PLSC 260 - Lecture 1

Some Definitions

Phytopathology is the study of plant disease. This includes:

  • Causes of disease

  • How pathogens attack (molecular, cellular, whole plant)

  • Plant response

  • How pathogens move around

  • How environment influences

  • How to manage and reduce effects

Disease is any time a part of a plant is doing something it shouldn’t do (not working quite right) because of a pathogen.

You cannot have a disease without symptoms.

Symptoms are the visible reactions of a plant to disease. May include:

  • Chlorosis; yellowing of tissue

  • Necrosis

  • Gall and Tumours

  • Cankers, lesions, pustules

  • Scabs

  • Wilts; part of the plant is dried/drooping

Healthy plants are those growing to the best of its genetic potential, after considering any environmental restrictions

Diseased plants must exhibit:

  • mulfunctioning cells/tissues by pathogenic or environmental factor that causes symptoms

  • abnormal changes in form, physiology, integrity of the plant

  • changes may result in partial impairment or death of plant or its parts

 

Disease Causes

Disease will result from either abiotic or biotic stresses.

Abiotic Stresses (caused by the non-living environment)

  • Nutrient deficiency

    • Unfortunately, some nutrient deficiencies have similar symptoms. However, symptoms can often determine that it’s an abiotic issue

    • Might not be too little in the soil, but something happening to the nutrients in the soil (too much water, chemical interaction, etc.)

  • Environmental conditions

    • Weather can cause stress: hail, frost, heat.

    • Physical damage counts as a symptom (such as lesions from hail)

    • Frost often causes banding (horizontal stripes of discolouration)

  • Chemical injury

    • Herbicides are designed to kill plants.

    • Insecticides too (wrong surfactants, etc.)

    • Phytotoxicity = the negative effects on a plant from a specific chemical substance (typically used when the effects are unintentional; i.e. on the crop)

 

To diagnosis a plant disease:

The first step is to rule out (or determine) abiotic stress. You’ll need to consider:

  • Crop rotation history

  • Herbicide history (residual pesticides can harm plants)

  • Fertiliser history

  • Crop seeding date

  • Weather conditions

  • Pesticides applied (which products, when, the crop stages, rate)

You should keep track of these things as a farmer.

 

If it’s not abiotic, there are 5 classes of biotic stress

  • Fungi

  • Bacteria and Mollicutes (prokaryotes)

  • Viruses

  • Parasitic higher plants (They don’t substantially exist in Canada. This class won’t discuss them)

  • Nematodes (animals, but cause disease, not direct physical damage like insects)

 

In Western Canada, most of the pathogens we face are fungal. However, dominant class causing issues typically varies by crop

 

Parasites and Pathogens

Parasite = an organism that lives on or in a host that derives its food from the host. It grows at the expense of the plant. Parasitism is the stealing of food done by a parasite. (an organism hitching a ride or using the plant as protection is not a parasite).

 

Pathogen = an organism that causes disease. Removes water or nutrients and inhibits plant development

  • Parasites are considered pathogens. May not only be taking nutrients, but also blocking plant systems or damaging structures

 

Pathogenicity is how much a parasite interferes with a plants’ essential functions (essentially how good it is at causing disease)

  • However! The severity of disease is not just from how much pathogen there. More pathogen doesn’t necessarily produce severe symptoms and little pathogen may cause significant symptoms if the plant is susceptible or conditions are variable. Pathogenicity is caused by a variety of factors, and parasitism is not always the important one

 

Host range = the number of host species a pathogen can use

  • Some pathogens only affect one species/genus/family. This is called a narrow host range

  • Others can affect multiple and is considered a wider host range

Some pathogens can only attack certain organs and/or ages of plants.

  • Some only grow on flowers, roots, leaves, etc

  • Some are vascular (in the xylem or phloem)

  • Some attack seedlings, others only when plant is mature

 

5 Strategies of Pathogens:

  • Biotrophic parasitism.

    • Pathogens use a living plant as a nutrient source

    • Penetrate and absorb nutrients from cells willing killing them

    • Two sub-categories:

      • Obligate parasites live their whole lifecycle inside the host

      • Non-obligate (facultative) parasites live part of lifecycle inside the host and part in dead matter

    • e.g. powdery mildew, rust infection

  • Necrotrophic parasitism

    • Pathogen is killing the plant tissues and then consuming them

    • Pathogen secretes enzymes (and sometimes toxins) to break down the plant’s cells

    • Lives in the dead cell

  • Hemi-biotrophic parasitism

    • Starts as biotrophic then switches to necrotrophic. Depends on the phase of the pathogen’s lifecycle

  • Saprophytic

    • Live on dead organic matter. Hosts don’t die

    • e.g. alternaria black spot, mushrooms

  • Symbiotic (endophytes)

    • These are good parasites! They live in the plant and use its nutrients, but provide other, often greater, benefits

    • e.g. mycorrhizae (for N fixation)

 

A diseased condition requires a host, a pathogen, and contact between them in a conductive environment. This is the basis of the disease triangle (host, pathogen, and environment).

 

Some environments will not see certain pathogens due to heat, cold, humidity, drought, etc. An environment bad for a pathogen is good for a plant.