Biotic stress occurs when living organisms like fungi, bacteria, viruses, nematodes or insects damage plants. Some examples of damaging biotic stresses include the potato blight that caused famine in the 1840s in Europe and grape phylloxera that led to the Great French wine blight in the 19th century. Biotic stresses can kill plants, damage leaves and reproductive organs, and reduce growth. They also physiologically alter plants by decreasing photosynthesis, increasing transpiration, reducing chlorophyll content, introducing toxins, and reducing the activity of the calvin cycle enzymes.

1. Biotic stress in plants
 Introduction
 Stress is a biological condition of a plant
resulting from the influence of some
environmental factors that adversely affect
its complete growth and development.
 Depending on the nature there are mainly
two types these are biotic stress and abiotic
stress.

2. Biotic stress
 This type of stress occurs as a result of
damage done to plants by other living
organisms such as
 Fungi
 Bacteria
 Virus
 Nematodes ,harmful insects and weeds etc

3. History
 Biotic stresses have huge effect on plants in
this way cause different loss. For example the
potato blight , an oomycete which caused
widespread famine in England,Ireland and
Belgium in the 1840.
 Another example is grape phylloxera coming
from North America in the 19 century which
lead to the Great French wine Blight.

4.  Plant affected by disease is known as host
,while the organism that produce the disease
is termed as pathogen.
 Some diseases are mention below
 Late bight of potato,black rust of wheat
 Mosaic of tomato and tobacco,citrus cancer
 Crown gall of vascular plants

5. General losses due to disease
 Killing of plants
 Damage to the leaf tissues
 Damage to reproductive organs including
fruits and seeds.
 Besides the pathogenic attacks, insects also
lead certain major damage economically
important plants.
 For example cotton is attacked by more than
60 species of harmful insects

6. Certain losses due to insects
 Reduced growth
 Premature defoliation
 Wilting of plants
 Serves as vectors for some pathogen making
the plants vulnerable for further pathogenic
infection

7. Physiological alteration in
host plant due to biotic stress
 Photosynthesis is tremendously decrease after
pathogenic infection.This is carried out by
 Reduction in photosynthetic area the plant
disease
 that attack green aerial tissues like leaf spot,
blight
 render harmful effects by destructing
photosynthetic leaf tissue or defoliation which
result in lessening the photosynthetic area
thereby reducing the photosynthesis.

8. Transpiration
 Rate of transpiration increases in plants after
certain disease infection leaf rust,apple
scab,downy mildew
 This is because these diseases result in
destruction of protection devices in plants
like cuticle,epidermis,increase permeability
of leaf cells resulting in uncontrolled loss of
water and loss of turgor to which wilting of
leaves follows.

9. Reduction in chlorophyll
content
 Chlorosis is one of the most common
symptoms of plant disease and refers to the
destruction of chloropyll per chloroplast
 It is resulted by viral infection e.g tobacco
mosaic disease etc

10. Effect of toxins
 In some fungal and bacterial diseases.
Photosynthesis is reduced because of toxins
 The toxins like tabtoxins inhibit some of the
enzymes that are directly involved in
photosynthesis

11. Reduction in activity of calvin
cycle
 After disease infection the activity of some key
enzymes of calvin cycle is reduced due to
changes in the concentration of soluble
carbohydrates in infected tissues which in turn
effects photosynthetic fixation of carbon
dioxide.
 In Arabidopsis, leaves infected by the biotrophic
fungus Albugo candida the reduction in the rate
of photosynthesis is parallel to decrease in the
amount of Rubisco enzyme present in the host
tissue.