Bacteria
Mode of infection:
- Unlike fungus, bacteria must find a part of entry into host plant, such as through natural openings like stomata and wounds.
- They move between plant cells and produce pectolytic enzyme to kill or degrade the host cells for food.
- Plant bacterial diseases occur in any place that is reasonably moist and/or warm and they affect all kinds of plants and plant parts.
Laboratory identification of bacteria:
- Morphology studies in size, shape, structure and odour.
- Cell wall chemical composition (Specific staining)
- Ability to use certain nutrients (biolog)
- Enzymatic action (Calalase)
- Pathogenicity to plants (Soft rot test)
Some physiological and biochemical methods of identification:
1) Gram Reaction
- Usually the first step in identifying bacteria
- Gram positive - Cell wall is thick and chemically simple
- Gram negative - Cell wall is thin, complex and multilayered.
- This test is essential for differentiating plant pathogenic bacteria into 2 broad groups based on their rate of decolorisation.
- Gram positive - Retain primary stain
- Gram negative - Decolorise easily
2) Potassium Hydroxide test:
An alternative to gram stain
In alkaline solution (3% potassium hydroxide)
Thin cell walls of Gram-negative bacteria are rapidly disrupted releasing DNA.
Thick cell walls of Gram-positive bacteria do not lyse in 3% KOH.
3) Substrate Utilisation - Biolog
An automated microbial identification system
Utilizes 96-well microplates containing 95 substrates
Types of Bacterial diseases
1) Bacterial Spots
Pathogens:
Pseudomonas mangifera
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| Xanthomonas vesicatori on tomato |
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| Xanthomonas phaseoli on beans |
Symptoms:
Tiny circular to enlarged spots on leaves
In bean - Leaves spots are water soaked and enlarged to angular spots on dying. Pods tend to be sunken reddish lesions and may be encrusted with bacterial ooze. Necrotic spot usually has a yellow halo around them due to toxin secretions. Severe infections would cause blight (defoliation and necrotic tissues fall out can leaed to tattered looking leaves).
Control:
Copper mixes e.g fungicides - Bordeaux mixture, Zineb for young plants and early infection. Antibiotics e.g agrimycin may be used but can be expensive.
2) Bacterial Soft Rot
Pathogen:
Erwinia carotova
Symptoms:
Soft rots on vegetable fruits and fleshy organ. Initially, small water soakspots on leaves, stems or fruits can be seen and enlarges rapidly in size within 1 - 2 days. Affected areas become soft and mushy and the surface is blistered and breaks easily to disintegrated masses or slimy cells. Affected stems or collars of plants will end in the collapse of the above ground parts.
Mode of spread:
The pathogen survive in the soil and on rotting plant materials, carried in soil water and insect bodies and spread to new host through a point of entry such as wounds and insect holes.
Control:
Sanitary of cultural practice materials. Cutting tools should be disinfected and diseased materials should be disposed off by burning. Do not apply fertilizers excessively high in nitrogen. Apply fertilizers high in calcium, phosphorus and potassium so as to help plants strengthen their tissues against easy entry of bacteria.
3) Bacterial Wilts
Pathogens:
Ralstonia solanacearum
Symptoms:
Sudden wilting of plants. Initial drooping of older leaves and chlorosis follow by wilting permanently. Young plants die rapidly. In the cross sections of infected plants, white/cream bacterial ooze is seen or when dipped in water, a bacterial stream is observed to exude from the cut end. The bacteria multiplies in the vessels and the slim plugs restrict the water transport resulting in wilts and death of the plants.
Shape:
Straight to curved rods, they are motile by means of one or many polar flagella. Gram negative.
Mode of spread:
Contamination of cutting tools, diseased materials in soil water etc.
Common Plant Disease Symptoms
Blight- A rapid discoloration and death of twigs, foliage or flowers. Possible casual agents are fungi, bacteria, insects, mechanical injury.
Canker- Dead area on bark or stem, often sunken or raised. Possible casual agents are fungi, bacteria, mechanical injury, heat or cold injury, chemical injury.
Chlorosis- Yellowing of foliage. Chlorosis is so generic that without additional details diagnosis is impossible. Possible casual agents are nutrient deficiency, low or excess light, growth regulator or herbicide injury, natural pigmentation, root dysfunction, soil pH problems.
Decline- Progressive decrease in plant vigour.
Dieback- Progressive dealth of shoot, branch or root starting at the tip. Possible casual agents are fungi, bacterial, virus, water stress, excess soluble salts, high temperature, wind, cold, insect injury/damage.
Gall or gall-like - Abnormal localized swelling or enlargement of plant part. It cold be caused by insects, mites, fungal or bacteria infection, or abiotic disorders caused by mechanical injury, even hormonal disorders.
Gummosis- Exudation of gum or sap. Possible causal agents are fungi or bacteria.
Leaf distortion- The leaf cold be twisted, cupped, rolled or otherwise deformed. Causal agents are fungal, bacterial, viral or other pathogen infection, herbicide or growth regulator injury, insect or mite damage, chemical phytotoxicity, mechanical damage.
Leaf scorched- Burning along the leaf margin and into the leaf from the leaf margin. Possible causes are root and stem dysfunction, water stress, excess soluble salts, herbicide or other chemical injury fungal, bacterial or other pathogenic infection.
Leaf spot- A spot or lesion on the leaf. Possible casual agents are fungus, bacterial or viral pathogens, chemical phytotoxicity.
Mosaic- Varying patterns of light and dark plant tissue. Possible causes include viral infection, reaction to hot or cold water on foliage, chemical phytotoxicity.
Necrosis- Dead tissue. Necrotic areas are also so generic that without additional details diagnosis is impossible.
Stunting- Lack of growth. Possible causes are fungal bacterial, viral or other wilt pathogenic infection, nutrient deficiency, water stress, nematode or insect injury, growth regulator damage.
Wilt- General wilting of plants or plant part. Fungal, bacterial, viral or other wilt pathogens, root rot or canker pathogens, water stress, excess soluble salts, high temperatures, wind, cold insect injury.
Witches broom- Abnormal broom-like growth of many weak shoots. Fungal, bacterial, viral infection, mite infestation, micronutrient deficiency, herbicide or growth regulator phytotoxicity.
Common Signs of Biotic Factors on Diseased/Damaged Plants
Signs showing presence of the actual organisms causing the disease. Example:
Fruiting bodies- Reproductive structures of fungi which could be in the form of mushrooms, puffballs, pycnidia, rusts or conks.
Mildew- Whitish growth produced by fungi composed of mycelium.
Mushrooms- Fleshy reproductive structures of fungi
Mycellium- Thread-like vegetative growth of fungi.
Rhizomorphs- Shoestring-like fungal threads found under the bark of stressed or dying trees caused by the Armillaria fungi. They may glow.
Slim Flux or Ooze- A bacterial discharge that oozes out of the plant tissues, may be gooey or a dried mass.
Spore masses- Masses of spores.
Insects and /or their frass (excrement) are also signs, although not signs of diseases.
Some examples of fungus and bacterial infection:
Host plant: Plumeria rubra
Signs and symptoms: The specimen has spots with dark ring on the upper leaf surface.
brown spots can be seen on the lower surface of the leaf as well. No odour. It is probably a fungus infection.
Host plant: Piper sarmentosum
Signs and symptoms: The specimen has black spots on it. The veins near the black spots has turned yellow. The spots are dry and odourless. It is probably a fungus infection.
Host plant: Costus
Signs and symptoms: The specimen is covered with a black layer on the upper surface of the leaf. Gray spots can be seen. There is no odour. The black substance can be scraped off. It is probably a fungus infection, sooty mold.
Host plant: Sanseveria sp.
Signs and symptoms: The specimen has turned squishy and slimy. When being cut across, the wound releases wet substance with odour. As there is no ringspots, it is probably a bacteria infection.
Potassium Hydroxide (KOH) solubility test:
Stringing effect, long viscous strand. Gram negative
Gram stain test:
Pink. Gram negative.
Catalase test:
Bubbles produced.
It is an obligate aerobic bacteria. The catalase enzyme converts hydrogen peroxide to H2O and )2. O2 is released.
Host plant: Pear fruit
Signs and symptoms: The entire specimen has turned black and squishy with an odour. White substance can be seen on the specimen under the magnifying glass. When being cut opened, liquid substance is released with an odour. It is probably a bacteria infection.
Potassium Hydroxide (KOH) solubility test:
No stringing effect. Gram positive.
Gram stain test:
Blue. Gram positive.
Catalase test:
No bubbles produced. Anaerobic bacteria does not produce O2. Hence no bubble is released.
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