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.

Types of pathogens

Nematodes

Bacteria

Viruses

Fungi

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

-

-

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

Factors For Successful Disease Development

Disease Triangle

• 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

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