21.10.
2016
Introduction to plant protection
- plant pathology,
- basic terms,
Lecture 1a
Plant pathology
• studies plant diseases
• tries to protect plants against unfavorable
environmental conditions and parasitic
microorganisms that cause disease
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The agents that cause disease in plants
are the same or very similar to
those causing disease in humans and animals
•viruses,
• bacteria,
• fungi,
• protozoa,
• and nematodes,
• unfavorable environmental conditions (lack or
excess of nutrients, moisture, and light, presence of
toxic chemicals in air or soil)
Plant pathology is the study of:
• the organisms and of the environmental factors
that cause disease in plants;
• the mechanisms by which these factors induce
disease in plants;
• the methods of preventing or controlling disease
and reducing the damage it causes.
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Plant pathology is an integrative science and profession
that uses and combines the basic knowledge of:
b o ta n y, my c o lo g y, b a c ter io lo g y, v ir o lo g y,
n ema to lo g y, p la n t a n a to my, p la n t p h y sio lo g y,
g en etic s, mo lec u la r b io lo g y a n d g en etic
en g in eer in g , b io c h emistr y, h o r tic u ltu r e,
a g r o n o my, tissu e c u ltu r e, so il sc ien c e,
f o r estr y, c h emistr y, p h y sic s, meteo r o lo g y,
and many ot he r br anc he s of s c i e nc e
First question:
What i s a he al t hy pl ant ? ? ?
npic.orst.edu
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Healthy, or normal plant
can carry out
its physiological functions to the best of its
genetic potential
the meristematic (cambium) cells of a healthy plant divide
and differentiate as needed,
different types of specialized cells absorb water and
nutrients from the soil;
translocate these to all plant parts;
carry on photosynthesis, translocate, metabolize, or store
the photosynthetic products;
produce seed or other reproductive organs for survival and
multiplication
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The ability of the cells of a plant or plant part to carry out one
or more of these essential functions
is interfered with/by
either a pathogenic organism or an adverse environmental
factor
the activities of the cells are disrupted, altered, or
inhibited,
the cells malfunction or die,
the plant becomes diseased
The symptoms of the disease
The visible changes
fyi.uwex.edu
www.apsnet.org
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The visible (measurable) adverse changes
in a plant
produced in reaction to infection by an organism
or to an unfavorable environmental factor,
are a measure of the amount of disease in the
plant
Disease in plants - can be defined as:
the series of invisible and visible responses of plant cells and
tissues to a pathogenic organism or environmental factor
that result in adverse changes in the form, function, or
integrity of the plant
partial impairment or death of plant parts or of the entire
plant
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Pathogenic microorganisms,
the transmissible biotic (= living) agents
Pathogens, usually cause disease in plants by:
• disturbing the metabolism of plant cells through
enzymes, toxins, growth regulators, and other
substances they secrete
• absorbing foodstuffs from the host cells for their
own use.
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Some pathogens
may also cause disease by growing and multiplying in
the xylem or phloem vessels of plants, thereby blocking
the upward transportation of water or the downward
movement of sugars
quizlet.com
Environmental factors cause disease in plants when
abiotic factors, such as:
• temperature,
• moisture,
• mineral nutrients,
•pollutants,
occur at levels above or below a certain range tolerated by
the plants
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Types of Plant Diseases
How many diseases affect cultivated and wild plants?
One crop plant ---- How many diseases?
One pathogen ----- one variety of a plant?
One pathogen ----- How many species of plants?
Examples?
How we can group plant diseases?
According to:
• to the plant organ they affect (root diseases, stem
diseases, foliage diseases),
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How we can group plant diseases?
According to:
• to the symptoms they cause (root rots, wilts, leaf spots,
blights, rusts, smuts),
How we can group plant diseases?
According to:
• to the types of plants affected (field crop diseases,
vegetable diseases, turf diseases, etc.).
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Useful criterion - the type of pathogen that causes the
disease
bacteria
nematodes
viroids
protozoon
mollicutes
fungus
viruses
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PARASITISM AND PATHOGENICITY
Parasite - an organism that lives on or in some other
organism and obtains its food from the latter.
Parasitism - the removal of food by a parasite from its
host.
Plant parasite - an organism that becomes intimately
associated with a plant and multiplies or grows at the
expense of the plant.
Pathogenicity - the ability of a pathogen to cause disease.
Symbiosis
both the organisms benefit from the association
• the root nodule bacteria of legume plants
• the mycorrhizal infection of feeder roots of most
flowering plants
Pathogenicity - the ability of the parasite to interfere with
one or more of the essential functions of the plant,
thereby causing disease.
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A few groups can parasitize plants:
• fungi,
• bacteria,
• mollicutes,
• parasitic higher plants,
• parasitic green algae,
• nematodes,
• protozoa,
• viruses,
• viroids.
These parasites are successful because they can invade a
host plant, feed and proliferate in it, and withstand the
conditions in which the host lives.
Biotrophs - can grow and reproduce in nature only in
living hosts
viruses, viroids, mollicutes, some fastidious bacteria,
nematodes, protozoa, and fungi (causing downy mildews,
powdery mildews, rusts)
Obligate parasites
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Nonobligate parasites
Other parasites (most fungi and bacteria) can
live on either living or dead hosts and on
various nutrient media
Semibiotrophs
Some nonobligate parasites live most of the time or
most of their life cycles as parasites, but, under
certain conditions, may grow saprophytically on
dead organic matter
Facultative saprophytes
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Necrotrophs
Llive most of the time and thrive well on dead organic
matter but, under certain circumstances, may attack living
plants and become parasitic
Facultative parasites
Parasites Saprophytes
* Tend to have narrow host range - * Tend to have broad host range
except viruses * Kills plant tissues to acquire
* Does not need to kill plant
nutrients for growth and complete
cells to complete lifecycle
* Intimate relationship with plant - lifecycle
continuous absorption of * Secrete enzymes and toxins
nutrients
* Grows inter-cellularly
* Grows inter- and intracellular
* Infected plants tend to be
stunted, poor vigor
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The degree of Usually no The severity of
parasitism of a correlation disease it can
pathogen between cause
Many diseases caused by weakly parasitic pathogens
are much more damaging to a plant than
others caused even by obligate parasites
slime molds and those causing sooty molds
cause disease by covering the surface of the plant
without parasitizing the plant
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Obligate and nonobligate parasites differ in the ways in which they
attack their host plants and get nutrients from the host
Many nonobligate parasites
secrete enzymes
disintegration of the cell components of plants
enzymes alone or with the toxins secreted by the pathogen
death and degradation of the cells
Obligate and nonobligate parasites differ in the ways in which they
attack their host plants and get nutrients from the host
The invading pathogen then utilizes the contents of the cells for its
growth
Many fungi and most bacteria act in this fashion, growing as
necrotrophs on a nonliving substrate within a living plant
This mode of nutrition is like that of saprophytes
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All obligate (and some nonobligate) parasites
do not kill cells in advance
get their nutrients either by penetrating living cells or by
establishing close contact with them
The association of these pathogens with their host cells
is an intimate one
continuous absorption or diversion of nutrients into the body
of the parasite
The depletion of nutrients
Restricts the growth of the host and causes symptoms
However, does not always kill the host
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HOST RANGE OF PATHOGENS
Pathogens differ:
•the kinds of plants that they can attack,
•the organs and tissues that they can infect,
•the age of the organ or tissue of the plant on which they
can grow.
Some pathogens are restricted:
• to a single species,
• to one genus of plants,
• others have a wide range of hosts, belonging to many
families of higher plants
Some pathogens grow especially:
• on roots,
• on stems,
• some mainly on the leaves
• on fleshy fruits or vegetables
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• Some pathogens, e.g., vascular parasites, attack specifically
certain kinds of tissues, such as phloem or xylem.
• Others may produce different effects on different parts of the
same plant.
• With regard to the age of plants, some pathogens attack
seedlings or the young tender parts of plants, whereas others
attack only mature tissues
DEVELOPMENT OF DISEASE IN PLANTS
“disease triangle”
Agrios 2005
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DEVELOPMENT OF DISEASE IN PLANTS “disease triangle”
Factors affecting the Pathogen:
1. Virulence
2. Population present
3. Life-stage of propagule
4. Vectors present
Agrios 2005
DEVELOPMENT OF DISEASE IN PLANTS “disease triangle”
Factors affecting the Host:
1. Level of resistance
2. Growth stage of plant
3. Genetic uniformity of neighboring plants
4. Plant Vigor
5. Plant Density
6. Structure of Plant
Agrios 2005
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DEVELOPMENT OF DISEASE IN PLANTS “disease triangle”
Factors affecting the Environment:
1. Conditions that favor disease
2. Conditions that promote plant growth
3. Conditions that affect dispersal
Includes levels of humidity, temperature, wind speed,
and periods of rain
Agrios 2005
DEVELOPMENT OF DISEASE IN PLANTS “disease triangle”
Factors affecting
Interpretation of the triangle:
- Length of each side is
proportional to the sum total of
the characteristics of each
component that favors disease
- if one side is zero, no disease
- if one side is very large,
disease potential is large
Quantification of the triangle’s area would represent the
amount of disease
Agrios 2005
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STAGES IN THE DEVELOPMENT OF DISEASE:
THE DISEASE CYCLE
In every infectious disease a series of more or less
distinct events occurs in succession and leads to the
development and perpetuation of the disease and the
pathogen.
This chain of events is called a disease cycle.
Closely related to the lifecycle of the pathogen
The primary events in a disease cycle are
in o c u latio n , p en etr atio n , estab lish men t o f
in f ec tio n , c o lo n iz atio n ( in v asio n ) , g r o w th an d
r ep r o d u c tio n o f th e p ath o g en , d issemin atio n
o f th e p ath o g en , an d su r v iv al o f th e p ath o g en
in th e ab sen c e o f th e h o st , i.e., overwintering or
oversummering (overseasoning) of the pathogen
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The primary events in a disease cycle are
in o c u latio n , p en etr atio n , estab lish men t o f
in f ec tio n , c o lo n iz atio n ( in v asio n ) , g r o w th an d
r ep r o d u c tio n o f th e p ath o g en , d issemin atio n
o f th e p ath o g en , an d su r v iv al o f th e p ath o g en
in th e ab sen c e o f th e h o st , i.e., overwintering or
oversummering (overseasoning) of the pathogen
Agrios 2005
Inoculation
Inoculation is the initial contact of a pathogen with a site
of plant where infection is possible.
The pathogen(s) that lands on or is otherwise brought into
contact with the plant is called the inoculum.
The inoculum is any part of the pathogen that can initiate
infection
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In fungi the inoculum may be spores, sclerotia (i.e., a
compact mass of mycelium), or fragments of mycelium.
In bacteria, mollicutes, protozoa, viruses, and viroids, the
inoculum is always whole individuals of bacteria,
mollicutes, protozoa, viruses, and viroids, respectively.
In nematodes, the inoculum may be adult nematodes,
nematode juveniles, or eggs.
In parasitic higher plants, the inoculum may be plant
fragments or seeds.
The inoculum may consist of a single individual of a
pathogen, e.g., one spore or one multicellular sclerotium,
or of millions of individuals of a pathogen, e.g., bacteria
carried in a drop of water.
One unit of inoculum of any pathogen is called a
propagule.
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Types of Inoculum
An inoculum that survives dormant in the winter or
summer and causes the original infections in the spring or
in the autumn is called a primary inoculum, and the
infections it causes are called primary infections.
An inoculum produced from primary infections is called a
secondary inoculum and it, in turn, causes secondary
infections.
Sources of Inoculum
In some fungal and bacterial diseases of perennial plants,
such as shrubs and trees, the inoculum is produced on the
branches, trunks, or roots of the plants.
The inoculum sometimes is present right in the
plantdebris or soil in the field where the crop is grown;
other times it comes into the field with the seed,
transplants tubers, or other propagative organs or it may
come from sources outside the field.
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Outside sources of inoculum may be nearby plants or
fields or fields many miles away.
In many plant diseases, especially those of annual crops,
the inoculum survives in perennial weeds or alternate
hosts, and every season it is carried from them to the
annual and other plants.
Fungi, bacteria, parasitic higher plants, and nematodes
either produce their inoculum on the surface of infected
plants or their inoculum reaches the plant surface when
the infected tissue breaks down.
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Viruses, viroids, mollicutes, fastidious bacteria, and
protozoa produce their inoculum within the plants; such
an inoculum almost never reaches the plant surface in
nature and, therefore, it can be transmitted from one
plant to another almost entirely by some kind of vector,
such as an insect.
Landing or Arrival of Inoculum
The inoculum of most pathogens is carried to host plants
passively by wind, water, and insects. An airborne
inoculum usually gets out of the air and onto the plant
surface not just by gravity but by being washed out by
rain. Only a tiny fraction of the potential inoculum
produced actually lands on susceptible host plants; the
bulk of the produced inoculum lands on things that
cannot become infected.
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Some types of inoculum in the soil, e.g., zoospores and
nematodes, may be attracted to the host plant by such
substances as sugars and amino acids diffusing out of the
plant roots.
Vector transmitted pathogens are usually carried to their
host plants with an extremely high efficiency
PREPENETRATION
Attachment of Pathogen to Host
Spore Germination and Perception of the Host Surface
Appressorium Formation and Maturation
Recognition between Host and Pathogen
Germination of Spores and Seeds
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PENETRATION
Pathogens penetrate plant surfaces by direct penetration
of cell walls, through natural openings, or through wounds
Penetration does not always lead to infection. Many
organisms actually penetrate cells of plants that are not
susceptible to these organisms and that do not become
diseased; these organisms cannot proceed beyond the
stage of penetration and die without producing disease.
PREPENETRATION
Agrios 2005
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PENETRATION
Agrios 2005
PENETRATION
Agrios 2005
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Infection
Infection is the process by which pathogens establish
contact with susceptible cells or tissues of the host and
procure nutrients from them.
Successful infections result in the appearance of
symptoms, i.e., discolored, malformed, or necrotic areas
on the host plant.
Some infections, however, remain latent, i.e., they do not
produce symptoms right away but at a later time when
the environmental conditions or the stage of maturity of
the plant become more favorable.
All the visible and otherwise detectable changes in the
infected plants make up the symptoms of the disease.
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Symptoms may appear as soon as 2 to 4 days after
inoculation.
In most plant diseases, however, symptoms appear
from a few days to a few weeks after inoculation.
The time interval between inoculation and the
appearance of disease symptoms is called the incubation
period.
Condition needed for a successful infection
•a pathogen comes in contact with its host
•the plant variety must be susceptible to the particular
pathogen and at a susceptible stage
•the pathogen must be in a pathogenic stage
•the temperature and moisture conditions in the
environment of the plant must favor the growth and
multiplication of the pathogen
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Invasion
•Some fungi, such as those causing apple scab and
black spot of rose, produce mycelium that grows only
in the area between the cuticle and the epidermis
(subcuticular colonization)
•others, such as those causing powdery mildews,
produce mycelium only on the surface of the plant but
send haustoria into the epidermal cells.
Invasion
•Most fungi spread into all the tissues of the plant
organs (leaves, stems, and roots) they infect, either by
growing directly through the cells as an intracellular
mycelium or by growing between the cells as an
intercellular mycelium
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Growth and Reproduction of the Pathogen
(Colonization)
•Fungi reproduce by means of spores, which may be
either asexual (mitospores, i.e., products of mitosis,
roughly equivalent to the buds on a twig or the tubers of a
potato plant), or sexual (meiospores, i.e. products of
meiosis, roughly equivalent to the seeds of plants).
Growth and Reproduction of the Pathogen
(Colonization)
•Parasitic higher plants reproduce by seeds.
•Bacteria and mollicutes reproduce by fission in which one
mature individual splits into two equal, smaller
individuals.
•Viruses and viroids are replicated by the cell,
•Nematodes reproduce by means of eggs.
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Dissemination of the Pathogen
A few pathogens, such as nematodes, oomycetes,
zoosporic fungi, and bacteria, can move short distances on
their own power and thus can move from one host to
another one very close to it.
Dissemination of the Pathogen
Almost all dissemination of pathogens responsible
for plant disease outbreaks, and even for disease
occurrences of minor economic importance, is carried out
passively by such agents as air and insects
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Dissemination of the Pathogen
• Dissemination by Air
• Dissemination by Water
• Dissemination by Insects, Mites, Nematodes, and
ther Vectors
• Dissemination by Pollen, Seed, Transplants,
Budwood and Nursery Stock
• Dissemination by Humans
Overwintering and/or Oversummering of Pathogens
Agrios 2005
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