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GAP Ministries

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Diseases in Plants

A distinct sequence of events occurs in the development of plant diseases. These events are known as the plant disease cycle. This cycle is unique to plants and is part of plant pathology, the study of plant diseases. Following is a very basic introduction to plant pathology and the plant disease cycle. This is not a scientific article but it will help the gardener or small scale farmer to better understand the causes and symptoms of diseases that affects plants. To be able to produce food in a sustainable way any small scale farmer or gardener will have to implement measures that will protect the plants from diseases, and that will prevent the spreading of diseases.
Stoma (Stomata) - A minute opening in the epidermis of plants occurring as a slit between two or more cells.
Pathogen - An organism that cause infectious disease in plants.
Plant Pathology - Is the scientific study of plant diseases caused by pathogens and environmental conditions.
Eukaryotic Cells - Are cells that comprise allmost all of the life kingdoms. They can be easily distinguished through a membrane-bound nucleus.

Types of Plant Pathogens
Viruses - They infect a plant via intracellular means. Viruses gain access to the plant body, infect other plant cells and survive and grow by living of the nutrients of the specific host.
Bacteria - Bacteria are microscopic single-celled organisms that reproduce by cell splitting.
Fungi - Fungi are eukaryotic organisms that may or may not produce reproductive spores
Nematodes - Nematodes are parasitic multi-celled worm like creatures.

The Plant Disease Cycle
The main events of stages comprising the disease cycle include the following: production and dissemination of the primary inoculum, primary infection, growth and development of the pathogen, secondary infection, and overwintering.
Production and Dissemination
The primary inoculum is the part of the pathogen that overwinters (over-seasons) and causes the first infection of the season, known as primary infection. The greater the amount of inoculum and the nearer it is to its host, the greater the potential for a disease epidemic. An example of primary inoculum is the cells of the fire blight bacterium, Erwinia amylovora, that overwinter in infected bark tissue along the margins of cankers. In spring, the bacteria multiply and are spread by rain or insects to suculent tissue of apples or pears where primary infection occurs. Dissemination refers to the spread or dispersal of the pathogen from an inoculum source to a host. Dissemination can occur by wind, splashing rain, insects, infested pruning tools, infected or infested transplants, and other means. Spread can occur over short distances within the tree canopy or from distant sources.
Primary Infection
This process occurs when the pathogen comes into contact with a susceptible host under favorable environmental conditions. Most fungal and bacterial pathogens require free water for spore germination; consequently, infection is favored by prolonged warm, wet periods with high relative humidity.
Growth and Development
The growth and development of a pathogen usually occurs on or within infected plant tissue. Fungi grow and spread within their host by means of mycelium. Most fungi produce spore-producing structures such as pycnidia or perithecia on or within infected tissue. These structures give rise to secondary inoculum (infections) which eventually causes additional, or secondary, infections during the season. Bacteria spread by rapidly increasing in numbers. Spread of bacterial cells usually occurs when fissures or cracks develop on infected tissue exposing the cells (secondary inoculum) to the environment. Each pathogen has it's own optimal conditions for growth and development which usually coincide with optimal condition for infection.

Secondary infection results from spores or cells produced following primary infection or from other secondary infections. The secondary infection cycle can be repeated many times during the growing season. The number of cycles is dependent on the biology of the pathogen and its host and the duration of environmental conditions needed for infection.

Overwintering or overseasoning is the ability of a pathogen to surivive from one growing season to the next.
(Source: Integrated Pest Management Programme North Carolina)

Illustration of a Plant Disease Cycle - Bacterial Leaf Spot
File:Black rot lifecycle.tifSource: wikipedia.org

A healthy plant is always its own best protection against diseases but even healthy plants are exposed to diseases. With pathogens (disease causing organisms) becoming more resistant to pesticides organic treatments and solutions to plant diseases are becoming more relevant for small scale farmers. We are in favor of organic treatment. Ou measuress in dealing with plant diseases are therefore organic in nature.