Plant Pathogenic Bacteria & Phytoplasmas Notes

Plant Pathogenic Bacteria & Phytoplasmas

• Plant & Environmental Health, PLNT2011, Dr Anthony Young.

Bacterial Overview

• Bacteria are among the oldest life forms, appearing approximately 3.5 billion years ago, which is about 3/4 of the Earth's lifespan.
• They inhabit most environments and are the most metabolically diverse life forms.
• Eukaryotic cells contain bacterial components such as chloroplasts and mitochondria.
• All other life forms depend on bacteria, which are essentially everywhere and essential.

What are Bacteria?

• Illustrates the position of bacteria relative to other kingdoms of life, including plantae, fungi, and animalia.

Bacterial Numbers

• Bacterial numbers are huge, approximately 10^{10} colony forming units.

Cell Division (Binary Fission)

• Bacteria reproduce through binary fission.
• Process:
  • A circular DNA strand replicates.
  • The cell membrane separates the cytoplasm.
  • A dividing cell wall is produced.
  • Complete division results in two daughter cells.

Diagnosing Bacterial Diseases

• To diagnose bacterial diseases:
  • Exclude other agents like viruses, fungi, and physiological factors.
  • Identify bacterial presentations such as spots, lesions, and blights.
  • Look for ooze.
• Field observations may be useful, but accurate diagnoses require specimens in a laboratory.

Ooze

• Checking for ooze is an important step in identifying bacterial diseases.

Types of Bacterial Diseases

Blights/Spots

• Can be vascular or topical, potentially leading to scorching. Spread by spatter of EPS (Extracellular Polysaccharides)
• Entry into the plant often occurs at stomata.
• Examples:
  • Psuedomonas savastanoi pv. phaseolicola: Halo blight of legumes.
  • Curtobacterium flaccumfaciens subsp. flaccumfaciens: Tan spot of legumes.

Specific Examples of Blights/Spots

• Pumpkin: Angular leaf spot caused by Acidovorax avenae subsp. citrulli.
• Tomato: Bacterial black spot caused by Xanthomonas vesicatoria.
• Green bean: Halo blight caused by Pseudomonas savastanoi pv. phaseolicola.
• Lettuce: Varnish spot caused by Pseudomonas cichorii.

Wilts

• Typically vascular diseases involving the plugging of vascular bundles.
• Often asymmetric and progressively systemic.
• Examples:
  • Ralstonia solanacearum: Bacterial wilt, Moko.
  • Candidatus Liberibacter: Greening, zebra chip.

Specific Examples of Wilts

• Banana: Banana Xanthomonas wilt caused by Xanthomonas vasicola pv. musacearum.
• Eggplant: Bacterial wilt caused by Ralstonia solanacearum.
• Potato: Bacterial wilt caused by Ralstonia solanacearum.
• Dieffenbachia: Anthurium wilt caused by Xanthomonas dieffenbachiae.

More Examples of Wilts

• Banana: Banana Xanthomonas wilt caused by Xanthomonas vasicola pv. musacearum.
• Eucalypt: Bacterial wilt caused by Ralstonia solanacearum.
• Potato: Brown rot caused by Ralstonia solanacearum.
• Potato: Bacterial wilt caused by Ralstonia solanacearum.

Tumours/Galls

• Affect parenchyma tissues, often involving genetic interactions, leading to decreased useful growth.
• Can be beneficial in some cases.
• Examples:
  • Agrobacterium tumefaciens: Crown gall.
  • Rhizobacterium spp.: Root-nodule formation.

Specific Examples of Tumours/Galls

• Pumpkin: Skin galls caused by an unidentified bacterium.
• Macadamia: Root gall of unidentified aetiology.
• Passionfruit: Stem galls caused by Rhodococcus sp.
• Grape: Crown gall caused by Agrobacterium tumefaciens.

Soft Rots

• Systemic or localized putrefaction of tissues, often affecting tubers or rhizomes.
• Can follow pest damage.
• Examples:
  • Pectobacterium carotovora: General soft rots.
  • Dickeya zeae: Pineapple ghost rot.

Specific Examples of Soft Rots

• Avocado: Subcutaneous soft rot caused by Pectobacterium sp.
• Pumpkin: One spot caused by an unidentified bacterium.
• Potato: Bacterial soft rot caused by Pectobacterium atrosepticum.
• Pineapple: Ghost rot caused by Dickeya zeae.

Diagnosing Bacterial Diseases: First Steps

• Host:
  • Identify the plant.
  • Describe the symptoms.
• Environment:
  • Assess if the plant is suitable for the area.
  • Determine if the prevailing conditions are usual.
  • Check if the area has a history of any given disease.
• Pathogen:
  • Determine the type of pathogen (e.g., bacterial, fungal).
  • Check if it is known to occur on this host.
  • Check if it is known to occur in this area.

Control of Bacterial Diseases

• Control is usually difficult, requiring a combination of measures:
  • Exclusion by quarantine: Use of clean planting material and equipment sanitation.
  • Eradication: Examples cited include Citrus canker eradication from Cape York Peninsula.
  • Prevention: Use of resistant cultivars, drip irrigation, wind breaks, hot water treatment of seed, etc.
  • Chemical control: Seed surface sterilization with sodium hypochlorite or hydrochloric acid; copper compounds as sprays on foliage.

Bacterial Wilt (Ralstonia solanacearum) in the Philippines

• Scoping study revealed issues:
  • No (or not enough) clean seed.
  • Contaminated water used for irrigation.
  • Contaminated soil.
  • Poor rotation selection.
• Research aimed at:
  • Grower education.
  • Production of clean seed.
  • Soil remediation through organic amendments (e.g., radish, sunflowers).
• Local officials both key asset and sometimes constraint to adoption.

Factors Promoting Bacterial Wilt in the Southern Philippines

• Infected seed.
• Alternate hosts.
• Pump (likely referring to irrigation systems spreading the pathogen).
• R. solanacearum itself.

Comic Strip: Bacterial Wilt Management

• A comic strip in a local language (likely Visayan or Tagalog) about managing bacterial wilt in potatoes. The characters discuss biofumigation, clean seeds, and other management practices.

Huanglongbing (Citrus Greening)

• Destroying citrus industries worldwide.
• Caused by a phloem-limited bacterium.
• Considered worse than citrus canker.

Signs & Symptoms of Citrus Greening

• "Blotchy Mottle" is a characteristic symptom.
• Caused by incomplete (asymmetric) vascular infections.
• Best symptom expression in Spring & Summer.
• Excellent sign for HLB (Huanglongbing).

Blotchy Mottle

• The bacterium is distributed unevenly within:
  • Phloem vessels.
  • Leaves.
  • Twigs.
• Not all phloem, twigs, branches, or trees are infected, which is a problem for surveillance.

Lerps

• Nymphs are protected by polysaccharide-wax ‘lerp’.
• Size, shape and aspect of lerps can aid identification.
• Over 300 species in Australia (with many not described).

Pathogen Environments

• Ca. Liberibacter must live with:
  • Plant cell constituents.
  • Psyllid gut constituents.
  • Other microbes.
• Genomes/proteomes of plant, psyllid, and Liberibacter.

Genomic Reductions

• Genome size is reduced in response to intimate interactions.
• Supply of metabolites relaxes selection pressures for endogenous pathways.
• Differential loss of genes may indicate evolutionary pathways.

Simplified Example of Genomic Reduction

• If Ca. Liberibacter has a long evolutionary association with a plant, it may lose genes encoding metabolic pathways of necessary metabolites provided by the plant.
• The portion of the genome encoding plant-provided metabolites decreases over time/selection, leading to genome reduction.

Loss of Vector-Provided Pathways

• Genes encoding vector-provided pathways will also be lost over time/selection, resulting in a smaller genome (e.g., 1.2 Mb genome).

Conclusions

• Bacteria are critical to all life.
• They can cause problems when they jump hosts.
• Most diagnostics can be achieved through molecular biology.