Disease Cycles

By Mary McKellar, Department of Crop and Soil Sciences, Cornell University

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.
7. Fungal spores serve as dormant structures that help the pathogen survive through the winter months when no host is available or when environmental conditions are unfavorable.	*Spores produced throughout the growing season are called secondary spores causing infections later in the growing season.*		2. Signals from the host as well as environmental factors (i.e. moisture, temperature) will trigger the germination of spores (resting structures).
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.			3. After germination, the fungus must enter the plant either through natural openings or by penetrating the exterior of the host plant.
	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).	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.

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 (122°F) 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 1840s). 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

Wilts
Leaf Spots and Blights
Fruit Rots
Root Rots
Damping Off
Seed Rots
Distorted Growth
Rust
Smut
Mildew

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 appropriate host through various mechanisms including: water (eg. rain or irrigation), insects and humans.
6. Bacteria may survive unfavorable conditions (eg. during the winter or when no host is available). in seeds or tubers, in infected plant debris or in the soil.		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.
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.		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.
	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.

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

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 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.
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.			3. Most nematodes mechanically penetrate a host using a specialized part of their mouth called a stylet.
	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).	4. Once penetration occurs, a 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.

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.

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 1840s). 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 Nematodes

Nematodes that attack roots

Root knots
Root galls
Root lesions
Excessive root branching
Injured root tips

In addition to above ground symptoms:

Reduced 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

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.

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.		2. The virus must gain entrance 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.
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.		3. Once inside the plant, the 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.
	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.

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 1900s. Breeding plants for disease resistance can be as simple as manually self-pollinating 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

Dwarfing and stunting
Mosaics
Ringspots
Leaf rolls
Yellowing
Streaking
Pox
Pitting of stem or fruit
Flattening and distortion of stem

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.