact galls.
When you find clubroot in the field it is important that you determine the distribution of the disease in the area so that you can develop an informed clubroot management plan for the field.
- Walk the area to determine the size of the clubroot-infested area. When you determine the area with symptoms, double the size. This will become the clubroot-infested patch.
- If the patch with symptoms is small, consider pulling plants with clubroot galls and removing them from the field. If possible, burn the infested tissue or dispose of it in a way that has no risk of spreading the disease. Removing the infected tissue will prevent or reduce the risk of increasing the pathogen levels in the soil when the galls decompose.
- Visit other areas in the field to check for clubroot symptoms.
- Check other canola fields on the farm for clubroot symptoms.
- Limit traffic through the clubroot-infested areas.
- Take time to remove soil from and clean all equipment before leaving the field to reduce the risk of moving the pathogen to new areas. When possible, visit this field last and clean equipment before working in other fields.
- Develop a clubroot management plan for the clubroot infested field(s) using The Saskatchewan Clubroot Management Plan as a guide.
For more information on disease scouting, please check out our Plant Disease Scouting 101 factsheet.