Plant Pathology is the study of diseases that affect plants. It involves studying the causes, mechanisms, and control of plant diseases. The document discusses key topics in plant pathology including disease cycles, types of pathogens and inoculum, modes of dissemination, and the relationship between disease cycles and epidemics. It provides definitions and examples of important terminology used in plant pathology.

1. Plant Pathology
By:
Tullu Tadessa

2. • Is this normal Crop
What happen to these crops?

3. Is this normal?

4. Plant Pathology?
Is the study of the;

5. Laboratory
1. microorganisms and environmental
factors that cause disease in plants;

6. is this normal or abnormal leaf?

7. Plant Pathology studies the:
 causes,
 mechanisms and
 control of diseases affecting plants.

9. 3. IMPORTANCE

10. 1. Increases man`s knowledge about plant diseases
2. Develop:
 methods,
 equipment and
 materials through which plant diseases can be
prevented or controlled

11. Mycotoxins
3. Diseased plant products may be poisonous and
unfit for consumption so the control of plant
diseases will help in species conservation, alleviating
losses to our food, fiber, fodder and other plant
products

12. poisonous and unfit for consumption

14. More food and better quality

15. TERMINOLOGIES OR
TERMS

16. Disease: ? suffering of plant.
Pathogen: ? disease causing agent.
Pathogencity:? ability to cause disease
Pathogenesis:? sequence of events in disease development
from the initial contact --- symptom appearance.
Host: ? An organism that supports the activities of a parasite

17. Susceptibility? Resistance? Tolerant plants?
Immunity?
Tolerant plants: ?
have little resistance to pathogens

18. Immune
Immunity:? implies the host is exempted from infection.

19. High resistance
Resistance: The inherent ability of a plant to prevent: or
resist the establishment and subsequent activities of a
potential pathogen.

20. Highly susceptible
Susceptibility: ?
The inability of a plant to resist the effect of a pathogen or
any other damaging factor. Environment can influence it.

21. Moderate resistance

22. SYMPTOM DIFFERS FROM SIGNS.
Symptom:?
the physical manifestation of the host`s response to a
pathogen
E.g. wilting, necrotic, rotting etc.
Sign: ?
Is the physical manifestation of the causal agent
(Pathogen)
e.g. bacterial ooze, spores or mycelia.

23. Disease and Health
What is health?
normal physiological functions.

24. B. What is disease?
•Malfunctioning of plant functions

25. CAUSE OF PLANT DISEASE

26. Two kinds of disease causants
Living (animate) things:
Non- living (inanimate) things:

27. CLASSIFICATION OF PLANT DISEASE

28. Diseases are classified in the following ways:
1. Occurrence on the plant
Localized: only on a particular part of plant
Systemic: spreads throughout the entire plant.

29. 2. Based on the method of dissemination
 Soil-borne disease
 Air - borne disease.
 Seed –borne diseases

30. 1. Soil-borne disease.
primary infection takes place from the soil.
Where they survive?
…..in soil or on infected plant debris lying in soil
How? either as resting spores or as mycelia strands
and rhizomorphs from which they attack the root
system of the host plants.
Example:
Armilaria mellea,
Pythium,
Fusarium
Verticillium,
Colletotrichum:

31. 2. Air - borne disease.
Pathogens infect most through the air and bring about
primary and secondary infections .
E.g. Stem rust, yellow rust and
Downey mildew, powdery mildew
develop thick walled resting structures like;
• Teliospores in rusts
• Cleistothecia in powdery mildews
• Oospores in downy mildews
disseminated by wind to infect plant shoots.

32. 3. Seed –borne diseases?
Pathogens survive, as dormant mycelium in the seeds
or other propagative structures of the host plant.
Mycelium grows;
the growing apex of the seedling.
activated during seed germination.
spread throughout the entire plant
mainly systemic.
E.g. Loose Smut
Cover Smut

33. 3. Based on the kinds of symptoms: Rust/wilt
4. Type of plants affected:
cereal diseases
coffee diseases,
forest diseases
fruit diseases.
5. Occurrence and geographic distribution
Endemic diseases: natural in a country or part of the
earth, from year to year.
Epidemic diseases: occur periodically but in a sever state
involving loss of major area of crop.
Sporadic diseases: occur in very irregular intervals and
locations and sparingly.

34. 6. Based on organ of plant:
root diseases,
stem diseases
foliage diseases and
fruit diseases.
7. Based on the major causal agent
infectious diseases: fungi, bacteria, virus, and nematodes,
non – infectious (abiotic agent):
climatic factors,
mineral,
toxicity,
air, pollination, and
cultural practices.

35. Disease Diagnosis
: determining the causal agent
How?
comparing the systems observed with standard
literatures: books, charts
describing symptoms of known disease on
specific plants.
Koch`s postulate

36. KOCH’S POSTULATE
Basic principle of disease diagnosis in plants Koch`s
postulate include;
Associate (1) Isolate (2)
Inoculate (3) Reisolate (4)

37. Chocolate spot Symptoms

38. Suspension
INFECTED LEAF
CUTTED

40. Disease Development in Plant (Pathogenesis)

41. Parasite: an organism that lives on or in some other organism
and obtained food from it.
Obligate Parasite
Facultative Saprophyte
Facultative Parasite
Obligate Saprophyte/ necrotrophs
Parasitism: the interaction between the parasite and its host.

42. The pathogen not only draws nutrient from the host to grow
and reproduce but its relationship with the host results in:
 synthesis of toxic metabolites,
 harmful to the host,
 resulting in shift of balance more and more
towards parasitism.
If alternations are beyond the tolerance level of the host, it
suffers:
 physiologically,
 cytologically and
 morphologically resulting symptoms & syndromes.

43. What are the basic Requirements for disease Development?

44. Disease is
dependent on
three components:
 host,
 pathogen &
 env`t.
Source: Robert
et al. (2006)
The area within
the triangle
represents the
interaction of
these components,
referred to as a
disease.

46. Since these interactions and their effects are not
spontaneous & time of interaction affect the result
of the interaction the
``disease triangle`` can be modified into a pyramid

47. The disease tetrahedron.
Source: Agrios (2005)

48. The whole chain of event leading to disease development is
known as pathogenesis.

49. Disease Cycle refers to appearance, development and
perpetuation of disease as a function of the pathogen.
The primary events in disease cycle are:
• Inoculation
• Penetration
• Establishment of infection
• Growth and reproduction
• Dissemination
• Overwintering/oversummering

50. Disease Cycle refers to appearance, development and
perpetuation of disease as a function of the pathogen.
The primary events in disease cycle are:
• Inoculation
• Penetration
• Establishment of infection
• Growth and reproduction
• Dissemination
• Overwintering/oversummering

51. INOCULATION
It is the coming in contact of a pathogen with a plant or host.
Inoculum
Any part of pathogen that infect plant

52. What`s Inoculum ?
Inoculum: the pathogens that lands on or are brought in
contact with plant.
Propagule: one unit of inoculum of any pathogen.
.

53. Inoculum -Any part of pathogen that infect plant
Inoculum differs in various organisms.
They includes as follows:
 Fungi: spores, sclerotia, fragment of the mycelium
 Bacteria: cell.
 Virus: Particles
 Nematods: adult nematods, juveniles, or eggs
 Higher parasitic plants: plant fragments or seeds
part of pathogens = Propagule

54. Sclerotia

55. Fungal spore
Viral Particles
Bacterial Cell

56. Nematode

57. Dropping of Banana due to
Nematode infection

58. Root Knot Nematode

59. There are two types of inoculum.
1. PRIMARY INOCULUM
Is the one that survives the off season period and causes the
original infection in the growing season and the infection it
causes is primary infection.
2. SECONDARY INOCULUM
Inoculum produced from this primary infection that actually
spreads the disease in the field under favorable condition is
called secondary incoulum and it in turn, causes secondary
infection.

60. Sources of inoculum
Inoculum in the absence of its host in the field survives in the
 plant debris,
 soil,
 insect vectors,
 tubers,
 transplant or other plant parts,
 perennial weeds and
 alternate host.

61. INFECTED
SEEDS
Plant Debris
insect vectors

62. 2. PREPENETRATION

63. What does it mean?
How pre-penetration occur?
What are the steps in pre -penetration?
What makes d/t from penetration?

64. Prepenetration
This phase include all the events prior to actually entry of
the pathogen. Such events include:
 Attachment of pathogens to host
 Germination of spores and seeds
 Hatching of eggs in nematodes
 Growth of germ tube
 Recognition between host and pathogens

65. I. Attachment of Pathogens to Host
:called adhesion, anchoring to the plant surface.
Adhesion:
attaching to the plant surface itself
serves the propagule to anchor to the plant surface
 using adhesive materials or mucilaginous
substances
 this happen before germination

66. ATTACHMENT OF INOCULUM

67. Two Types of attachment?
1. placing directly into cells of plants by their vectors.
e.g. viruses,
mullicutes,
fastidious fungi,
bacteria &
protozoa.
2. first brought into contact with the external surfaces of the
plants, attached & penetrate.
e.g. almost all;
fungi,
bacteria,
parasitic higher plants

68. The mucilaginous substances are mixture of:
 polysaccharides,
 lipids
 glycoproteins and
 fibrilar materials, which, when moistened, become
sticky and help the pathogens to adhere to the plant.
Spores, seeds germinate and germ tubes also produce
mucilaginous substances helping adhere to the cuticle of
the host plant.

69. II. Germination of Spores and Seed and Perception of
the Host Surface

70. Spore germination and
production of conidiophore

71. Spore germination of apple scab

72. What triggers spore germination?
1. Nutrient availability
Necrotrophic and saprophytic pathogens; the determine
factor is nutrient availability.
Biotrophic microorganisms: lack of nutrients importance
stimulate for germination

73. 2. Physical stimuli and hydration
The surface must be hydrated (water). Hydration alone seems
to be sufficient to induce spore germination in rice blast.
3. Contact Stimulation
It is important and followed by germination of spores or
formation of germ tube.
This is simply contacting or touching the surface and
microorganisms can read the letter of the surface.

74. Factor affecting spore germination?
influenced by biological, environmental and physical
factors.
Biological factors: chemical substances secreted by plant
surface and other microorganisms around the plant surface.
Plant exudates
- inhibit or stimulate germination of spores.
E.g. Fusarium spp spores are stimulated to germinate by root
exudates.
Non-parasitic microorganisms in the rhizosphere
and phyllosphere - also influence spore germination.

75. Environmental factors such as light,
temperature, moisture, and humidity stimulate
the germination of spores of these fungi, other
fungi, and other organisms on plant surfaces.

76. (III) GROWTH OF GERM TUBES
Germ tubes form appressoria which are developed as a result
of thigmtropism with little contributions by chemicals from
the host. Once an appressorium forms on the leaf surface,
infection hyphae are produced that penetrate the leaf and
initiate the infection process. Such hypha with narrow
diameter, known as a penetration peg, grows from the
bottom surface of the appressorium which produces a
haustorium, a specialized feeding cell.

77. Spore or Conidia
Condium germinating and forming
an appressorium on the leaf surface
Conideophores and conidia.
Ascospore

78. NEMATODE

79. What are the factors that affect growth of germ tube of
fungi?
…influenced by leaf surface topography. This response is
referred to as thigmotropism and results in germ tube
growth perpendicular to the parallel arrangement of leaf.

80. Thigmotropism is well understood for appresorium
formation.
Fungi always responds to contact stimuli and
reorient itself according to the leaf surface
orientation.

81. Recognition between host and pathogen
(Host-pathogen interaction)

82. Host component signals
 Recognition factors/ structures/pathways
 Acting as signals for recognition by and activation
of pathogens are numerous
 Fatty acids of cuticle
 Galaacturonan molecules of pectin
 Phenolics – strigol
 Phenolics and sugars

83. Pathogens Component
Elicitors of recognition
Act as elicitor of recognition by the host plant
Released from the pathogen before or during entry.
– B-glucans
– Chitins
– Chitosan

84. Contact - spore germination - germ tube production-
approserium formation – haustrioum- penetration pig –
penetrate and infection

85. 3. PENETRATION

86. This is the actual entry of the pathogens into
their host plants. Pathogens penetrate plant
surface in different ways:
(i) direct penetration through intact plant
surfaces,
(ii) Through natural openings, and
(iii) Through wounds.

87. Infection
• Infection is the establishment of a pathogen in
the tissues or cells obtaining nutrients from
them.

88. Invasion
• Pathogen invades their host indifferent ways
and to different extents.

89. Colonization
• This is the growth and reproduction of
pathogens.

90. Dissemination of Pathogens
• Distribution implies the spread of a pathogen
into new geographic area and its establishment
there.
• Dissemination or disposal is what happens
between takeoff of a spore and its disposition; it
does not include its germination or infection of
the plant.
• Spread implies that the pathogen reaches and
infects plants.
• Contamination is the movement of inoculums to
susceptible plant.

91. Types of Dissemination
Active/autonomous dissemination: disseminate
through the agency of soils, seeds or plant parts
within crop husbandry.
Passive dissemination: general and common
because the pathogens are dispersed through
external agents (i) air (ii) water (iii) insects (iv)
other animals (v) man and (iv) fungi.

92. Air: Puccinia graminis
Water e.g.
Flash of water: Fusarium, Collectotrichum,
Irrigation water:
Xanthomonas malvacearum,
Plasmodiophora brassicae

93. Insects:
• Aphids, whiteflies and leafhoppers: vectors of
viruses and mullicutes.
• Flies, aphids, ants, beetles, wasps and bees: carry
Erwinia amylovora.
• Xanthomonas stewartii dispersed by flea beetles,
• Pseudomonas savastoni carried by olive fly.

94. • Mites: disseminate Nigrospora oryzae
• Nematodes: Corynebacterium alopeuriis carried by
Anguinatritici.
• Other animals move through crop area: Spider,
mice, frog, and birds, dogs: Mycosphaerella linorum.
• Man: e.g Striga asiatica root parasite of maize,
sorghum and other cereals was introduced in USA
on seeds imported from Asia and Africa..
• Fungi: Chytrid, Olpidumbrassicae can transmit
lettuce virus (TNV) etc.

95. RELATIONSHIP BETWEEN DISEASE
CYCLE AND EPIDEMICS
Three types of pathogens are involved in disease development and they are
described as follows:
Monocyclic Pathogens: complete only one disease cycle or part of one in a
year.
cause simple interest disease
Polycyclic pathogens: complete 2-30 diseases cycles per year.
cause compound interest diseases.
e.g. rust, mildews
Polyetic or multiyear pathogens: not completing a disease cycle within a
year but may take several years before the incolulums it produces on the
tree can be disseminated and initiate new infections.
e.g. Dutch elm disease, pear decline and citrus tristeza.

96. PLANT DISEASE EPIDEMIOLOGY
Epidemiology: the outbreak and spread of disease in a
plant population.
Rapid epiphytotics:
• Non-systemic pathogen
• high rate of multiplication
• high birth rate.
• affected more by environmental factors
• easily dispersed.
• controlled by fungicides.

97. Slow Epiphytotics:
• develops slowly
• occur in perennial long-lived plants such as fruits
trees.
• has low death rate.
• systemic in their attack
• spread slowly over a long period
• checked only by removal of the source of inoculum
through regulation and cultural methods.
• dangerous because once developed it becomes
difficult to eradicate the source of primary
inoculum.

98. Factors that affect development of
epidemics
The factors that affect disease development include;
Host Factor:
Level of genetic resistance or susceptibility, degree of genetic
uniformity of plant, types of crop and age of plants
Pathogen factors: virulence level, virulence level,
reproduction pattern of pathogen, ecology of
pathogens, mode of spread of pathogen:
Environmental Factors
Human activities: site selection, selection of propagative
material, cultural practices, disease Control measures,
introduction of new pathogens

99. PATHOGENS WEAPONS OF ATTACK