General Disease Cycle of Plant Pathogenic Fungi
The disease cycle occurs during the growing season of the host plant. An appropriate host as well
as appropriate environmental conditions must be present for disease to occur.
1. Fungi are spread to an appropriate host through various
mechanisms including: air
transport, water, insects and
humans.
2. Signals from the host as well
as environmental factors (i.e.
moisture, temperature) will trigger the germination of spores
(resting structures).
7. Fungal spores serve
as dormant structures
that help the pathogen
survive through the
winter months when no
host plant is available or
when environmental
conditions are unfavorable.
Spread to Host
Dormancy
6. During reproduction fungi
produce thousands of spores.
These spores act as “seeds”
that allow a fungus to spread
from one host to another and
under the proper conditions
germinate and cause infection.
Growth and
Reproduction
Penetration into Host 3. After germination, the fungus
must enter the plant either
through natural openings or by
penetrating the exterior of the
host plant.
Spores produced
throughout the
growing season are
called secondary
spores causing
infections later in the
growing season.
Infection
Invasion
4. Once inside the plant, the
fungus must come in contact
with susceptible cells for infection to take place. Once infection
occurs, symptoms may be visible
immediately or there may be a
latency period.
5. Once inside the plant, fungi
will generally grow and actively
spread from cell to cell. Some
fungal infections may be found in
one area (localized) while others
will spread throughout the plant
(systemic).
Created by Mary McKellar, Department of Crop and Soil Sciences, Cornell University
Breaking the Disease Cycle
Interruption of the disease cycle of a plant pathogenic fungus at key points can lead to disease control.
Cultural Control Methods
 Make sure that you do not over water and that your plants have plenty of aeration. Wet humid environments provide conditions
suitable for spore dispersal and germination.
 Prevent spread of fungi from one plant to another by using clean tools and containers.
 At the end of the growing season, remove dead plant material that may contain spores that would allow a fungus to survive to
the following growing season.
 Take caution when adding diseased plant material to your home composting system. Home composting systems may not hit
high enough temperatures (122oF) to destroy fungi in diseased plant material.
Chemical Control Methods*
 Protectant fungicides are applied to the foliage of a plant. These fungicides will kill spores as they attempt to germinate (Most
fungicides purchased from garden centers fall under this category).
 Systemic fungicides are applied to a plant after infection has started. These fungicides can penetrate plant tissue and kill the
fungus.
* Always read and follow label directions prior to using.
Fungi Facts

Although most plant diseases are caused by fungi (85%), only a small fraction of
fungi in the environment cause disease.

Many fungi are non-pathogenic or beneficial and are responsible for decomposition and recycling organic matter in the environment.

Plant pathogenic fungi have played a powerful role in human and natural history.
They have caused famines and mass migrations of human populations (Irish Potato Famine caused by the fungus Phytopthora infestans 1840’s). They have
changed the landscape of our country indefinitely (Chestnut Blight caused by the
fungus Cryphonectria parasitica 1904). They have caused the loss of prominent
landscape features in every town (Dutch Elm Disease caused by the fungus Ophiostoma ulmi 1921.

The role of fungi in plant disease has been recorded as far back as 700 B.C. The
Romans would celebrate the holiday “Robigalia” that involved sacrifices of reddish
colored dogs and cattle in an attempt to appease the god that caused rust on their
crops.
Some Disease Symptoms
Caused by Plant
Pathogenic Fungi
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Wilts
Leaf Spots and Blights
Fruit Rots
Root Rots
Damping Off
Seed Rots
Distorted Growth
Rust
Smut
Mildew
Created by Mary McKellar, Department of Crop and Soil Sciences, Cornell University
General Disease Cycle of Plant Pathogenic Bacteria
The disease cycle occurs during the growing season of the host plant. An appropriate host as well
as appropriate environmental conditions must be present for disease to occur.
1. Bacteria are spread to an
unfavorable conditions
(eg. during the winter or
when no host is available). in seeds or tubers,
in infected plant debris or
in the soil.
Dormancy
5. Populations of bacteria can
increase rapidly. Bacteria reproduce by dividing. Under the
right conditions, bacterial cells
can divide every twenty to fifty
minutes. Given this, a single
bacterial cell could give rise to
one million bacterial cells in
less than one day.
Growth and
Reproduction
2. In order for infection to occur, the bacteria most enter the
plant. Penetration of the plant
by bacteria is through natural
openings (eg. stomates) or
wounds.
Spread to Host
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6. Bacteria may survive
appropriate host through various mechanisms including:
water (eg. rain or irrigation),
insects and humans.
Penetration into Host
Infection
3. Once inside the plant, the
bacteria must come in contact
with susceptible cells for infection to take place. Once infection
occurs, symptoms may be visible
immediately or there may be a
latency period.
Invasion
4. Infections by bacteria are generally systemic. Spread of bacteria
throughout the plant is passive. Bacteria take advantage of the vascular
system of plants and move via the
xylem or phloem.
Created by Mary McKellar, Department of Crop and Soil Sciences, Cornell University
Breaking the Disease Cycle
Interruption of the disease cycle of a plant pathogenic bacteria at key points can lead to disease control.

Use only disease free seeds and other propagative plant materials that are certified or from a reputable breeder.

Diseased plant tissue or whole plants should be removed from the garden throughout the growing season in order to prevent bacteria from being introduced from one plant to another.

Use only clean tools and containers to prevent the spread of bacteria from one plant to another.

Take care when watering to prevent water splashing. Bacteria can be spread from one plant to another through splashing of water on foliage. Soaker hoses that supply water directly to the plants roots may help to overcome this problem.

Handle plants to the best of your ability so that they do not become wounded.

Insects may serve as a means of spreading bacteria. Keep an eye out for insects in your garden. If necessary, control
insects that may be potential vectors.

Take caution when adding diseased plant material to your home composting system. Home composting systems may
not hit high enough temperatures (155-165oF) to destroy bacteria in diseased plant material.

Currently there are no chemicals available to home gardeners for the treatment of plant diseases caused by bacteria.
How do humans spread plant diseases?

Using infected seeds, bulbs, tubers, nursery stock or transplants.

Using contaminated containers and tools.

Unintentional or intentional wounding of plants.

Transporting contaminated soil from one field to another.

Excessive handling of diseased and healthy plants. (This is especially true for
some viruses)

On a larger scale, by importing food and other items that may carry plant pathogens.
Some Disease Symptoms Caused
by Plant Pathogenic Bacteria
Bacteria cause many of the same symptoms
as fungi.
 Leaf spots
 Leaf blights
 Soft rots of fruits, roots and storage crops
 Wilts
 Distorted Growth
 Scabs
 Cankers
Created by Mary McKellar, Department of Crop and Soil Sciences, Cornell University
General Disease Cycle of Plant Pathogenic Nematodes (microscopic worm-like organisms)
The disease cycle occurs during the growing season of the host plant. An appropriate host as well as appropriate
environmental conditions must be present for disease to occur.
1. Nematodes are spread to
an appropriate host through
various mechanisms including:
water (rain and irrigation) and
humans.
7. Nematodes will survive
Dormancy
6. A single nematode can
do little damage to a plant
root. However, when
large numbers build up in
the soil over several
growing seasons, many
root areas can be attacked
at once and significant
damage can result.
Growth and
Reproduction
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Spread to Host
the winter and unfavorable
conditions in roots of dormant
plants, in infected plant
debris, in seeds and in bulbs
or as eggs in the soil.
2. Signals from the host will
trigger hatching of nematode
eggs in soil . Nematodes go
through various stages during
their life cycle. Only the second juvenile stage can infect
plants.
Penetration into Host
3. Most nematodes mechanically
penetrate a host using a specialized
part of their mouth called a stylet.
Infection
4. Once penetration occurs, a
Invasion
nematode must come in contact
with susceptible cells for infection to take place. Once infection
occurs, symptoms may be visible
immediately or there may be a
latency period.
5. Nematodes will actively move
from one cell to the next by their
own power. Most infections by
nematodes are localized
(contained within a small area of
the plant).
Created by Mary McKellar, Department of Crop and Soil Sciences, Cornell University
Breaking the Disease Cycle
Interruption of the disease cycle of a plant pathogenic nematodes at key points can lead to disease control.

Crop rotation with plants that are not attacked or only lightly attacked can help control nematodes by limiting their population
growth.
 There are chemicals available for the control of nematodes (nematicides). However, they are extremely toxic and the use of
these chemicals is slowly being phased out. Furthermore, nematicides are not available for home gardening use.
What is a nematode?
Is it an insect, worm or toad? No, nematodes are microscopic wormlike organisms belonging to the Animal kingdom. They reside
primarily in soil. Most are saprophytic feeding on organic matter
while some are pathogens of plants and others are pathogens of
animals.
Companion Planting to Control
Nematode Populations
Several studies indicate that marigolds produce materials that
are toxic to root knot and other parasitic nematodes. Planting
marigolds in spots where nematode populations are high can reduce their number. However, marigolds only reduce populations
around their own roots. For this reason they would not help other
crops in a garden. One approach may be to plant sections of a
garden with marigolds on a rotating basis decreasing populations
of nematodes with each rotation.
Some Disease Symptoms Caused by
Plant Pathogenic Nematodes
Nematodes that attack roots
 Root knots
 Root galls
 Root lesions
 Excessive root branching
 Injured root tips
In addition to above ground symptoms:
 Redcuced growth
 Yellowing of foliage
 Excessive wilting
Nematodes that attack above ground plant parts
 Galls
 Necrotic lesions and rots
 Distortion of leaves and stems
 Abnormal development of floral parts
Created by Mary McKellar, Department of Crop and Soil Sciences, Cornell University
General Disease Cycle of Plant Pathogenic Viruses
The disease cycle occurs during the growing season of the host plant. An appropriate host as well
as appropriate environmental conditions must be present for disease to occur.
1. Viruses are spread to an
appropriate host through two
main mechanisms: insects and
humans.
6. Viruses only survive the winter
or unfavorable conditions in living
plant material such as the tops and
roots of perennials, vegetative
propagating organs (bulbs, tubers)
and some seeds. Some viruses can
survive in their insect vector.
Growth and
Reproduction
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Dormancy
5. Viruses cannot survive outside a living host. They are referred to as obligate intracellular
parasites. Without a host, a virus
is incapable of reproducing. A
virus uses the enzymes and other
molecules in a host cell to create
new viruses. Once new viruses
are produced, they burst from
the cell that they just used and
spread to new cells.
2. The virus must gain entrance
Spread to Host
inside of the plant for the infective
process to begin. Penetration of a
host plant by a virus is through
wounds created by the insect
(vector) that carries the virus.
Penetration into Host
3. Once inside the plant, the
Infection
viruses must come in contact
with susceptible cells for infection to take place. Once infection
occurs, symptoms may be visible
immediately or there may be a
latency period.
Invasion
4. Infections by viruses are generally
systemic. Spread of a virus throughout the plant is passive. Viruses take
advantage of the vascular system of
plants and move via the phloem.
Created by Mary McKellar, Department of Crop and Soil Sciences, Cornell University
Breaking the Disease Cycle
Interruption of the disease cycle of a plant pathogenic nematodes at key points can lead to disease control.

Use disease free seeds, bulbs and tubers. Seeds and other vegetative propagating organs can harbor viruses.

Remove infected plants because they can serve as a source of the virus that can then be spread to other plants.

Insects are the major means of spreading viruses. Keep an eye out for insects in your garden. If necessary, control insects that may be potential vectors.

Smokers wash your hands! The tobacco in the cigarettes that you are smoking may contain Tobacco Mosaic Virus
(TMV). You can transmit TMV to your plants if you do not disinfect your hands after smoking.

No chemical substances are yet available for controlling virus diseases of plants.
Breeding Plants for Disease Resistance
The value of breeding plants for disease resistance was recognized in the early
1900’s. Breeding plants for disease resistance can be as simple as manually selfpollinating a plant or cross pollinating two plants or as complex as introducing
genes into a plant tissue culture using genetic engineering techniques. The use of
resistant plant varieties is the least expensive, easiest and one of the most effective means of controlling plant diseases. Furthermore, planting disease resistant
varieties eliminates the use of toxic chemicals to control disease.
Much success has been achieved in breeding many vegetables and ornamentals
that are resistant to one or more diseases. Less effort has been spent in creating
disease resistant varieties of trees. This is primarily due to the difficulty in quickly
replacing susceptible varieties with resistant ones as well as keeping resistant
ones from being attacked by new races of a pathogen that are likely to develop
over the long life span of a tree. However, considerable effort has been made to
breed clones of the American Elm (Ulmus americana )using Asiatic species that
are resistant to Dutch Elm Disease. Several varieties of American Elm resistant to
Dutch elm disease are now available including ‘Pioneer’ and ‘Heritage’.
Some Disease Symptoms
Caused by Plant
Pathogenic Viruses
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Dwarfing and stunting
Mosaics
Ringspots
Leaf rolls
Yellowing
Streaking
Pox
Pitting of stem or fruit
Flattening and distortion of
stem
Created by Mary McKellar, Department of Crop and Soil Sciences, Cornell University
Sources
General Plant Pathology
Agrios, G. (2005). Plant pathology. (5th ed. ed.). Academic Press.
Fungi
Kendrick, B. (2001). The fifth kingdom. Newburyport, MA: R. Pullins Company.
Sinclair, W., & Lyons, H. (1987). Diseases of trees and shrubs. Ithaca, NY: Cornell University Press.
Plant Pathogenic Bacteria
Fletcher, J. and A. Wayadande. 2002. Fastidious vascular-colonizing bacteria. The Plant Health Instructor. Retrieved from http://
www.apsnet.org/edcenter/intropp/PathogenGroups/Pages/Fastidious.aspx.
Nematodes
Companion planting. (2001). Retrieved from http://www.uri.edu/ce/factsheets/sheets/compplant.html.
Lambert, K. and S. Bekal. 2002. Introduction to Plant-Parasitic Nematodes. The Plant Health Instructor. Retrieved from http://
www.apsnet.org/edcenter/intropp/PathogenGroups/Pages/IntroNematodes.aspx.
Plant Pathogenic Viruses
Gergerich, R.C., and V. V. Dolja. 2006. Introduction to Plant Viruses, the Invisible Foe. The Plant Health Instructor . Retrieved from
http://www.apsnet.org/edcenter/intropp/PathogenGroups/Pages/PlantViruses.aspx.
Plant Pathogens and Composting
Hoitink, H. A. J., and Fahy, P. C. 1986. Basis for the control of soilborne plant pathogens with composts. Ann. Rev. Phytopathology
24:93-114.
McKellar, M.E. and Nelson, E.B. 2003. Compost-induced suppression of Pythium damping-off is mediated by fatty acid metabolizing seed colonizing microbial communities. Applied and Environmental Microbiology 69(1): 452-460.
Created by Mary McKellar, Department of Crop and Soil Sciences, Cornell University