This document discusses plant pathology, which is the scientific study of plant diseases caused by pathogens and environmental conditions. It addresses the causes of plant diseases, including living organisms like fungi, bacteria, viruses and nematodes, as well as non-living factors. The disease cycle and factors affecting disease development are also examined. Plant diseases can cause significant economic losses by reducing crop yields and quality. Understanding plant pathology is important for preventing diseases and maintaining food supply.

1. Unit 3
Plant pathogen

2. Plant pathology
deals with the nature, causes and control of
plant diseases. It is a science which looks into
the characteristics of the diseases, their causes,
plant-pathogen interactions, factors affecting
disease development in individual plants and in
populations, and various means of controlling
diseases.
is the scientific study of diseases in plants
caused by pathogens (infectious organisms) and
environmental conditions (physiological factors).

3. Plant pathology
• The art of plant pathology deals with the
application of the knowledge gained from
studying it as a science. This includes the
following:
a. diagnosis or recognizing particular diseases by
their symptoms and sign; diseases assessment
and forecasting;
b. recommendation of appropriate control
measures; and
c. field application of suitable of control measures

4. Plant pathology
The ultimate objective of plant pathology is to
prevent or minimize plant diseases not only
to increase food production but also to
maintain the quantity and quality of the
harvested fresh commodity until it reach the
final consumer. We also need to protect and
preserve plant used for fiber, drugs and
aesthetics.

5. ECONOMIC IMPORTANCE OF PLANT DISEASES
• It has been said that men and animals exist on
earth as guests of the plant kingdom because
only the green plant can convert the energy
from the sun into food. We depend on plants
not only for food but also for our clothing and
shelter needs and for numerous luxuries.
When diseases kill plant, all other form of life
on earth is adversely affected.

6. ECONOMIC IMPORTANCE OF PLANT
DISEASES
• The study of plant diseases is important as they cause
loss to the plant as well as plant produce. The various
types of losses occur in the field, in storage or any time
between sowing and consumption of produce. The
diseases are responsible for direct monitory loss and
material loss. Plant diseases still inflect suffering on
untold millions of people worldwide causing an
estimated annual yield loss of 14% globally with an
estimated economic loss of 220 billion U. S. dollars.
Fossil evidence indicates that plants were affected by
different diseases 250 million year ago. The Plant
disease has been associated with many important
events in the history of mankind of the earth

7. Diseases that had caused enormous economic
losses
The following are examples of plant diseases in the Philippines that have caused
enormous economic losses:
• Cadang-cadang disease of coconut
first observed in 1918; have caused the country a loss of over $200M.
• Downy mildew of corn
the nemesis of corn; loss can be as high 95% amounting to over P170M
annually; now controlled by chemical seed treatment using metalaxy, discovered in
1978; caused by the fungus Peronosclerospora philippinensis (weston) shaw.
• Rice tungro disease
affected 70,000 has in 1971; 1.22M cavans rough rice lost valued at
P30,357,000
• Coffee rust
destroyed the coffee industry in Batangas province.
• Citrus decline
destroyed citrus plantations in Batangas.

8. Types of Crop Losses
• Reduction in yield
-Leaf spots/blight reduce photosynthetic capacity of plants
-Root pathogens
-Fruit rots and fruit spots – reduce quantity of harvestable and marketable
fruits
• Losses from deterioration during storage, marketing or transport
-The amount of lost food daily is enough to feed the world’s population
• Reduction in quality of produce
-Citrus fruits with scabs
-Moldy cereals and other commodities
-Reduced strength and undesirable discoloration in wood pulp
-Poor germination of infected seeds Losses from produce contaminated with
toxins that cause various disorders and/or death to animals including man.
 Aflatoxin
-Produced by Aspergillus flavus; carcinogenic to animals and man;
commonly found in stored corn, sorghum, copra, root crops etc.

9. Types of Crop Losses
 Ochratoxin
-A mycotoxin produced by A. ochraceous causes cancer of the liver.
 Yellow rice toxins
-Formed by Penicillium spp.; caused several deaths in Japan.
 Estrogenic factor in corn
-Produced by Fusarium graminearum; causes testes of young male swine to
have atrophy and to have uterus of female pigs to enlarge and abort.
 Fumonisins
-Formed by Fusaruim spp. In corn grains; caused esophageal cancer in man
and toxic to animals like horses. Losses due to predisposition of host to attack by other
pathogens
Example:
 Nematode injuries on roots serve as point of entry to other pathogens.
 Leaf pathogens weaken plants which can become a host susceptible to root-rotting
pathogens.
 Severely defoliated trees can be readily attacked Armillaria mellea and other fungi.

10. Types of Crop Losses
• Losses from increased cost of production and handling
 Cost of disease control is an added cost.
 Cost of culling disease commodities for marketing and processing.
 Infected and stained wood chips need longer time to bleach to
obtain white paper product.
• Vital Processes Affected
 root absorption
 uptake of water and minerals
 photosynthesis
 respiration
 transport of photosynthates
 reproduction

11. Importance of Plant Diseases
Plant diseases damage plants and plant products
therefore it is directly related to importance of
plant as:
• Food
• Shelter
• Clothing
• Medicine
• Aesthetics
• Improving environment
• Luxuries

12. Negative Impact of Plant Diseases
• Plant diseases endanger food supply
• Reduce the quantity and quality of plant produce
• Cause financial losses
• Limits the kind of plants and industries in an area
• Make plants poisonous to human
• Increase cost of production due to control
measures

13. CONCEPTS OF PLANT DISEASES
Disease – abnormal condition that negatively affects the
structure of function of all part of organism
- dis order of structures/functions of an organism
- is a common occurrence but a wholly satisfactory
definition of the term has eluded plant pathologists for so
long.
• Important 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
favourable.

14. THE DISEASE TRIANGLE

15. FACTORS FOR SUCCESSFUL DISEASE
DEVELOPMENT
1. Properties of pathogen
• Level of virulence
• Adaptability
• Dispersal efficiency
• Survival efficiency
• Reproductive fitness
2. Properties of host
• Susceptibility
• Growth stage and form
• Population density and structure
• General health
3. Properties of environment
• Temperature
• Rainfall/dew
• Leaf wetness period
• Soil properties
• Wind

16. CAUSES OF PLANT DISEASES
• Plant Disease Agents
1. Living Organisms - includes fungi, bacteria,
viruses and nematodes
2. Non-living agents – including unbalanced soil
fertility, toxic chemicals, air pollution, frost,
drought, sunburn, wind and hail.

17. LIVING ORGANISMS - PLANT
PATHOGENS
1. Fungi-Fungi can cause a variety of symptoms including leaf
spots and blights, root rots, seedling blights, seed
discoloration, wilts, and stem rots

18. LIVING ORGANISMS - PLANT
PATHOGENS
2. Bacteria- Typical symptoms of bacterial diseases include leaf
spots, water soaking, and soft rots of plant tissues

19. LIVING ORGANISMS - PLANT
PATHOGENS
3. Viruses- Typical viral symptoms include mosaic patterns on
leaves, deformation of plant tissues, stunting, seed discoloration,
and reduced yield.

20. LIVING ORGANISMS - PLANT
PATHOGENS
4. Nematodes- Most important plant‐parasitic nematodes feed on plant roots
and directly interfere with water and nutrient uptake by the plant. Root injury
causes aboveground symptoms similar to those produced by other conditions
that damage root systems

21. DISEASE CYCLE
STAGES OF PLANT DISEASES
1. INOCULATION
-the arrival of pathogen on the host
-initial contact of a pathogen with a site of plant where infection is
possible
• Inoculum – any part of the pathogen that can initiate infection
Examples: Fungi – Spores, sclerotia or hyphae
Bacteria - Mollicutes, protozoa, viruses or viroids: whole individual
Nematodes: Adults, juveniles or eggs
Sources of inoculum
• Branches, trunks, roots of plant
• Plant debris, soil in the field
• Seeds, transplants, tubers, other propagative organs
• Sources outside the field (nearby plants or fields)
• Perennial weeds, alternate hosts

22. DISEASE CYCLE
2. PENETRATION -initial invasion of the host on the
pathogen
• Direct penetration through intact plant surfaces –
Fungi and Nematodes
• Penetration through natural openings – E.g.:
Stoma, hydathode (open pores at margins and
tips of leaves), nectarthode & lenticels (openings
on fruits, stems & tubers – Fungi & Bacteria
• Penetration through wounds – Fungi, bacteria,
mollicutes, viruses & viroids

23. DISEASE CYCLE
3. INFECTION – establishment of pathogen with
vulnerable cells or tissues of the host and obtain
the nutrients from them. Successful infection will
produce symptoms.
4. GROWTH AND REPRODUCTION
• colonization
• the pathogen will grow and multiply within the
infected host
• Successful colonization results in the appearance
of symptoms

24. DISEASE CYCLE
5. DISSEMINATION OF PATHOGEN – transport of spores
of infectious bodies from one host to another host at
various distance resulting in the spread of the disease
Pathogens are disseminated by several way:
• By air
• By water
• By insects, nematodes, and other vectors (during
feeding)
• By seed and transplanting process
• By human

25. Epidemiology
Epidemic
 (Layman’s view) is a wide spread, explosive disease
outbreak.
 Van der Plank (19630) – Epidemic is an increase in
disease incidence within the population with time.
Epiphytotics
 Refer to epidemics of plant disease
Endemic
 Disease is one that is native or indigenous to a
particular place.

26. Terminology
Pandemic disease
It is one of worldwide or widespread
occurrence throughout a cotenant or a region.
Sporadic disease
Occur at irregular intervals.
Exotic disease
it is one which had been introduced from
some other area.

27. Factors Affecting the Development of
Epidemics
1. Plant susceptibility
2. Pathogen virulence
3. The duration and intensity of the various
environmental factors

28. Variability in Plant Pathogens
 Plant pathogen that causes plant diseases varies from fungi,
bacteria, virus, and others. And these pathogens can be further
classified according to taxonomy
Terminology
 Physiological specialization – within the species of a pathogen
there exist certain individuals that are morphologically similar but
differs with respect to their physiology, biochemical characters and
pathogenicity and are differentiated on the basis of their reaction
on certain host genera or cultivars
 Physiologic race – individuals within the species of a pathogen that
morphologically similar but differ with respect to their
pathogenicity on particular set of host varieties.

29. Variability in fungi
1. Mutation
2. Recombination
3.Heterokaryosis
4.Parasexualism - is the process by which
genetic recombination can occur within fungal
heterokaryon
5. Heteroploidy

30. Variability in Bacteria
1. Conjugation- Transfer of DNA from one
bacterial cell to another
2. Transformation- DNA taken up from external
environment by absorption
3. Transduction - Transfer of bacterial genes
with a bacteriophage

31. Variability in Viruses
1. Recombination
2. Reassortment
3. Mutation

32. Variability - it is the property of an organism to change its
characters from one generation to the other. Variation -
when progeny of an individual show variation in
characters from parents such a progeny is called a variant.
Pathotype - A pathotype is a population of a parasite
species in which all individuals have a stated pathosystem
character (pathogenicity or parasitic ability) in common.
Biotype - progeny developed by variant having similar
heredity is called a biotype or a subgroup of individuals
within the species, usually characterized by the
possession of single or few characters in common.