Plant Pathology
Topic 2043
By Katie Wagar
Contents
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Introduction
Damage caused by plant pests
Plant Disease ID
Plant Pathology
Nematodes
Plant Pathology Introduction
• Plant diseases are important to humans,
because they cause damage to plants and
plant products.
• The yield and quality of plants are reduced
by a wide array of plant diseases
– A plant disease is the complex of symptoms
caused by a pathogen on a plant.
Plant Pathology Intro
• A plant pathologist is a person who studies
plant diseases and works to diagnose and
control them.
– Some plant diseases are easily controlled by one
or another method.
• For other plant diseases, however the cost of
control is as high or higher than the
expected value of the crop.
Questions?
DAMAGE CAUSED BY PLANT
PESTS
• Plant diseases can be caused by fungi,
bacteria, viruses, nematodes, insects and
several other organisms. These are called
pathogens. Various organisms can transport
(vector) pathogens from infected to healthy
plants.
– As agriculturalists, we are concerned with these
organisms because of the damage they do to
crops.
Damage caused by plant pests (plant
pathogens)
Plant pathogens can cause various symptoms to appear on affected
plants
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Dwarfing of growth
Yellowing of foliage
Leaf spotting
Blasting of grain heads
Stem cankers
Fruit rot
Seed decay
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Damping off
(destruction of
seedlings near the soil
line)
Wilt
Defoliation
Root rot
Galls
Causes
• The organisms that cause these symptoms
include fungi, bacteria, nematodes and
viruses.
Fungi
• Fungi are microscopic plants that lack
chlorophyll and conductive tissues.
– Fungi produce diseases like stem rust, corn
smut, powdery mildews, brown rot and damping
off.
– Fungi reproduce mainly by means of spores.
– Fungi are particularly damaging to plant
propagation operations.
Bacteria
• Bacteria are microscopic, single-celled
organisms.
– They cause diseases such as galls, leaf spots, soft
rots, scabs and systemic disorders.
– Bacteria are a significant cause of plant disease,
because they can multiply very rapidly when
proper environmental conditions are present.
– Bacteria are as damaging to plant propagation
operations as are fungi.
Viruses
• Viruses are pathogenic particles that infect
most higher plants and animals.
– In plants they cause such symptoms as stunting,
leaves with yellow mosaic patterns, flower break
and vein clearing (veins are chlorotic--i.e., lack
green color; without chlorophyll).
– Virus can multiply only in living cells.
– Nematodes are very small round worms
belonging to the animal phylum Nemata.
Questions?
Plant Disease Identification
– Beginning students may not be able to accurately
identify the exact pathogen causing the plant
symptoms.
– Much experience is needed to become an expert
plant pathologist.
– However, the ability to collect samples, observe
symptoms and record observations will be an
excellent experience for a beginning student.
Questions?
Plant Pathology
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Affect plants
Are Parasitic
Types of pathogens
Controls for Bacterial
infections
• Plant Breeding
• Examples of Bacterial
plant diseases
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Fungi
Examples of Fungi
Viruses
Other methods of
control
• Chemcial substances
Affect Plants
• There are many ways in which plant disease
pathogens can affect plants
– They can suppress the chlorophyll content.
– They can reduce the leaf area.
– They can curb the movement of solutes and
water through the stems.
– They sometimes reduce the water-absorbing
capacity of the roots.
Affect Plants
– They suppress the translocation of
photosynthates away from the leaves.
– They sometimes promote wasteful use of the
products of photosynthesis as in the formation
of galls.
Are Parasitic
• Most pathogens are parasitic - they invade the
host and obtain food from it.
– Many are submicroscopic, making identification
difficult.
– By definition, plant pathogens are capable of
spreading from one host to another.
– The most important plant pathogens are bacteria,
fungi, viruses, and nematodes.
– Most of the plant pathogens have a saprophytic (living
in dead or decaying organic matter) existence apart
from their host plants during most of the year.
Types of Pathogen
• Bacteria are small, single celled, microscopic
organisms.
– Of the 1600 known bacterial species about 200
have been found to cause plant disease.
– Most plant infecting bacteria are rod-shaped and
most have thread-like structures (Flagella) that
propel them through liquids.
Types of Pathogen
– Anything that moves and comes in contact with
bacteria may spread them to other areas.
•This includes farm equipment, rain, plant material,
seeds, birds, insects, nematodes, and people.
•To start an infection in a plant they must enter a
natural opening or a wound..
Types of Pathogen
– Bacteria are significant as pathogens because of
their ability to multiply rapidly.
•They divide by binary fission. (One bacterium
divides in half and becomes two bacteria.)
•Under proper environmental conditions this division
takes place every twenty minutes.
•At this rate 1 bacterium can give rise to 17 million
cells within 12 hours if food, moisture, and
temperature are favorable.
Types of Pathogen
– Bacterial diseases can be seen as galls, leaf spots,
soft rots, scabs and systemic disorders
Controls for Bacterial Infections
• Controls for bacterial infections include the
use of antibiotics, Bordeaux mixtures, and
fixed coppers.
Plant Breeding
• Plant breeding has produced many plant
varieties that are resistant to bacterial
infection
Examples of Bacterial Plant
Diseases
• Examples of bacterial plant diseases are
crown gall, fireblight, walnut blight, deep
phloem canker of walnuts, soft rots of
vegetables, and bacterial wilt of cucumbers.
Fungi
• Fungi are small, usually microscopic, plants
that lack chlorophyll and conductive tissues.
– Unlike green plants, they do not photosynthesize
their own food, so they depend on living or dead
plant or animal tissue.
– Only about 8,000 fungi species are known to
cause plant diseases of the 100,000 species on
earth.
Fungi
– All plants can be attacked by some type of fungi.
– Each of the parasitic fungi can attack one or
many kinds of plants.
– Some fungi grow and multiply by living on their
host plant during their entire life.
– Other fungi can multiply on dead organic
matter as well as living on plants.
Fungi
– Fungi reproduce mainly by means of spores.
•These spores are special reproductive bodies made up
of one or a few cells.
•The spores perform the same job as seeds in higher
plants.
•Some fungi produce up to 5 types of spores to
complete a single life cycle.
Fungi
•Dissemination of fungi can be by wind, rain, insects,
irrigation or flooding, contaminated seed, infected
plants, animals, tillage equipment and pruning shears
and knives.
•A few have motile spores and others can grow to
neighboring plants by their hyphae (thread-like strands).
•The vegetative body (Mycelium) of fungi is made up of
very small filaments or threads called hyphae.
– These branch and grow in all directions through their food
supply.
– They absorb food from the cells of their host plants.
Fungi
•Fungi are controlled by several methods.
– Chemical sprays (fungicides) have been used with success
for many years.
– Soil pasteurization and the use of fumigants works well in
some instances.
– The development of resistant species and cultivars
continues to show success and more promise.
– Crop rotation, good soil drainage, proper handling of the
crop, and low temperature storage are all helpful in
controlling various fungi.
Examples of Fungi Diseases
• Examples of fungus diseases are stem rust of
wheat, corn smut, powdery mildews, rusts,
brown rot, damping off, and Dutch Elm
disease.
Viruses
– Viruses are pathogenic particles that infect most
higher plants and animals.
•Virus particles are extremely small (20 to 250 millimicrons).
•Viruses are not cells, nor do they consist of cells.
•They can be seen only with an electron microscope.
•They cannot grow or multiply except when they are within a
host cell or insect vector cell.
Viruses
•These particles move from one plant to another by
vectors (carriers).
– Also virus is spread to new plants when any asexual method
is used for propagation (once virus is in the mother plant,
all plants started from it will likely have virus).
– Most commonly aphids, leafhoppers, thrips, or mites are
the vectors.
Viruses
•Viruses can often be identified by their symptoms:
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Mosaic patterns on leaves.
Yellows (leaves lacking chlorophyll).
Stunting.
Ringspots.
Flower break.
Vein clearing (veins are chlorotic).
•These symptoms may be accompanied by other
symptoms elsewhere on the same plant.
•The best way to control a virus is to keep it out of an
area through quarantine, inspection, and
certification programs.
Other Methods
– Other methods of control include:
– Use virus free tubers, budwood, scionwood and other
propagating materials.
– Breeding resistant varieties.
– Eradication of diseased plants.
– Controlling insect vectors is rarely effective.
– Removal of weeds which serve as virus hosts.
Chemical Substances
• As yet, no chemical substances (viricides) are
available for controlling virus diseases.
– Examples of virus diseases are Tobacco mosaic,
Curly Top of Sugar Beets, Barley Yellow Dwarf,
Necrotic Ring Spot of Stone Fruit, Tristeza
Disease of Citrus, and Blackline of Walnuts.
Chemical Substances
– Nematodes are plant parasites belonging to the
animal kingdom that are studied in plant pathology.
•Nematodes are small round worms (1/64" to 1/8").
•They are, in general, eel-shaped, round in cross section,
with smooth unsegmented bodies.
•They have no legs or other appendages.
•Plant parasitic nematodes characteristically have a stylet
(spear) as a feeding apparatus.
•Their method of feeding is to puncture the cell with their
stylet, secrete fluids (saliva) into the cell, then withdraw the
cell contents.
Chemical Substances
– They are spread by any way that soil is moved
(e.g., farm equipment, water, animals, wind,
nursery plants). Major damage to plants occurs
from nematodes feeding on roots.
– The above ground symptoms shown by a plant
infected with nematodes is not very specific.
•Reduced growth, nutrient deficient leaves (yellow),
excess wilting are symptoms.
•Reduced yields and poor quality products are the
results.
Chemical Substances
– Below ground symptoms are more distinctive;
these include the following:
•Galls produced on roots and tubers.
•Generally a lack of feeder roots.
•Root lesions.
•Injured root tips.
•Excessive root branching (sprangling).
Chemical Substances
– Control measures are very seldom complete,
usually only reducing populations.
– Controls that are successfully used include:
•The use of fumigants.
•Use of resistant varieties.
•Crop rotation to non host plants.
•Summer fallow.
Chemical Substances
– At present all chemicals must be used pre-plant;
none that are available for use today can be used
after the crop is planted (post-plant).
•Most commonly found nematodes include
Root-Knot, Root Lesion, Cyst, and Burrowing
Nematode:
Questions?
Nematodes
COMMON
NAME
SCIENTIFIC
NAME
Cyst Nematodes
Heterodera
Root Lesion
Nematode
Pratylenchus
Root Lesion
Nematode
Pratylenchus
Burrowing
Nematode
Radopholus
PRINICPAL HOST
Small grains, corn, carrot,
crucifers, soybean, pea, hop,
wheat, irish potato, tomato,
eggplant
1700 hosts including: forage, small
grains, grasses, fruit, vegetables,
field crops, nursery, weeds
Potato, Corn, peanut, cotton,
pineapple, avocado, tobacco, olive,
apple, wheat, oats, strawberry,
tomato, lily, walnut, grape, fig,
citrus, rose
Citrus, Ornamentals, avocados
Nematodes
– Nutrition affects the rate of growth and the state of
readiness of plants to defend themselves against
attack by pathogens.
•High nitrogen fertilization increases the susceptibility of
some plants to bacterial and fungal diseases.
•Proper nutrition of phosphorous, potassium, calcium,
and micronutrients have been shown to aid a plants
resistance to certain diseases.
•In general, plants receiving a balanced nutrition, are
more capable of protecting themselves than plants with
either excessive or deficient amounts of nutrients
Questions??