Laboratory
Outline
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A symptom is a
visible reaction of a host plant to a pathogen that
results from an abnormal physiological process. Symptoms
can occur as a direct result of pathogen activity, such
as decay due to the release of an extracellular enzyme,
or as an indirect result, such as nutrient deficiency
symptoms resulting from roots that are unable to absorb
nutrients because of infection with a pathogen. The
specific symptoms described below are grouped into
general categories based on appearance of the infected
plant.
1. Color Changes
| a. Chlorosis.
A normally green tissue becomes yellowish. This
is one of the most common symptoms of plant
disease. |
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b. Albinism.
A normally green tissue looses chlorophyll and
becomes white. |
| c. Anthocyanescence.
Plant tissues become purple or red in color due
to the build-up of anthocyanins. |
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d. Silvering/Bronzing.
Plant tissues have a silver- or bronze-colored
metallic luster due to a slight separation of the
upper epidermis. |
| e. Interveinal
chlorosis. Leaf veins remain green
while the area between veins is chlorotic. |
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f. Veinclearing.
Leaf veins become clear or whitish while the rest
of the leaf is green. |
| g. Mosaic.
A variegated pattern consisting of light green,
dark green, and yellow-colored plant tissue.
Mosaics are characteristic of some virus
diseases. |
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h. Mottle. Similar
to a mosaic but the demarcation between adjacent
colors is less distinct. |
| i. Virescence.
Development of chlorophyll in tissues or organs
in which it is normally absent. |
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2. Death of Plant Tissues
| a. Necrosis.
Tissues become brown, black, gray, or tan in
color. Necrotic tissue may be dry and brittle
(leaves) or wet and mushy (roots, tubers, etc.). |
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b. Rot.
Disintegration and decomposition of plant tissues
due to the activity of extracellular enzymes.
Tissue can be soft and wet or dry and powdery. |
| c. Canker.
A localized, sunken, dead area on the trunk,
stems, or twigs of woody plants. |
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d. Blasting. Flowers on
infected plants that are sterile as a result of
disease. |
| e. Mummification.
Transformation of fleshy fruits into dry,
shriveled, hard structures that are highly
resistant to decay. |
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f.
Damping-off.
Death of a plant before (pre-emergence
damping-off) or after emergence (post-emergence
damping-off) of the seedling. |
3. Abnormal Growth Increase
| a. Galls.
Localized swellings or overgrowths on a plant. |
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b. Fasiculation.
Excessive branching or adventitious development
of twigs, roots, flowers, or fruits around a
common point. Also known as witches' broom. |
| c. Phyllody.
Development of leaves at sites from which ovules
normally develop. |
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d. Fasciation. A
flattening and widening of stems, flowers, or
fruits. |
4. Abnormal Growth Decrease
| a. Stunting.
A general reduction in the size of an infected
plant. |
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b. Rosette. A
short, bunchy growth around a common point due to
a failure of internodes to elongate. |
5. Wilting
| Loss of turgidity such that
plants becomes flaccid. |
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6. Defoliation and/or Fruit
Drop
| Leaves or fruits drop prematurely
due to the formation of abscission layers
following infection by a pathogen. |
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7. Replacement of Host Tissue
| Host tissues are colonized and
replaced by pathogen tissues. |
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