Practical 7 - Phytobacteriology

INTRODUCTION

Bacteria is a group of microorganisms that are classified as fungi. They are small and can  only be seen clearly using the highest magnification of a light microscope and objective lens with oil immersion.

Bacteria can be divided by shape into 3 main types which are coccus, bacillus and spiral. However, most plant pathogenic bacteria are rod shaped except Steptomyces (filamentous). 

           

They reproduce by binary fission that occurs every hour, after 24 hours, 1 bacterium cell could reproduce approximately 16,800,000 new bacteria cells. Their growth in a group of bacteria cells that comes from a single cell is term as colony.

1) MORPHOLOGY

(a) Shapes of bacteria.

 

  

(b) Shapes of bacteria colonies produced on NA using dilution streak plate technique.

(c) The color of Gram negative bacteria (red) and Gram positive bacteria (purple)

Gram negative bacteria (red)

Gram positive bacteria (purple)

2) DISEASE SYMPTOMS

(a) Citrus canker, Xanthomonas axonopodis pv. citri

• Bacteria infecting citrus leaves, branches and fruits
• Symptoms: sunken spot surrounded by yellow halo. Leaves are not wrinkled like those infected by scab.

(b) Bacteria wilt, Ralstonia solanacearum

• Broad host range: chili, brinjal (eggplant), groundnut, okra, tomato, etc.
• Bacteria infecting vascular tissues and causing symptom of overall wilt in host plant.

(c) Soft rot of vegetable, Erwinia caratovora

• Bateria infecting various vegetables including cabage, brassica and carrot.
• Infected tissues turned watery, rot and produce bad odor.

(d) Stem rot of corn, Erwinia chrysanthemi pv. zeae

• Bacteria infecting corn stem, especially at the nod showing symptom of brown and watery rotting.

(e) Black rot of crusifer, Xanthomonas campestris pv. campestris

• Bacteria infectingvarous crucifer plants such as brassica and cabbage.
• Symptom: dry rot starting from the leaf edge.

(f) Leaf blight of paddy, Xanthomonas oryzae pv.  oryzae

• Symptom: dry spots starting from the edge of leaves due to infection on hydathode openings.

(g) Leaf streak of paddy, Xanthomonas oryzae pv. oryzicola

• Symptom: necrotic narrow stripes parallel to leaf veins.

3) DISEASE CYCLES

Erwinia sp.

Xanthomonas oryzae (campestris) pv. oryzae

Ralstonia (Pseudomonas) solanacearum

Pseudomonas syringae pv. phaseolicola

Xanthomonas axonopodis (campestris) pv. citri

4) BACTERIAL ISOLATION FROM PLANT TISSUES

(a) Direct isolation

• A pen knife with 70% alcohol or 10-20% chlorox and canker spots was cut from diseased tissues.
• The cut tissues was surface sterilized and was placed on the provided NYGA or NA media.
• The plates  was incubated at 27-30 'C for 24-48 hours.

(b) Dilution streak

• Some of the cut out diseased tissues was crushed from (a) above in distilled water for 10 min.
• The provided Petri plates containing appropriate  medium. 

(5) DIAGNOSTIC TEST USING CUT PLANT TISSUES