PLSC 260 - Lecture 3

Bacteria

Overview + Anatomy 

  • cause fewer diseases than viruses or fungi but the diseases are often more severe

  • Either Proteobacteria (purple bacteria) or Gram-positive bacteria

  • Prokaryotes. (Meaning they’re: single celled, genetic info not membrane bound, so no defined nucleus, mitochondria, or chloroplasts. Have cytoplasm with DNA and ribosomes. Use asexual reproduction

  • Shape 4 possible shapes: spheres (cocci), pleomorphic rods (irregular), spirals (spirillium), bacilli (rods). Bacilli is most common for plant pathogens

  • Chromosomes are coiled

  • Sometimes have appendages: flagella (tail), fimbriae, pili (little hair-like structures)

  • Cell walls and then sometimes a thick gummy material. If it’s thin it’s called a slime layer. If thick it’s called a capsule

  • Groups are called biofilms, which can be large enough to be visible

 

Categorised into:

Gram-Positive = one layer of cell wall and no outer membrane

Gram-Negative = two layers of cell wall and an outer membrane layer

Staining a bacteria will determine which it is.

·       If gram-positive they’ll turn purple (stain gets into cell)

·       if gram-negative they’ll turn pink (stain can’t get into cell)

o   Proteobacteria are gram-negative which is counter-intuitive since they’re called “purple bacteria”

Reproduction

  • asexual via binary fission (one cell will split itself into two)

  • Horizontal DNA tranfer (from one cell to another)

  • Plasmids also help swap DNA without using chromosomes

Gram-Positive

  • spread easily. Windblown soil and sand (cauding wounds) or water droplets

  • Need to enter a plant via wounds, vectors (insects), or in/on seeds

  • Prefer humidity

Examples:

·       Bacterial Blight of Soybean

·       Bacterial Leaf Streak (Black Chaff)

o   spread by rain splash, irrigation, plant-to-plant contact (sometimes changing seeding rate can help this), insects (sometimes)

·       Goss’s Wilt

o   Mostly in corn

o   can overwinter in crop residues or soil

o   enters via plant wounds

o   rain and wind driven infection

·       Fireblight

o   apples and pear crops

o   overwinters on branches

o   infection through open flowers

o   pollinators spread disease (but you need them for pollination too, so you can’t just kill them off)

·       Various blights and wilts

 

Black Leg in potato

-              several bacteria species

-              gram-negative, necrotrophic, non-sporing, rod-shaped

 

Generally by looking at these examples, wind and water are key factors in spreading bacteria along with wounds as points of entry

 

Mollicutes

  • These are a subcategory of proteobacteria and a major group of gram-negative bacteria

  • Phytoplasmas and Spiroplasmas are the important genera

Physiology

  • cell membrane, but no wall

  • Very small

  • 4% of Phytoplasmas and Spiroplasmas are disease-causing and are responsible for 700 plants diseases

  • Phytoplasmas are pleimorphic or filamentous in shape

  • Spiroplasmas are helical in shape (like a coil)

Location

-              Most common in tropical/subtropical areas

-              CAN ONLY LIVE IN PHLOEM. (or insect vectors) However, symptoms can be elsewhere

-              Can’t be cultured

Reproduction

-              While inside the host, they use binary fission or budding

-              Spread mostly by insects

-              Phytoplasma’s cannot be cultured (thus they fails Koch’s postulates: see below)

 

Examples

Corn Stunt

  • Not in Canada because we don’t have corn leafhoppers (an insect)

  • Economically very bad for corn

Citrus Stubborn Disease

  • Reduce fruit production by up to 50%

  • Also in some Brassicas

  • Spread by multiple leafhopper species

 

Koch’s postulates

These were criteria for establishing a microbe as causing a disease

1.        Where the organism is, disease is always also present

2.        Must be able to isolate and culture the organism

3.        Must be able to infect a healthy host

4.        Must see the same symptoms in the plant and re-isolate the same organism

Phytoplasma Disease Examples:

Aster Yellows

  • in around 100 plant species (vegetables, field crops, legumes, oil seed, very problematic in canola). Severity depends on crop

  • Sometimes there are different names for disease in different plants even if it’s caused by the same pathogen (Tree peony yellows disease is the same as aster yellows)

  • In canola the symptoms are: phyllody (flowers turned into leaves. slashes yield), purpling pods, blue-green colours

  • In potatos: purple leaves (called “purple top”), small tubers, aerial tubers, odd hair sprouts. this is an issue everywhere

  • Phytoplasma is not entirely bad. Phytoplasma makes poinsettias free-branching, which creates the shape we aesthetically like.

    • If they have Poinsettia mosaic virus (PnMV), the treatment will result in the loss of the phytoplasma too

 

Liberibacters

These are similar to phytoplasma and mollicutes. Few are known, but those that are have very detrimental impacts

  • Huánglóngbìng (citrus greening). 74% decrease of citrus production

  • Potato zebra chip. Ruined flavour, burnt taste when cooked, aerial tubers, stunted plants. Can cause complete field failure

  • Ash trees Cottony ash psyllid (an insect) often carry liberibacter. This is why most of the ash trees have been cut down in Saskatoon in the past decade

Controls

  • A couple bactericides. but hard to use, expensive, lots not approved, not very effective

  • Cultural control. Mostly good irrigation practices (so not overyl moist) and crop rotation. Also disease free propagation materials, certified seed, pruning, sanitation,

  • Some biological control, but rare

Mostly these pathogens are transmitted by insect vectors so in our disease management triangle, we target the vector corner (in theory, targeting one corner should impact the other two). We do this through IPM (will be discussed in later lectures)