Classification of diseases – general symptoms and
method of control of plant diseases mechanical, Chemical and biological
methods, defiance mechanism in plant.
The
study of plant diseases is covered under the science of phytopathology, which
is commonly called plant pathology. Under this study the plant pathologist
study the plant diseases caused by fungi, bacteria, virus and very small microbes
(viroid etc) parasitic plant and nematode. They also study plant disorders
caused by a biotic problem.
Diseases
There is several way of
defining word diseases according to different schools of thought. But all the
come to the point that diseases is a malfunctioning process that is caused by
continuous irritation. This process results in suffering. Hence disease can
simply define as a pathogenic process. Disease
or disorder:
These two terms are synonymous. The malfunctioning of plant body is known as
disease or disorder. The term disease includes all types of harmful
physiological changes in the plant while non infectious changes due to abiotic
factors are termed as disorders.
DISEASE TRIANGLE
Three critical factors
or conditions must exist for disease to occur: a Susceptible host plant, a virulent
aggressive pathogen, an active
vector and the right mix of environmental conditions. The relationship of
these factors is called the disease triangle.
If only a part of the triangle exists, disease will not occur. Understanding
the disease triangle helps us understand why most plants are not affected by
the many thousands of diseases that exist.
Diseases
Pathogen Susceptible
host plant
Infectious diseases are caused by living
organisms called pathogens.
Noninfectious diseases caused by environmental stress and damage by weather and
other environmental factors also will be covered.
Indirectly,
environmental factors that cause a plant to be stressed may result in the organisms.
Because of this, diagnosing plant diseases can be tricky. The real cause of a
problem may be the stress factors, with the disease simply being a secondary factor.The
classification of diseases is very diversity and here are some of the ways in
which diseases can be classified -;
Modes
of spreading of diseases:
·
Soil
borne diseases: - Inoculums of the diseases causing pathogen remains in soil
and penetrate the plant resulting in diseased condition e.g. Root rot, wilt.
·
Seed
borne diseases: - The micro organisms are carried along with seeds and cause
diseases when congenial condition occurs. E.g. Damping off.
·
Air
borne diseases: - The micro-organisms are spread through air and attack the
plants causing diseases. E.g. Blight, rust, powdery mildew.
·
Diseases spread by insects: The viral diseases
are spread by insects. The insets which carry the viruses are known as vectors.
Extent of occurrence and geographical location:-
·
Endemic diseases; Endemic means common in and
limited to a particular locality. These diseases are more or less constantly
present in a particular area
·
Epiphytotic (in plants) or epidemic (in
animals) diseases-;
these diseases occur incidentally and occasionally in a particular locality.
·
Sporadic diseases -; these occur at very irregular intervals
and locations.
Some school of thought
has come to conclusion that diseases can either be infectious or non infectious
Infectious diseases:- these are diseases caused by living agents, the
pathogen may spread by contact of a diseased plant with healthy plant. Example
pathogen caused by fungi, bacteria, protozoa etc.Non-infectious disease:- they do not spread from plant to plant,
caused due to improper condition of the soil, air or mechanical injury.
Example, tip-rot, tip-burn of buddy, black heart of potatoes.
DISEASE CYCLE
There are five
stages in disease development: inoculation, incubation, penetration, Infection
and symptoms
i.
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.
ii.
Penetration – Is the entrance of the pathogen
into a plant and this usually takes place through wounds, natural openings
e.t.c.
iii.
Infection – Implies the establishment of
the pathogen within the tissues of the plant.
iv.
Incubation – This is the interval of
time between the infection by the pathogen and production of disease
v.
Symptoms. As the pathogen consumes
nutrients, the plant reacts by showing symptoms. Symptoms are evidence of the
pathogens causing damage to the plant. Symptoms include mottling, dwarfing,
distortion, discoloration, wilting, and shriveling of any plant part.
Evidence of disease shown by plant is called symptom
Symptoms
due to visible pathogen
The
pathogen is visible when they are in larger size or in sufficient mass, such
symptoms are as follows:
i)
Mildew:
- Pathogen is seen as a growth on the surface of the host. Downy mildew and
powdery mildew.
ii)
Rust: -
Rusty symptoms are seen on the host epidermis Red, Green, yellow and black
rust.
iii)
Smuts: - Sooty or charcoal like powder appears
on floral organs, smut symptoms also found on stem leaves and roots.
iv)
White
blisters: Numerous white blisters, like pustules are seen.
v)
Scab:
Crust like lesion on the diseased organs.
vi)
Sclerotia: A compact, often hard, mass of
dormant fungus mycelium.
vii)
Blotch:
Superficial growth giving the fruit a blotched appearance.
viii)
Fruiting
bodies: Relatively large spore bearing structures which are either fleshy or
woody.
ix)
Exudations:
mass of bacteria oozes out to the surface of affected organ.
x)
Tar
spots: Raised, black coated fungus bodies with the appearance of a flattened
out drop of tar on leaves.
Symptoms due to some effect on
host plant
ü Colour changes, Discoloration,
Chromosis, and Chlorosis.
ü Overgrowth Galls, Curl, bladder,
witches broom, hairy root,
ü Dwarfing stunted growth
ü Necrosis Spots, strips, blight,
damping off, scald, scorch, rot.
ü Anthracnose: ulcer like lesions on
stems, leaves, pods.
ü Dieback: Drying of plant organ from
the tip backwards.
ü Wilts: Drying or wilting of entire
plant
ü Miscellaneous symptoms. Change in
habit, dropping of leaves, flowers, fruits, destruction of organs etc.
Methods
of controlling plant diseases
Mechanical
methods
Mechanical control means controlling
pests and diseases with the help of mechanical measures.
- Keep insects away from the plants. Many flying insects
(aphids, butterflies, flies, capsids, thrips, etc.) can be kept away from
the crop with the help of insect netting. By covering the soil or
substrate with polythene, cloth or with special collars around the stem
base it is possible to protect plants against larvae that eat roots or
sub-soil stems. These measures also stop the development of larvae and
pupae that need soil for completion of their life cycle, and keep them
from further spreading.
- Trap insects. With the help of sticky traps,
insect-o-cutors, trap plants, pheromone traps etc. it is possible to trap
winged insects.
- Use temperature treatments to kill harmful organisms.
There are different methods:
Hot water
Seeds, bulbs, tubers and cuttings can be immersed in hot water to kill potential pests such as insects, mites, nematodes, fungi and bacteria.
Hot air
A hot air treatment can also kill harmful organisms in plants, bulbs and seeds.
Solarisation
If the soil is covered with transparent polythene for several weeks in summer, temperatures can rise so high through solar radiation that pest organisms are killed.
Steaming
Treatments with steam will disinfect soil, substrate, crates, etc. - Use flooding as a technique. If a piece of ground can
be flooded for a sufficient period of time, most of the harmful organisms
will die due to lack of oxygen.
- Remove infested plant materials. Place them in a bag,
and destroy
Biological control
Biological
control is the control of pests and diseases with natural enemies. Within
biological control three groups of beneficial organisms can be distinguished.
The most common examples are given for each category below.
|
Predators
|
predatory mites, bugs, beetles and
gallmidges
|
|
Parasites
|
parasitic wasps and parasitic flies
|
|
Micro-organisms
|
nematodes, fungi, bacteria and
viruses
|
It
is vital to pay attention to the following points.
- Introduce
beneficials at the right time. The earlier the beneficials are introduced
the lower the numbers that are needed and the better the effect. Some
beneficials can even be introduced preventatively.
- Pay
attention to the quality of the material
- Obtain
the material from a recognised supplier who guarantees its quality and
quantity.
- Keep
in mind the indicated storage temperature as well as the use-by-date.
Begin with Integrated Pest Management in a period where the incidence of
pest infestation is still low. That way, the use of pesticides can be
limited and natural enemies will have a greater chance of establishment in
the crop.
- Become
acquainted with the biology of the beneficials.
- Encourage
spontaneously occurring natural enemies to enter the crop by creating
optimal conditions for them in which they thrive, e.g. humidity.
- Use
attractive plants or banker plants when possible (Datura, Ricinus). They
may also attract pest insects, but when managed well they can be good
sources of predators and parasites.
- Make
sure that harvesting, pruning or deleafing of the crop does not
substantially reduce the population of beneficials.
- Ensure
that other natural enemies are not harmed by the application of natural
products e.g. plant extracts and minerals, which contain components with a
controlling or protecting effect against pests and/or diseases. These may
be less selective than is indicated.
Chemical
control
Within the concept of IPM, chemical
pesticides are mainly used as a corrective measure. Nevertheless, the following
guidelines should be respected:
- Use
selective pesticides. These are pesticides that are non-toxic or only
slightly toxic to beneficials or do not inhibit their development or
reproduction.
- Use
selective application techniques. Some pesticides may be applied without
actually damaging the beneficial population, even if they are not
particularly safe for beneficials. This can be achieved by applying
pesticides through the drip system, drenching, using seeds treated with a
seed coating, and restricting applications to young plants.
- Use
pesticides with a short persistence. Some chemicals are quite harmful at
the moment of application, but the effect may not last more than a couple
of days. Soon afterwards, beneficials can be introduced again, or natural
enemies from the surroundings can enter in the treated area again and
survive.
- Ensure
that the plant propagator does not use pesticides with a long persistence.
These products can stop or inhibit the development of beneficials.
- Avoid
the use of pesticides with a long persistence prior to beginning
biological control.
- Consult
the Koppert Side-Effects Guide for detailed information about the
compatibility of pesticides with most beneficials and the persistence of a
possible harmful effect. This guide is also available on the Internet.
- Avoid
dusting any products, as dusting inhibits the development of beneficial.
- Be
aware that pesticides or their vapour may drift from the area of
application into an area with biological control. When this happens, the
population of beneficial can be affected. During spot treatments, screen
off the infested area if possible. Proper communication with neighbours in
combination with the right action (open/close vents, avoid spraying under
windy conditions) can mitigate these effects.
Plant defense mechanism
Plants defense mechanism. These are the
processes where by the plant undergoes different chemical and mechanical
resistance after the entrance of the pathogens. Pathogens may perforate in
intracellular spaces (the apoplast) after entering though stomata or hydathodes
(bacteria) enter plant epidermal cells or extend hyphae into the plant cells
(fungi). Innate immune receptors in plants defect the presences of microbial
pathogens and triggers defense mechanism responses to terminate (restrict
pathogens grow). The following now are the chemical defense mechanism exhibited
by most of the plants.
Elicitors
of defense responses, these are any substance that has the capabability of
activating defiance response in plants. Including components of the cell
surface as well as the excreted metabolites. Example gene product “b”
proteins/peptides elicitors. Perceptions of elicitor signals. Have including
binding proteins, oligosaccharide binding sites through specific glucan binding
sites in soya beans root plasma membranes also the higher affinity binding
sites for chitin fragments in tomato, rice.
Plant defense to pathogens; plants respond to
infection using two branched invite immune system. Recognitions and response to
molecules common to many classes of microbes many plants associated microbes
are pathogens that impair plants growth and reproduction.
(Basal diseases resistance)
Response to pathogen virulence factors. Basal
defense this is triggered trans- membrane receptor that recognizes conserved
molecules released by variety of (unrelated) microbes, including cell
fragments, chitin or peptides motifs in bacteria flagella.
Secondary defense;
this through again sting virulence effectors proteins produced by pathogens.
Effecter triggered immunity which is mediated by resistance proteins
Phases
of triggered immunity, this involve there phases which are resistance phase,
susceptibility.
Concepts
of regarding pathogens recognition and defense example gene for gene resistance
and guard hypothesis. Gene for gene resistance example for restance to occur
complementary pairs of dominant genes one in the host and other in a pathogens
are required (incomptility) a loss or alterations to either the plants
resistance (R) gene or pathogen aviluence (AVr) gene leads to diseases
(compatibility).
Plants
diseases resistance genes, this include This include encored proteins that recognize AVr-gene depended lingands that activate signating cascades that coordinate the initial plant
defense responses to impair pathogens ingress. Common feature of resistance
protein is a laucime rich repeat.
The following are the mechanism in plant
defense
Spine
Stabbiness
corns in handy where you are plant leaves (and the related stipules) have
evolved to draw blood, too many plants have spines but they are perhaps most
memorably marshaled by the cacti who. Them in abundance, spines not only defend
the juicy stems of cacti against ravenous succulent eaters, but also shade them
from the relentless desert sun. (Cactus that grow in non-desert environment
wear aligher covering of spine)
Chemical signaling
Plants that are attached by browsers or
insects pests or subjected to stressful conditions. Such as drought or
microbial infections may warn other plant of the impending crises by releasing
volatile organic compound (VOCs) which precipitate physiological reactions on
nearly plants. They may increases concentrations of toxic compounds to award of
the enemy or they may release compounds of their own that attract the enemy
predator. Some recent experiments have shown that plants also communicate
through chemical release by their roots and crane via networks of fungal
symbiotic.
Plant
defense mechanism
Trichome
if you have had the misfortune of brushing against stringing nettle you knows
the plain induced power of the tiny trichome. Nettle other plants grow a
bristling fur of these pointed structure of shield there selves from browsing.
Had the proverbial “very hungry cartipillar” made the mistake of wandering out
atrichome-bearing plant. He would not have been hugely for much longer he would
have been impaled or eviscerated. Some plants like nettle have associated gland
that injects poison into the trichrome inflicted would. Some tropical nettle
can cause permanent derve damage or death.
Cyrpsis
Plants sensitive plants example
(mimosa pudica) close its leaves when they are touched making them appear dead
and therefore unappetizing. These plants are often sold as curiosities and
featured at botanical gardens watching plants reacts in real. Time serves as
visceral and memorable demonstration the facts the plant are in facts example:
Erasmus Darwin Charles. Also all living plants cells have the ability to detect
invading pathogens and respond with indivisible defenses including productions
of toxic and chemicals.
Thorns
Thons
are basically pointy branches or stem their defensive functions is somewhat
self-evident. They stab by and contrary what (the band) poison would have you
believe not a single rose has are.Prickle
Roses
actually have prickes, not thons prickle are actually pointed protuberances
from the plants epidermis. Think of them as razor-sharp frecicles while they shield
the plant that bear them from some depredations, crtain species of plants
hopper, small enough to squeeze between the slup up sap actually mimic their
spiky appearance to avoid predators.
Idioblast
Not all
plants bear their defense on the surface. If thons spines prickles and
trichomes are spear brigade, idioblasts are the landmines. Specialized cell
that contain variety of defense compounds from razor-shap to pain induced
chemical. Idioblast detonate when the first line defense has been branched. The
different brachia, common house plant, contain idioblast that fire
barded calcium oxalate crystals into the mouth of predator and then release an
enzymes analysis to reutilization venom. This can cause paralysis and thus
loose of speech hence the common name “dumb cane”.
Commensalism
Some plants have opted to have mercenones several
species of South Africa acacia tree both house and feed aggressive ants. The stinging little soldiers make their
support inside swallower thons and feed of blood bodies produced by the plant
especially for them
The arts savagely defends their “giving trees”
agaist all comers, be they animal, vegetable or fungus. They even snip of the foliage of any plants
that have nerve to encroach upon their acacias personal space. In experiments
where that ant colonize were removed the trees died
Poison
Everyone
knows that some plants are poisonous but what interpret “a poison” to one organism may very well be an inert
compounds to another birds for example are unfazed by uroshiol, the toxic oil
produced by poison and fact seem to have barriers produced by plants in their
own devious ends from the chrysanthemum.
Dirked pyrethrums used as insecticides to the castor bean derived Rican,
which Walter white of breaking band attempted to use to eliminate his enemies
of several occasions.
Presences of preformed barrier such
as cell wall wax, epidermal cuticles and bark.
This prevent plant from invasion also give the plant strength and rigid
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