here we are going discussing about the causes of the plant diseases and also their brief description

1. Lectuer 3
Prepared by
R MOHANAPRIYA
Assisstant Professor (Plant Pathology)
JSA College of Agriculture and Technology,
Ma. Podaiyur, Cuddalore district, Tamilnadu

2. Causes of the Plant Diseases
I. Living / Animate / Biotic agents/ Infectious
-Eukaryotes
-Prokaryotes
II. Non-Living / Inanimate / Abiotic agents/ Non Infectious
-Deficiencies
-Toxicities
-Environmental conditions
III. Mesobiotic

4. Definition
FUNGI
 Eukaryotic
 Achlorophyllous
 Heterotrophic
 Sexually & Asexually
 Cell wall – Chitin & Cellulose
Eg: Damping off, Downy mildew, Powdery mildew, Rust, Leaf
spot, Leaf blight, Smut etc.,

5. PROTOZOA
• Heterotrophic
• Animal, fungi, flagellate & plant
• Eukayrote
• Motile
• Microscopic
Eg: Club root of cabbage, potato powdery scab

6. CHROMISTA
 Introduced by Cavalier-Smith , 1981
 Means colored
 Eukaryotic
 Uni or multi cellular
 Cellulose
 Photosynthetic – Chlorophyl C
 Sexual and asexual
 Eg: late blight of potato, damping off, & mildew diseases

7. BACTERIA
 Prokaryotic
 Unicellular
 Motile/Non-Motile
 Microscopic
 Lacks chlorophyll
 Rigid cell wall
 Asexually by binary fission
Eg: Bacterial leaf blight of rice, Citrus canker, Apple fire blight etc.,

8. VIRUS
 Obligate parasite
 Nucleoproteins
 RNA / DNA
Eg: Cassava mosaic virus, sugarcane mosaic virus,
banana bunchy top virus, cauliflower mosaic virus etc.,
VIROID
 Naked nucleic acid without protein coat
Eg: Potato spindle tuber viroid, coconut cadang cadang viroid
disease.

9. PHYTOPLASMA
 Pleomorphic
 Lack cell wall
 Non-Motile
 Sensitive – Tetracycline &
Chloramphenicol
 Resistant – Penicillin
 Cannot be culture on artificial
media
 Require sterol
 Gram negative
 No spores & flagella.
Eg: Sesame phyllody, little leaf
of brinjal, sugarcane grassy
shoot, rice yellow dwarf etc.,
SPIROPLASMA
 Spiral to Helical shape
 Cell wall less
 Rotary motility
 Sensitive – Tetracycline
 Resistant – Penicillin
 Can be culture on artificial
media
 Require cholesterol
 Gram negative
 No spores & flagella.
Eg: Citrus stubborn, corn stunt
etc.,

10. Fastidious vascular bacteria / Rickettesia like
organism
 Gram negative
 Pleomorphic
 Cell wall
 Rod shape
 Cannot be grown in simple culture media
 Confined to either Xylem or Phloem
Eg: Xylem ltd. FVB – Pierce’s disease of grapevine
Phloem ltd. FVB – Citrus greening

11. ALGAE
 Eukaryotic
 Uni or multi cellular
 Sexually & Asexually
Eg: Red rust of guava, sapoto, mango, and tea.
PHANEROGAMIC PARASITE
- Flowering plants, draw nutrition and water from crop plants
- Partial / total parasites
 Partial stem parasite - loranthus
 Partial root parasite - striga
 Total stem parasite – cuscuta
 Total root parasite - orobanche

12. NEMATODES
 Bilaterally symmetric
 Ubiquitous
 Ecto or Endo parasitic
 Migratory or Sedentary
Eg: Wheat ear cockle nematode, rice white tip nematode

13. ABIOTIC DISEASES/ DISORDERS
o Abiotic disorders are caused by nonliving factors, such as
drought stress, sunscald, freeze injury, wind injury, chemical
injury, nutrient deficiency, or improper cultural practices, such
as overwatering or planting conditions.
o Zn def. - Khaira disease
o Ca def. – blossom end rot in tomato & crucifers
o Boron – top sickness in tobacco & heart rot in sugar beet

14. KOCH’S POSTULATES
1. Must be found associated with every instance
2. Must be isolated and brought into pure culture
3. Healthy host is inoculated with microorganism under
optimum condition
4. Must be reisolated and on comparison with original culture,
found identical.