Agriculture

1. Concepts and Classification of plant diseases

2. Diseases: Any abnormal condition that alters the appearance
or function of a plant.
Diseases: Dis + ease = Uneasiness
It leads to the death of infected Plant
If the malfunctioning or abnormality of
plant enhances its beauty or economic
value, then it is not said to be a disease.
e.g. Broken tulips
Diseases: Abnormal changes in
physiological processes which
disturb the normal activity of the
plant organ. (Julius Kuhn, 1858)

3. Susceptible Host
Virulent Pathogen Favorable environment
Time
“All the pathogens
are parasite but all the
parasites are not
pathogens”

4. Symptom: Visible effect of pathogen on the host plant or response
of the host plant to the pathogen.
1. Morphological Symptom
2. Histological Symptom
Syndrome : Series of symptoms / More number of symptoms for
single disease.
Sign: Visible structure of the pathogen produced in or on the
diseased tissues.

5. Sign
Syndrome
Symptom
Late Tikka leaf spot
Grapes powdery mildew

6. Symptoms
1. Colour change
i) Chlorosis
ii) Chromosis
iii) Albinism
2. Necrosis – Degeneration of Protoplasts
3. Hypertrophy – Enlargement of cells
Hyperplasia – Excess cell division
4. Hypoplasia – Reduced development of hole plant
(e.g. Stunting, dwarfing, etc.)

7. Hypertrophy & Hyperplasia
Hypoplasia
Banana Bunchy top
Necrosis Chlorosis

8. Classification of plant diseases
1. Type of infection
a. Localized diseases: These diseases
are limited to a definite area of an organ
or part(s) of a plant. e.g. leaf spots and
anthracnoses caused by fungi.
b. Systemic diseases: In these diseases
the pathogen spreads from a single
infection point to infect all or most of
the host tissues. e.g. Downy mildews
caused by fungi and mosaics and leaf
curls caused by viruses.

9. 2. Type of perpetuation and spread
a. Soil-borne diseases:
e.g. Damping off caused by fungi
Pythium spp. and root rot caused by
Rhizoctonia spp.
b. Seed-borne diseases:
e.g. Loose smut of wheat caused by
Ustilago nuda tritici (internally
seed-borne) and blast of rice caused
by Pyricularia grisea (externally
seed-borne).
c. Air-borne diseases:
e.g. Early leaf spot and late leaf spot of
groundnut caused by Cercospora
arachidicola and Cercospora
personata respectively.

10. 3. Extent of occurrence and geographic distribution
a. Endemic diseases: When a disease is more or less
constantly occurring year after year in a moderate to severe
form in a locality then it is called as an endemic disease.
E.g. Wart disease of potato caused by Synchytrium
endobioticum in Darjeeling
b. Epidemic or epiphytotic diseases: An epidemic or
epiphytotic refers to sudden outbreak of a disease
periodically over a widespread area in a devastatingly severe
form causing extensive losses or complete destruction.
E.g. Late blight of potato (Phytophthora infestans)

11. c. Sporadic diseases: Sporadic diseases are those,
which occur at irregular intervals over limited areas or
locations.
E.g. Fusarium wilt of cotton (Fusarium
oxysporum f.sp. vasinfectum)
Wheat loose smut (Ustilago nuda tritici)
d. Pandemic diseases: A disease is said to be
pandemic when it is prevalent throughout the country,
continent or world involving mass mortality.
E.g. Late blight of potato and wheat stem rust.

12. 4. Multiplication of inoculum
Based on the multiplication of inoculum, diseases are classified as
a. In simple interest disease (monocyclic epidemics) the disease
increase is like simple interest in money. Here inoculum comes from a
reservoir and hence amount of inoculum for a given season's crop is
fixed. So there is no repetition of the disease cycle within the crop
season. Hence the disease spread will be slow.
e.g. Soil pathogens like Pythium sp., Rhizoctonia sp. and Sclerotium sp.
b. In compound interest disease (polycyclic epidemics) the disease
increase is like compound interest in money. Inoculums gets multiplied
several times (every 7 to 15 days for wheat rust) during crop growth in
a season. So the disease spread is fast.
e.g. Wheat stem rust, rice blast, powdery mildew diseases

13. 5. Based on causal agents
A. Parasites:
i. Biotic agents: These are also called as animate or living
organisms.
1. Prokaryotes
a. True bacteria or bacteria e.g. Citrus canker.
b. Rickettsia-like bacteria (RLB) or FVB
e.g. Citrus greening, Pierce's disease of grapevine
c. Mollicutes or wall-less prokaryotes
i. Phytoplasma (Mycoplasma-like organism -MLO)
e.g. Sesame phyllody
ii. Spiroplasma e.g. Corn stunt, Citrus stubborn

14. 2. Eukaryotes (organisms with membrane
bound true nuclei)
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. hart rot of coconut
iii. Algae e.g. red rust of mango
iv. Chromista (False Fungi)
b. Plants - Parasitic flowering plants or
phanerogamic parasites Broomrape of tobacco.
c. Animals e.g. Nematodes

15. Character Prokaryote cell Eukaryote cell
Common size ranges
(dia )
0.3 to 2.5 µm
mycoplasmas are 0.1 to 0.25µm
2 to 20 µm
Nucleus membrane
bound
No Yes
Number of
chromosomes / nucleus
One Generally more
than one
Chromosome
replication by mitosis
No Yes
Cell organelles None Present
Chloroplasts None Occurs in plants
only
Cell wall and envelope
components
Peptidoglycans (except
mycoplasmas) teichoic acid,
lipotechoic acid,
lipo-proteins,
lipopolysaccharides.
Cellulose and other
polysaccharides (in
plants only)

16. ii. 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.
1. Viruses e.g. tobacco mosaic
2. Viroids e.g. spindle tuber of potato
iii. Non-parasites or Abiotic agents: These are also called
as non-infectious or physiological disorders. These
diseases occur because of disturbances in the plant system by
the improper environmental conditions.

17. 6. Based on Kinds of symptoms produced
Wilt in Cotton – Fusarium oxysporum f.sp. Vasinfectum
Soft rot in Fruits – Pectobacterium carotovorum sub sp.
Carotovorum
Anthracnose in chilli – Glomerella cingulata
(Colletotrichum capsici)
Rust in Sorghum – Puccinia sorghi
Grain smut in Sorghum – Sporisorium sorghi
Powdery mildew in Wheat – Erysiphe graminis
Downy mildew in Grape vines – Plasmopara viticola
Damping off of Vegetables – Pythium aphanidermatum

18. 7. Based on Host plant affected
Cereal diseases
Oil seed crops diseases
Fiber crops diseases
Vegetable crops diseases
Fruit crops diseases
Plantation crops diseases
Flower crops diseases
Spice and condiment crops diseases
Medicinal and Aromatic plants diseases
8. Based on Host plant affected
Root diseases
Stem diseases
Foliar diseases
Fruit diseases
Seed diseases

19. Deficiency
Disease
Difference between disease and deficiency

20. Difference between disease and deficiency
Disease will produce dark
margins around infected
portion

21. Thank you