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Getting to Know Plant Diseases

By Barbara Ziesman, Provincial Specialist, Plant Disease, A.Ag, PhD

February 2020

This year is the International Year of Plant Health! It's the perfect time to talk about plant diseases, which have the potential to threaten plant health and crop yield if not managed proactively.

Plant diseases can be caused by both biotic (living) and abiotic (non-living) factors. When we refer to plant diseases, we often think about those that are caused by living organisms, such as fungi, bacteria or viruses. However, similar symptoms can also be caused by abiotic factors, such as adverse environmental conditions. It's important that you're able to determine what is causing the disease symptoms.

In Saskatchewan, though viruses and bacteria can be a concern in some situations, most plant diseases are caused by fungi or fungal-like organisms and can be broken into two main groups: monocyclic and polycyclic diseases. Monocyclic diseases are those that only have a single disease cycle per season, whereas polycyclic diseases have many disease cycles per season. Polycyclic diseases, such as stripe rust, can increase exponentially over the growing season, making routine scouting very important.

A disease will only occur when the pathogen is present, the host plant is susceptible and the environmental conditions are conducive. These three factors together make up what is known as the plant disease triangle. The level of disease will increase when each point of the triangle becomes more favourable to disease development. This is a major reason why we often see disease levels fluctuate from year to year. Even if inoculum levels, which are the part of the pathogen that can infect the host (such as the fungal spores), are constant and the same variety is grown there will be some differences in disease levels due to differences in the environmental conditions. As a result, having knowledge of the pathogen's biology, host range and the environmental conditions that favour disease development can improve the management of the disease and assist producers in determining when to use management tools, such as fungicides, to reduce disease levels.

Plant disease triangle
Plant disease triangle.

Once you understand how the disease develops, you can start thinking about disease management. Most diseases are best managed through an integrated approach that involves multiple strategies. Some of the key management strategies for plant diseases are as follows:

  • Extended crop rotations: Diverse and extended crop rotations will break disease cycles and allow pathogen levels in the field to decline before the next susceptible crop is grown. This is a great way to influence the pathogen side of the disease triangle.
  • Genetic resistance: Plant breeders have successfully introduced resistance traits against major diseases traits into many crop varieties. Choosing the variety with the highest level of genetic resistance will effectively reduce disease levels and is a good way to influence the host side of the disease triangle.
  • Fungicide applications: Fungicides can be used to protect the plants from disease development. Fungicide application decisions should be based on disease risk. Scouting and the use of risk assessment tools can help guide these decisions and ensure that fungicides are only used when they are needed and will result in an economic return.
  • Use good-quality seed: This is important to ensure that crops get off to a good start. Ensuring healthy, vigorous plant stands will help reduce susceptibility to diseases, especially early-season and root diseases. Test seed lots for germination and pathogen presence. When seed-borne pathogens are present, you can use seed treatments to protect against seed and seedling diseases.
  • Seeding rate: Seeding rate is a way to influence the environmental side of the disease triangle. Seeding rate can influence the canopy density, which can, in turn, influence the microclimatic conditions.
  • Seeding date: For some diseases, early seeding is encouraged to reduce the likelihood that the crop will be in a susceptible stage when the pathogen is present and environmental conditions will favour disease development.
  • Residue management: For residue-borne diseases, the pathogen survives in infected residue. Fine chopping and other residue management strategies can be used to increase the rate of decomposition and help reduce pathogen levels more quickly.
  • Prevention: This strategy can be used to prevent the introduction of new pests into a field or area. This is an effective strategy to minimize the spread and introduction of soil-borne diseases such as clubroot.

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