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How do plant diseases spread?

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

Plant diseases can cause havoc in crop production systems, resulting in yield and/or quality losses, depending on the disease. The levels of disease present and the corresponding economic losses will fluctuate from year to year and will be influenced by the amount of pathogen (disease-causing organism) present, the environmental conditions and the level of susceptibility of the host plant. These factors make up the disease triangle.

One thing that is consistent across all diseases is that the pathogen needs to be present in the field, or in the area for a disease to occur. If the pathogen is not present, the disease will not occur. Having an understanding of how plant diseases spread and how pathogens move in the environment will be useful in determining how best to prevent the introduction of the pathogen if it is new and is not currently present in the field and will also be helpful in assessing disease risk.

Pathogens can be spread in a number of ways, including spore movement on wind currents, spore dispersal via rain splash, movement in water films within the soil, and within or on the seed. Based on how pathogens are introduced into a field and how they overwinter, plant diseases can be categorized into three main groups (some diseases can be included in more than one category):

  1. Soil-borne diseases;
  2. Residue-borne diseases; and
  3. Seed-borne diseases.

Soil borne diseases

Clubroot galls on canola roots
Small and large clubroot galls on
canola roots

The pathogens that cause soil-borne diseases overwinter in the soil and cause disease below ground. Some soil-borne pathogens can also be seed-borne and be moved between fields via infected seed (such as Fusarium species), whereas others primarily move via the movement of infected soil. Two important soil-borne diseases include aphanomyces root rot, caused by Aphanomyces euteiches, and clubroot, caused by Plasmodiophora brassicae. The pathogens that cause these diseases can be spread in any way that soil can be moved, including via vehicles, field equipment, footwear, soil and wind erosion, and earth tag on seed. Some of this soil movement can be reduced by the implementation of biosecurity protocols on the farm or as part of your daily practices. Minimizing soil movement via wind and water erosion can be accomplished by adopting soil conservation practices such as direct seeding or zero tillage.

An example of a soil-borne disease to take note of is the possible spread of clubroot via earth tag on potato seed or tubers that were grown in clubroot-infested soil. This can represent a long-distance mode of spread for clubroot. As a result, it is recommended that potato producers avoid purchasing seed potatoes from areas where clubroot is known or suspected to occur. This is particularly important for farms where canola or other susceptible crops such as mustard, camelina or brassica vegetables are grown in rotation with potatoes.

Residue-borne disease

Blackleg symptoms in canola
Blackleg symptoms in canola

A number of important diseases fall into this category, including blackleg of canola, mycosphaerella blight, ascochyta blight (both a residue and seed-borne disease) and fusarium head blight. Pathogens can overwinter on infected plant residue from the previous year(s). In the spring, the pathogen will produce spores that can be spread by wind or rain splash (to the host plant or neighbouring fields) to initiate infection. The distance of spread will vary depending on the pathogen and the spore type. Some spores, such as those produced by the stripe rust pathogen, can be moved over extremely long distances, while others, such as those moved by rain splash only, will move very short distances.

For most residue-borne diseases, crop rotation will be an effective way to reduce the amount of pathogen within your fields. Typically the rotations should be extended to give enough time for the infested residue to break down. For fusarium head blight, a one- to two-year break is recommended, while a three-year break (four-year rotation) is recommended for blackleg control.

Seed-borne diseases

Ascochyta blight symptoms on chickpea seed pot
Ascochyta blight symptoms on a chickpea seed pod

For seed-borne diseases, the pathogen will be present on or within the seed. These diseases will be moved between fields and into new areas when you plant seeds that are carrying the pathogen either on the seed surface or within the seed. When the infected seed is planted, the pathogen may result in seed decay (resulting in no germination) or seedling infection through the roots or other parts of the developing seedling and result in seedling death or weak plant with poor vigour.

Examples of diseases that move in this way include smuts and bunts of cereals, and seed decays, seedling blights and root rots (caused by a number of pathogens including Fusarium species, Ascochyta species and Cochliobolus sativus).

Some of these pathogens also cause other diseases and can overwinter in other ways. For example, Fusarium species also overwinter on infected cereal residue and can cause fusarium head blight (spread by rain splash from above-ground residue). When Fusarium-infected seed is planted, it can cause seed and seedling diseases. For fusarium, there is no correlation between seed infection in the year the infected seed is planted and fusarium head blight.The best way to prevent seed-borne diseases is to plant disease-free seed or to use a seed treatment registered to control the pathogen. Purchasing certified seed is one way to restrict the seed-borne diseases, but not all pathogens are monitored as part of the certification process. The best way to know the levels of seed-borne pathogens on or in your seed is to have it tested at a seed testing lab. For some pathogens of pulse and cereal crops, there are general rules of thumb that can be referred to when determining whether a seed treatment is needed.

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