Agents of plants diseases.pptx

MAJOR PATHOGENS OF CROP PLANTS
Pathology Of Plantation Crops And Spices
Presented by,
Aleena Rose K P
MSc Botany

Important Plant Pathogenic Organisms-
 Different Groups -
Fungi, Bacteria, Fastidious Vesicular
Bacteria, Phytoplasmas, Spiroplasmas,
Viruses, Viriods, Algae, Protozoa And
Phanerogamic Parasites
 Examples Of Diseases Caused By Them
CONTENTS :

INTRODUCTION
• Plant pathology or phytopathology is the science, which deals with the plant
diseases.
• It is concerned with health and productivity of growing plants.
• Phytopathology ( Greek Phyton = plant + Pathos = disease, ailments + Logos =
discourse, knowledge) is the branch of agricultural, botanical or biological science
which deals with the cause, etiology (study of causation or origination), resulting
in losses and management methods of plant diseases.

INTRODUCTION
• Plant diseases are recognized by the symptoms (external or internal) produced by
them or by sick appearance of the plant. The term plant disease signifies the
condition of the plant due to disease or cause of the disease.
• Plant disease is mainly defined in terms of the damage caused to the plant or to its
organ.
A plant is said to be “diseased” when there is a harmful deviation from normal
functioning of physiological process.

INTRODUCTION
• The learned men during Vedic period were aware that the diseases are caused by
microbes. The book "Vraksha Ayurveda" written by Surapal in ancient India
contained information on plant diseases. This is the Indian book, which gave first
information on plant diseases. He divided plant diseases into two groups viz.,
internal and external. Plant diseases like rust, smut, downy mildew, powdery
mildew and blight were mentioned in the Bible.

INTRODUCTION
• The Greek Philosopher, Theophrastus (370-
286 B.C.) was the first to study and write about
the diseases of trees, cereals and legumes. In
his book 'Enquiry into plants' Theophrastus
has recorded his observations, imaginations and
experiences but they were not based on any
experiments.

CLASSIFICATION OF PLANT DISEASES
• Classified on the basis of type of pathogenic or non-pathogenic causes of the disease.
Causes of plant
diseases
Pathogenic Parasites
Biotic
Prokaryotes
Eukaryotes
Mesobiotic
viruses
viroids
Non –Pathogenic Non-parasites Or
Abiotic Agents
Mineral deficiencies or toxicities
Soil acidity or alkalinity, etc.

PARASITES
• They include both biotic and mesobiotic agents.
• The diseases are incited by parasites under a set of suitable environment.
• Association of definite pathogen is essential with each disease.
 Biotic agents: They are also called as animate causes. They are living organisms.
 Mesobiotic agents: They include viruses and viroids. They are infectious agents.
They can be crystallized and are considered non-living. But their multiplication in the
living plants ensures that they are living. Hence they are called as mesobiotic agents.

PARASITES: MESOBIOTIC AGENTS
• Viruses and viroids are small infectious agents that cause diseases in plants. Both viruses and
viroids are composed of genetic material, either DNA or RNA, and require a host cell to replicate
and spread.
• Viruses are relatively complex in structure, consisting of a nucleic acid core surrounded by a
protein coat called a capsid. Some viruses also have an outer envelope composed of lipids and
proteins. The capsid provides protection for the nucleic acid core and aids in the virus's ability to
infect cells.

• One example of a viral plant disease is the tobacco mosaic
virus, which infects tobacco plants and many other crops.
Symptoms of the disease include yellowing and curling of
leaves, mottled green and yellow, and stunted plant growth.
• Another example is the Tomato spotted wilt virus, which can
infect a wide range of plant species, including tomatoes,
peppers, and lettuce. Symptoms of the disease include browning
and death of leaf tips, yellowing of leaves, necrosis, and stunted
plant growth.

• Viroids are even simpler in structure compared to viruses.
They consist of a short RNA molecule that lacks a protein coat
but can still infect plants. Viroids are much smaller than
viruses and are composed of a small piece of circular RNA.
• Viroids are responsible for diseases such as potato spindle
tuber viroid (PSTV), which can infect potato plants and cause
stunted growth, distorted leaves, and tuber malformation.

• The method of disease action for both viruses and viroids is similar.
• Once infected, the genetic material of the virus or viroid is inserted
into the plant's cells. The genetic material then replicates, often
disrupting the normal functioning of the plant's cells and tissues. This
disruption can cause a variety of symptoms, including stunted growth,
wilting, leaf yellowing, and necrosis.

PARASITES : BIOTIC AGENTS
Biotic agents include:
1. Prokaryotes
a) True bacteria or bacteria (Facultative parasites)
e.g. Citrus canker.
b) Rickettsia-like bacteria (RLB) e.g. Citrus
greening, Pierce's disease of grape
c) Mollicutes or wall-less prokaryotes
i. Mycoplasma-like organism (MLO) e.g.
Sesame phyllody, egg plant little leaf.
ii. Spiroplasma e.g. Corn stunt, Citrus
stubborn.
2. Eukaryotes
a) Protists (Unicellular, coenocytic or multicellular
with little or no differentiation of cells and
tissues).
i. Fungi e.g. wilt of cotton
ii. Protozoa e.g. heart rot of coconut
iii. Algae e.g. red rust of mango
b) Plants - Parasitic flowering plants or
phanerogamic parasites - Broomrape of tobacco.
c) Animals (Multicellular, extensive differentiation
of cells and tissues) e.g. Nematodes –Root knot
nematode.

1. Prokaryotes
a) True bacteria or bacteria
• Bacteria are microscopic, unicellular prokaryotes, which lack chlorophyll. These
microorganisms are with a primitive nucleus lacking a clearly defined membrane.
• The bacteria are smaller than fungi and measure about 0.5 to 1.0 x 2.0 to 5.0 μm in
size. More than 1,600 bacterial species are known. Majority of them are saprophytes.
• Several species cause diseases in human beings and animals. About 200 species of
bacteria cause diseases in plants.
• First report of plant disease by bacteria was made by T.J. Burrill of the University of
Illinois. He showed that fire blight of apple and pear is caused by a bacterium,
Erwinia amylovora.

Fire blight of apple and pear is caused by a bacterium, Erwinia amylovora.

a) Bacterial disease signs (difficult to observe, but can include):
a) Bacterial ooze
b) Water-soaked lesions
c) Bacterial streaming in water from a cut stem
b) Bacterial disease symptoms:
a) Leaf spot with yellow halo
b) Fruit spot
c) Canker
d) Crown gall

1. Prokaryotes
b) Rickettsia-like bacteria (RLB)
Fastidious vascular bacteria are otherwise called Rickettsia - Like
bacteria, Rickettsia like organisms (RLO), or fastidious
prokaryotes or rickettsia -like walled bacteria.
They are small bacteria with a cellular ultrastructure of typical
gram- negative bacteria. They are very exact in their nutritional
requirements, refusing to grow on routine bacteriological media.
They have a cell wall unlike MLO (Mycoplasma Like Organisms)
and spiroplasma. MLO is restricted to phloem tissues where as
RLB are restricted mostly to xylem or phloem.
A common habit for both is the insect body fluid (haemolymph).
Both the groups are dependent on insect vectors for transmission.

1. Prokaryotes
• They produce phytoalexins, which induce characteristic symptoms of the disease.
• They are cultured in artificial media e.g., Pierce's disease of grapevine, almond leaf
scorch, phony disease of peach and plum leaf scald. Xylem restricted RLB can be
more successfully cultured than phloem-limited bacteria.
• Penicillin is effective against RLB. Sulpha drugs also inhibit them.
• The RLB can be divided into three groups:
1. Xylem-limited RLB
2. Phloem-limited RLB and
3. Non-tissue restricted RLB

Pierce's disease of grapevine
Xylella fastidiosa is an aerobic, Gram-
negative bacterium of the genus Xylella.
It is a plant pathogen, that grows in the water
transport tissues of plants (xylem vessels) and
is transmitted exclusively by xylem sap-
feeding insects such as sharpshooters and
spittlebugs.

Almond Leaf Scorch (ALSD) – Golden Death
This disease is caused by the bacterial pathogen
Xylella fastidiosa and is transmitted by xylem-
feeding insects.
ALSD-affected trees produced a reasonable yield
and did not die over a 3-year period. Because
almond orchards are kept in production for
approximately 25 years, longer-term data are
needed to fully evaluate the merits of retaining
ALSD-affected trees.

1. Prokaryotes
2. Xylem-limited RLB
Examples of Xylem-limited RLB:
Pierce's disease of grapevine, almond leaf scorch, phony disease of peach, wilt of
periwinkle, Sumatra disease of cloves, elm leaf scorch, alfalfa dwarf, plum leaf
scald. The RLB causing phony disease of peach is named as Xylella fastidiosa.
Symptoms: Symptoms include marginal necrosis of leaves, stunting of plants,
decline in vigour and reduction in yield.

1. Prokaryotes
1. Phloem-limited RLB
Examples of phloem limited RLB include:
citrus greening, clover club leaf (CCL), white clover disease, clover rugose leaf
curl, potato leaflet stunt, little leaf of Sida cordifolia and stunting of dodder.
Symptoms: Stunting, yellowing of young leaves, virescence of floral parts,
premature death of the entire plant.

1. Prokaryotes
c) MOLLICUTES OR WALL-LESS PROKARYOTES
1) PHYTOPLASMA
• Phytoplasma lack cell wall and are bounded by a unit membrane.
• They are pleomorphic. They have fried egg appearance of colony. They are
filterable through 450nm membrane. They have both DNA and RNA.
• They cannot be grown on artificial media.
• They produce symptoms like little leaf, phyllody, spike, yellows, stunting,
witches’ broom etc. They are mostly transmitted by leafhoppers. They are
insensitive to penicillin and sensitive to tetracycline.
• e.g. phyllody of sesame, little leaf of brinjal.

1. Prokaryotes
c) MOLLICUTES OR WALL-LESS
PROKARYOTES
2) SPIROPLASMA
• Spiroplasma is helical, wall-less
prokaryotes requiring cholesterol
for growth and cause diseases in
plants, insects and rats.
• They are insensitive to penicillin
and sensitive to erythrocin and
tetracycline.
• e.g. corn stunt, citrus stubborn. Corn Stunt

2. Eukaryotes
a) PROTISTS
1. FUNGI
Fungi are eukaryotic, achlorophyllous organisms that may
reproduce sexually and asexually and whose filamentous
branched somatic structures are typically surrounded by cell
walls containing chitin or cellulose.

2. Eukaryotes
a) PROTISTS
1. FUNGI
Fungal disease signs:
•Leaf rust (common leaf rust in corn)
•Stem rust (wheat stem rust)
•Sclerotinia (white mold)
•Powdery mildew
Fungal disease symptoms:
•Birds-eye spot on berries (anthracnose)
•Damping off of seedlings (phytophthora)
•Leaf spot (septoria brown spot)
•Chlorosis (yellowing of leaves)

2. Eukaryotes
a) PROTISTS
2. PROTOZOA
• Protozoa (trypanosomatid flagellates) belonging to the class Mastigophora, order
Kinetoplastida and family Trypanosomatidae have been known to parasitize plants.
Protozoa attacking plants move by flagella.
• The Mastigophora, or flagellates, are characterized by one or more long slender
flagella at some or all stages of their life cycle. The flagella are used for locomotion
and food capture.
• They are also used as sense organs. The body of the flagellates has a definite long,
oval or spherical form, which is maintained by a thin, flexible membrane cover.

2. Eukaryotes
a) PROTISTS
2. PROTOZOA
• Flagellates apparently cause:
• the phloem necrosis disease of coffee,
• the heart rot disease of coconut palm
• the Marchitez suppressive (sudden wilt or wither) disease of oil palm, Marchitez
suppressive is one of the important diseases in oil palm.
• Phytomonas staheli was described from sieve tubes of coconut and oil palm.

2. Eukaryotes
a) PROTISTS
3. ALGAE
Algae are eukaryotic, unicellular or multicellular
organisms and mostly occur in aquatic
environments. Many algae thrive as terrestrial or
subterranean algae. The size of algae ranges from
1.0mm to many centimetres in length. They
contain chlorophyll and are photosynthetic.
They reproduce by asexual and sexual processes.
The study of algae is called phycology or
algology.

2. Eukaryotes
b) . PHANEROGAMIC PARASITES
Phanerogamic parasites are flowering plants or seed plants, which lead a parasitic life on other living
plants. They parasitize a great number of economic plants and cause considerable loss in yield. The
phanerogamic parasites invade stem or root of the host plants. Some of these parasites possess
chlorophyll, which manufacture carbohydrates to a limited extent and depend on the host for mineral,
salts and water.These are generally called as semi or partial parasites.
Some of the parasites, which do not have chlorophyll, depend entirely on the host plants for their
food materials. They are called holo or total parasites.

2. Eukaryotes
b) . PHANEROGAMIC PARASITES
Nearly 2,500 species of
phanerogamic parasites in 11
families have been recorded
throughout the world. Among them
Orobanchaceae, Scrophulariaceae,
Loranthaceae, Convolvulaceae and
Lauraceae are important.

2. Eukaryotes
c) . ANIMALS / NEMATODES
Root-knot nematodes (Meloidogyne species) are well known because of the conspicuous
“knots,” or gall-like swellings, they induce on roots. More than 2,000 kinds of higher plants are
subject to their attack. Losses are often heavy, especially in warm regions with long growing
seasons. Certain species, however, such as the northern root-knot nematode (M. hapla), are
found where soil may freeze to depths of nearly a meter. Vegetables, cotton, strawberry, and
orchard trees are commonly attacked. Garden plants and ornamentals frequently become infested
through nursery stock.

2. Eukaryotes
Root-lesion nematodes (Pratylenchus species), cosmopolitan in distribution, are endoparasites
that cause severe losses to hundreds of different crop and ornamental plants by penetrating roots
and making their way through the tissues, breaking down the cells as they feed. They deposit
eggs from which new colonies develop. After a root begins to decline in vigor, nematodes move
into the soil in search of healthy roots. Lesions form in the root as fungi and bacteria enter
damaged tissues, and root rot often occurs. Annual crops may succumb early in the season, but
perennials and orchard trees may not decline for several years.

2. Eukaryotes
The golden nematode of potatoes (Heterodera rostochiensis) is a
menace of the European potato industry. Great efforts have been
made to control it. The speck-sized golden cysts that dot infested
plant roots are the remains of female bodies. Each cyst may
contain up to 500 eggs, which hatch in the soil over a period of
up to 17 years. A chemical given off by potato and tomato roots
stimulates hatching of the eggs.

2. Eukaryotes
The golden nematode of potatoes (Heterodera rostochiensis)
Root-knot nematodes (Meloidogyne species)

2. Eukaryotes
Many important ectoparasites feed on plant roots—dagger nematodes (Xiphinema), stubby-root
nematodes (Trichodorus), spiral nematodes (Rotylenchus and Helicotylenchus), sting nematodes
(Belonolaimus), and pin nematodes (Paratylenchus). Leaf, or foliar, nematodes (Aphelenchoides
species) and bulb and stem nematodes (Ditylenchus dipsaci) cause severe losses in vegetable and
ornamental bulb crops, clovers, alfalfa, strawberry, sweet potato, orchids, chrysanthemums,
begonias, and ferns.

ABIOTIC AGENTS
Abiotic agents include: They are also called as non-infectious or physiological disorders. When no
pathogen is found, cultured from or transmitted from a diseased plant, then the disease is said to be
caused by a non-living or environmental factor. These diseases occur because of disturbances in the
plant system by the improper environmental conditions in the air or soil or by mechanical influences.
They are listed below:
1. Too low or too high temperature
2. Lack or excess of soil moisture
3. Lack or excess of light
4. Lack of oxygen
5. Air pollution (Toxic gases in the atmosphere
etc.)
6. Mineral deficiencies or toxicities
7. Soil acidity or alkalinity
8. Toxicity of pesticides
9. Improper agricultural practices.

REFERENCES
1. PLANT PATHOGENS AND PRINCIPLES OF PLANT PATHOLOGY.pdf - Google Drive
2. Signs and symptoms of plant disease: Is it fungal, viral or bacterial? - Field Crops (msu.edu)
3. plant disease - Nematode diseases | Britannica

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