introduction to plant pathology - magadh university
TRANSCRIPT
Introduction to Plant Pathology
Dr. Ravi Kumar Singh
University Department of Botany
Magadh University
Bodhgaya - 824234
What is a plant disease?
• A plant disease is any abnormal condition that alters
the appearance/ phenotype or function of a plant.
• It is a physiological process that affects plant
functions.
• Disease may also affect / reduce yield and quality of
harvested product.
• Disease is a process or a change that occurs over time.
What is a plant disease?
• Symptoms. Any visible / detectable changes in color,
shape, and/or functions of the plant in response to a
pathogen or disease-causing agent is a symptom.
• Signs of plant disease are physical evidence of the
pathogen, for example, fungal fruiting bodies. Signs
also can help with plant disease identification.
What causes plant disease?
• Infectious plant diseases are caused by living
organisms that attack and obtain their nutrition from
the plant they infect. The parasitic organism that
causes a disease is a pathogen.
• Numerous fungi, bacteria, viruses, and nematodes
are pathogens.
• The plant invaded by the pathogen and serving as its
food source is referred to as a host.
Role of the environment
• A favorable environment is critically important for
disease development – even the most susceptible
plants exposed to huge amounts of a pathogen will
not develop disease unless environmental conditions
are favorable.
The Disease Triangle
Plant pathogens - fungi
• Can cause plant, human, and livestock diseases
•Most cannot be seen without a microscope
• Lack chlorophyll
• Composed of growing structure of delicate, threadlike filaments called hyphae
• Reproduce by forming spores
Plant pathogens - bacteria
• Bacteria population can increase in number in short time period
• Cells clump together in masses called colonies
•Obtain food from dead or decaying organic matter or living tissue
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• Spread plant to plant by wind-driven rain
•Gain entrance through natural plant openings or injuries
Plant pathogens - viruses
•Most familiar, causes human and animal diseases such as influenza, polio, rabies, smallpox, and warts
•Ultramicroscopic, measure about one-millionth of an inch in size
• Cause some destructive plant diseases
• Survive only in living cells
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• Transmitted by insects which are called vectors
Plant pathogens - nematodes
• Round, slender, threadlike worms
• Some are parasites on animals, insects, fungi, and plants
• Plant-parasitic nematodes have a stylet
•Most live in the soil and feed in or on plant roots
Disease cycle
Survival Inoculum produced
Dispersal
Infection Colonization Symptoms
Production of survival
structures
Fungi Bacteria Viruses Nematodes
Survival Crop residue
Soil
Alt. hosts
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Crop residue
Soil
Alt. hosts
Insect vectors
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Alt. hosts
Insect vectors
Crop residue
Soil
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Dispersal Wind
Rain
Insects
Wind
Rain
Insects
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Insects
Tillage
Equipment
Water run-off
Infection Directly
Wounds
Insect feeding
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Wounds
Insect feeding
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Insect feeding
Directly
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Comparison of disease cycles
InoculumSource of inoculum varies for each disease
• Present in the soil sticking to equipment or tools
• Present in weeds or other crops in the area
• Present in or on the seed
• Carried by wind or water
• Carried by insect vectors
• Carried in by animals, birds, and people
Spread of inoculum: preferably in two ways:1. Plant placed in soil that contains a pathogen2. Inoculum moves from its source to host plant
Penetration of inoculum and infection
• Infection occurs when a pathogen successfully enters a plant and grows, reproduces, and spreads within the plant
• Pathogens enter a host through natural openings, wounds on plant surfaces, or by penetrating directly into the plant
PenetrationMycelial
growthSporulation
Pustule
formation
Spore
germination
Secondary cycles
• Some diseases have only one cycle during the growing season (often root rots)
• Some diseases develop secondary or repeating cycles during the growing season (often foliar diseases)
• Number of cycles depends on the pathogen, susceptibility of the host, and environmental conditions
Pathogen survival
Pathogens survive season to season in:
• Soil• Crop residue• Weed or noncrop hosts• Seed or vegetative plant parts• Insects • Mild climates
Koch’s PostulatesFour criteria that were established by Robert Koch to identify the
causative agent of a particular disease, these include:
the microorganism or other pathogen must be present in all
cases of the disease
the pathogen can be isolated from the diseased host and grown
in pure culture
the pathogen from the pure culture must cause the disease
when inoculated into a healthy, susceptible laboratory animal
the pathogen must be reisolated from the new host and shown
to be the same as the originally inoculated pathogen
Koch’s Postulates: limitations
Microorganisms that are unable to be cultured
Two or more pathogens work in synergy to cause a disease
Symptoms and diseases can be causes by any one of several
microbes
Asymptomatic: not exhibiting any symptoms of diseases
Key Points• Understanding the difference between a sign and a symptom
is key in identifying a plant disease
• A plant disease cannot develop if a susceptible host, pathogen,
and favorable environment do not occur simultaneously
• The major plant pathogens responsible for disease
development in plants are fungi, bacteria, viruses, and
nematodes
• The disease cycle describes the interaction of the pathogen
with the host
Parasitism and Disease Development
Factors for successful disease development
Stages in disease development
• Three important components of plant disease :
– Susceptible host
– Virulent pathogen
– Favourable environment
• For disease to occur, all three of must be present.
Disease Triangle
Pathogen
Level of virulence
Adaptability
Dispersal efficiency
Survival efficiency
Reproductive fitness
Important Properties
Host
Degree of susceptibility
Growth stage & form
Population density & structure
General health
Environment
Temperature
Rainfall / Dew
Humidity
Soil properties
Wind
Inoculation
Penetration
Infection
Growth and Reproduction
Dissemination
Important Stages in Disease Development
Inoculation : The arrival of pathogen on the host
Inoculum : Any part of the pathogen that land on the host and can initiate infection
Inoculation
Types of inoculum:
Primary inoculum : An inoculum that survives the dormant period & caused the first infections
Primary infections : Infection caused by primary inoculum
Secondary inoculum : An inoculum produced from primary infections
Secondary infections: Infection caused by secondary inoculum
• Landing or arrival of inoculum on the host surface
• Germination of spores and seeds (fungus)
• Hatching of nematode eggs
• Attachment of Pathogen to Host (virus and bacteria)
• Recognition Between Host and Pathogen (Pathogen
recognize the host)
Inoculation Process
The initial invasion of a host by a pathogen.
There are several way through which pathogen get penetrated in host
1. Direct Penetration
• Direct with haustoria
• Direct, subcuticular only
• Direct intercellular mycellium
• Direct, intercellular mycellium with haustoria
• Direct with appressorium (A), penetration peg (PP) and intracellular mycelium (IM)
Penetration
• Stomata
• Lenticels (opening on fruits, stems and tubers –filled with loosely connected cells – allow the passage of air)
• Hydathodes (permanently open pores at the margins and tips of leaves)
Penetration2. Penetration Through Natural Openings
3. Penetration Through Wounds
• Through various types of wounds
• Natural cracks between main and lateral roots
• Openings made by fungus
Penetration
Establishment of pathogen with vulnerable cells or tissues of the host and obtain the nutrients from them
Successful infection will produce symptoms
Infection
Incubation period : The time interval between inoculation and the appearance of disease symptoms
Latent infection : The state in which a host is infected with a pathogen but does not show any symptoms
The pathogen will grow and multiply within the infected host.
• Fungi – through spores
• Viruses – replicated by the cell
• Nematodes – reproduce by means of eggs
Growth and Reproduction
• Pathogens are disseminated by several ways:
• By air
– While airborne – spores touch wet surfaces –get trapped – air movement stops / rains
– From the air – brought down by rain drops
– Winds also helps spreading by blowing away rain splash droplets containing pathogens
Dissemination
• By water
– Pathogens in the soil – dissemination by rain / irrigation water that moves on the surface / through soil
• By Insects, Nematodes & other Vectors
– Transmitted by insects during feeding, movement from plant to plant
– Nematodes can also transmit virus internally
Dissemination
• By Seed and Transplanting process
– Transported to other fields
– Are sold and transported to other areas
• By human
– Through handling of diseased and healthy plants
– Tools (pruning shears)
– Transporting contaminated soil on their feet / equipment
– Contaminated container
– Using infected seed / nursery stock
Dissemination