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CAUSES OF PLANT
DISEASES
CHAPTER 2
Factors Affecting
Abiotic
Biotic
• Climatic, temperature & other
physical factors
• Insects, nematodes, mites, fungus,
bacteria &virus
Abiotic Factors
Caused by non living factors such as:
Nutritional abnormalities
Pesticide exposure
Environmental pollutants
Extreme weather conditions
1. Nutritional Abnormalities
Often nutrient abnormalities show up as
discoloration of foliage
Common discoloration symptoms include:
Yellowing
Chlorosis
Nutrient Deficiencies
Plants require several major
(N, P, Ca, Mg) and minor (iron,
copper, Zn) elements for
normal growth.
Deficiency or lack of any of
these essential nutrients
results in disease symptoms
in the plant.
The symptoms in this photo
occur due to a lack of
molybdenum (minor plant
nutrient)
Mineral Toxicity
Presence of excessive available
amounts of certain minerals in
the soil can lead to mineral
toxicity to the plants.
The extent of injury depends on
the mineral, its concentration, &
the species of the plant.
The plant in this photo has
received too much copper, which
has proved toxic and led to
yellowing.
2. Pesticide Exposure
Some pesticides, if improperly used, can cause serious
damage to plants.
However, the most common type of chemical injury to
plants is due to soil residues or spray drift of herbicides.
Examples of pesticide toxicity are:
2,4-D damage to beans and tomatoes
glyphosate (Roundup) damage to fruit trees
The common
symptoms of
herbicide exposure
are curling and
cupping
This plant was
exposed to the
herbicide 2,4-D
3. Environmental Pollutants
High levels of fluoride in water or fluorine gas in the
air can cause symptoms like this.
The damage is concentrated towards the margins of
the leaf where fluorine tends to accumulate.
Air Pollution
Certain chemicals, such as O3, SO2, and NO2 are released
into the air from factories, power plants, and automobile
exhausts.
These chemicals can accumulate in the atmosphere
insufficient concentration to cause damage to plants.
Ozone damage appears in the form of chlorosis, spots, and
bleaching of young leaves.
This is common in certain regions of the country where
there is a high ozone concentration in smog
Some of the air pollutants responsible for acid rain cause
damage to vegetation in certain regions.
Ozone Damage on Morning
Glory Plant
Ozone Damage of
marijuana leaves
4. Extreme Weather Conditions
Extremes of weather can also lead to plant injury
Cold injury
Low temperatures, like frost or freeze, can
damage the exposed or sensitive organs
(buds, flowers, young fruits) or may kill the
entire plant.
As the new branch tips began to expand, a
period of cold was severe enough to kill the
growing tips
Heat injury
In this case the temperature was high
enough and damage the plant tissue
When plants or plant parts are exposed to
high temperatures for prolonged periods,
symptoms of scorching or scalding may
develop
High/Low Soil Moisture
Due to excessive watering, poor drainage or flooding may
cause plants to turn yellow and be stunted.
Potted indoor plants may show poor development or root
rots.
At the other extreme, low moisture or drought conditions
can lead to poor development, wilting, and death of plants
High/Low Light Intensity
High light intensity is usually not a problem but low light
conditions, especially for indoor plants, can lead to
etiolation (tissue are yellowish)
A normal and an etiolated shoot under sun and
artificial lighting
Biotic Factors
Caused by living factors such as :
Fungi
Bacteria
Viruses
Nematodes
Parasitic Plants
1. Fungi
Largest pathogen group
More than 8000 pathogenic species
Vegetative growth through
production of hyphae
Reproduce via spores
Fungal Hyphae
2. Bacteria
About 200 pathogenic species
Can be seen with a light
microscope
Simple, unicellular
Reproduce by binary fission
3. Virus
Can only be seen using an electron
microscope
Extremely simple- nucleic acid
with a protein coat
Reproduce by taking over host
reproductive machinery
Often associated with insect
vectors
4. Nematodes
Parasitic worms
Very complex compared to other
pathogens
Usually seen only with a light
microscope
Reproduce by eggs
5. Phytoplasmas
A prokaryotic organism that lack a
cell wall and survive in the phloem
of plant
Round or elongate
Usually seen only with an electron
microscope
Reproduce by binary fission