Fire blight is a destructive bacterial disease affecting apple and pear trees. It's characterized by wilting, blackening, and "scorched" appearance of blossoms, shoots, and branches. It spreads rapidly, especially in warm, moist conditions, posing a significant threat to orchards. Control measures include pruning infected parts, using resistant varieties, and applying antibiotics or copper-based sprays during the growing season.

1. TRIBHUWAN UNIVERSITY
Institute of Agriculture and Animal Science
(IASS)
An assignment on the topic:
Micro-organisms and plant diseases with prime focus on signs and
Symptoms.
Submitted By: Submitted To:
Name: Monika Gaire Sanjay Mahato
Class roll no: 22 Department of microbiology
3rd
semester IAAS, Rampur Campus
Agriculture microbiology

2. INTRODUCTION
Bacterial disease
Bacterial diseases can be grouped into four broad categories based on the extent of damage to plant
tissue and the symptoms that they cause, which may include vascular wilt, necrosis, wilt and
tumours. Vascular wilt results from the bacterial invasion of the plant's vascular system. Plant
diseases caused by bacterial pathogens place major constraints on crop and forest production and
cause significant annual losses on a global scale. Numerous bacterial diseases affect crop
production in many countries and regions. These include fire blight in fruit trees, bacterial wilt in
banana, and bacterial blight in rice and crown gall in many perennial plants. In some cases, the
epidemics caused by bacteria can cause significant economic burden on crops.
PATHOGEN BIOLOGY
The taxonomy of plant pathogenic bacteria is currently in flux based on recent advances on
how bacteria are classified. Most plant pathogenic bacteria belong to the following
genera: Erwinia, Pectobacterium, Pantoea, Agrobacterium, Pseudomonas, Ralstonia,
Burkholderia, Acidovorax, Xanthomonas, Clavibacter, Streptomyces, Xylella,
Spiroplasma, and Phytoplasma. Plant pathogenic bacteria cause many different kinds of
symptoms that include galls and overgrowths, wilts, leaf spots, specks and blights, soft rots,
as well as scabs and cankers. In contrast to viruses, which are inside host cells, walled
bacteria grow in the spaces between cells and do not invade them. The means by which plant
pathogenic bacteria cause disease is as varied as the types of symptoms they cause. Some
plant pathogenic bacteria produce toxins or inject special proteins that lead to host cell death
or they produce enzymes that break down key structural components of plant cells and their
walls. An example is the production of enzymes by soft-rotting bacteria that degrade the
pectin layer that holds plant cells together. Still others colonize the water-conducting xylem
vessels causing the plants to wilt and die. Agrobacterium species even have the ability to
genetically modify or transform their hosts and bring about the formation of cancer-like
overgrowths called crown gall. Bacteria that cause plant diseases are spread in many ways—
they can be splashed about by rain or carried by the wind, birds or insects. People can
unwittingly spread bacterial diseases by, for instance, pruning infected orchard trees during
the rainy season. Water facilitates the entrance of bacteria carried on pruning tools into the
pruning cuts. Propagation with bacteria-infected plant material is a major way pathogenic
bacteria are moved over great distances. No matter how the bacterial pathogens are
disseminated, they require a wound or natural opening, such as stomata, to get inside a plant
host. Once inside they then kill host cells, by the means described above, so that they can
grow. Between hosts they may grow harmlessly on plant surfaces and then can overwinter
or survive unfavorable environmental periods or the absence of a susceptible host by either
going dormant in infected tissue, infested soil or water, or in an insect vector.

3. SIGN AND SYMPTOMS
1. Vascular wilt:
Caused by bacterial invasion of the plant's vascular system, leading to replication and closure that
inhibits the movement of water and nutrients through the xylem. Affecting herbaceous plants,
symptoms include wilting and death of aboveground parts, with bacterial ooze seeping out of
infected leaves. Symptoms include drooping, wilting, or death of the aerial plant structure.
Examples include bacterial wilt of sweet corn, alfalfa, tobacco, tomato, and cucurbits, as well as
black rot of crucifers.
2. Necrosis:
Caused by pathogens secreting toxins, leading to symptoms such as leaf spots, stem blights, or
cankers.
3. Soft rot:
Caused by pathogens secreting enzymes that decompose cell wall structures, resulting in the
softening and watery texture of plant tissue. Pathogens like Erwinia, Pseudomonas, Bacillus, and
Clostridium cause soft rot diseases by breaking down host tissue, resulting in watery, soft, foul-
smelling tissue and slimy bacterial ooze.
Common in vegetables like potatoes, carrots, eggplant, squash, and tomatoes.
4. Tumors (Abnormal Growth):
Caused by bacteria promoting uncontrolled replication of plant cells, resulting in the formation of
abnormally large structures.
5. Symptom variation:
While most bacterial diseases exhibit major symptoms, some may produce a range or combination
of symptoms.
6. Diagnosis:
Identifying bacterial pathogens in plants generally isn't too difficult based on symptoms.
However, identifying the specific causative agent in animal species requires laboratory techniques
such as isolation and placement of the pathogen.
7. Leaf Spots and Blights:
Commonly seen as spots on leaves, blossoms, fruits, and stems, advancing rapidly if it's a blight.
Leaves of dicotyledonous plants develop water-soaked spots with a fishy odor, often with bacterial
ooze present. Chlorotic halos may surround the lesions, which can coalesce, causing necrotic
tissue. Monocotyledonous plants exhibit streaks or stripes.
8. Cankers: Pseudomonas and Xanthomonas cause canker diseases in stone fruit, pome fruit trees,
and citrus. Symptoms include sunken, dark brown lesions on trunks, stems, twigs, and branches,
sometimes with gum exudation. Severe cases lead to leaf stress and dieback.
9. Bacterial Galls: Produced by various bacteria such as Agrobacterium, resulting in overgrowths
like crown gall or root gall.

4. FIRE BLIGHT OF APPLE
INTRODUCTION
Fire blight is a common and very destructive bacterial disease of apples and pears that can occur
in commercial orchards and residential plantings. The disease is caused by the bacterium Erwinia
amylovora, which can infect and cause severe damage to many plants in the rose (Rosaceae)
family. On apples and pears, the disease can kill blossoms, fruit, shoots, twigs, branches and entire
trees. While young trees can be killed in a single season, older trees can survive several years, even
with continuous dieback. Many landscape trees and shrubs in the rose family are also susceptible
to fire blight. Because precise conditions are needed for infection, disease appearance may seem
erratic from year to year. However, when conditions are favorable, fire blight can quickly cause
severe damage.
SIGN AND SYMPTOMS
Blossom & Spur Blight
Blossom Blight:
1. Wilting of blossoms
2. Blackening of blossoms

5. 3. Scorched appearance of affected blossoms
Spur Blight:
1. Blackening and necrosis of shoots
2. Blackening and necrosis of spurs
3. Infection often spreads from the blossom to affect spurs
Cankers
1.Spread from Blossoms
Infections often spread from blossoms to supporting spurs and branches.
2.Stem Lesions or Cankers
These infections result in the formation of stem lesions or cankers.
3.Appearance
Initially, cankers appear sunken with a dark brown to purple color.
4.Girdling Branches
As cankers grow, they encircle branches, causing them to become girdled.
5.Tissue death
Tissues above the infection sites die due to the constriction caused by the cankers.
Shoot Blight
1.Bacterial Buildup
Bacterial cells accumulate during the blossom and spur blight phases.
2.Infection of Rapidly Growing Shoots
The bacteria subsequently infect rapidly growing shoots.
3.Leaf Symptoms
Infected leaves initially blacken along the midrib and veins, then turn completely brown.
4.Attachment of Blighted Leaves
Blighted leaves remain attached to shoots, giving trees a scorched appearance.
5.Shoot Symptoms
Infected shoots wilt from the tip and develop a crook or bend at the growing point, known as a
"shepherd's crook."
6.Presence of Bacterial Ooze
Droplets of bacterial ooze may be present during warm, humid weather.
Rootstock/Trunk Blight
1.Development of Trunk Infections
Trunk infections can occur near the rootstock graft union.
2.Pathogen Movement
Pathogens move internally within water-conducting tissue or via infected water sprouts to reach
these sites.

6. 3.Bark Symptoms
Bark at the infection sites becomes water-soaked, discolored, and cracked.
4.Wood Discoloration
The wood beneath the infected bark develops a reddish-brown discoloration.
5.Rapid Tree Death
Trunk infections often lead to rapid tree death as the disease progresses.

7. DISEASE CYCLE
1. Early Spring Infection: In early spring, the bacteria become active as temperatures warm up,
and they ooze from cankers or are carried by wind, rain, or insects to flowers, where infection
begins.
2. Infection of Flowers: The bacterium enters through natural openings in flowers or wounds
caused by frost, insects, or other mechanical damage. Once inside the flower, the bacteria multiply
rapidly, causing the flower to turn brown and wilt.
3. Bacterial Growth and Spread: As infected flowers wilt, the bacteria can spread further through
rain splash, insects, and even bees visiting flowers, leading to the infection of nearby healthy
flowers and shoots.
4. Systemic Infection: In severe cases, the bacteria can move systemically through the vascular
system of the tree, causing extensive damage to branches and even the main trunk.
5. Environmental Conditions: Warm, humid weather during bloom favors the rapid spread of fire
blight, as it provides ideal conditions for bacterial growth and movement.
6. Secondary Infections: Besides flowers, other plant parts like young shoots, fruit, and even
rootstocks can become infected, leading to secondary infections and further spread of the disease.
7. Canker Formation: Infected tissues may develop cankers, which serve as reservoirs of the
bacteria, enabling them to survive through the winter and initiate new infections in the following
spring.
8. Control Measures: Management strategies include cultural practices like pruning out infected
branches, applying bactericides during bloom, and selecting resistant apple varieties.

8. MANAGEMENT STRATEGIES
Monitoring: As with most pathogens, monitoring for physical signs of bacteria is impractical.
Instead, monitor infection risk potential using disease forecast models such as Maryblight,
developed at the University of Maryland. This model measures temperature, rainfall and leaf
wetness to determine the risk of blossom, canker, shoot and trauma blights. The need for chemical
control depends on a combination of orchard risk factors and weather conditions during bloom.
Infection is most likely to occur after 200 degree days (base 65˚F) have accumulated since the first
blossom has opened in the orchard.
Control Strategies
Cultural/ Biological:
 Removal of cankers during dormant pruning.
 Avoid more susceptible cultivars when planting new orchards.
 Plant on well-drained soil.
 Maintain proper orchard nutrition in order to discourage excessive tree vigor.
 Inspect orchards weekly in the summer for fresh infections and remove.
 When removing infected plant material, cut infected branches at least 12 - 18” below the lowest
evidence of disease.
 When removing infected plant material during the growing season, prune only on sunny, hot
days when rain is not predicted.
Chemical
 Dormant to early season copper applications reduce epiphytic bacterial populations.
 Apply recommended fungicides at the green tip stage to reduce the amount of inoculum on the
outer surfaces of infected trees.
 Control insect vectors in the orchard.
 Use biological control materials in conjunction with antibiotic materials to reduce resistance
development.
 Apply antibiotic sprays at bloom to help prevent the disease from becoming established in an
orchard.
 Avoid overuse of materials that can lead to the development of resistance in this pathogen.