Transcript 09 Phytopathogenic microorganisms

Microbiology, Virology, and Immunology
Department
Phytopathogenic
microorganisms
History of plant pathology
Causes of plant disease
Bacteria
Fungi
Viruses and viroids
Disease cycles
Control
Plant associated bacteria may be beneficial
or detrimental.
Epiphytes – microbes which are on all
plant surfaces.
Endophytes – microbes live inside plants.
Phytopathogens – organisms that cause
infectious disease
Populations of 106 CFU (colony-forming units/milliliter)
or higher are normally required for bacteria to function
as biological control agents or cause infectious
disease.
Plant pathology (also phytopathology)
is the scientific study of plant diseases caused
by pathogens (infectious organisms) and
environmental conditions (physiological
factors).
Organisms that cause infectious disease
include fungy, bacteria, viruses, viroids,
phytoplasmas, protozoa, parasitic plants
Once inside the plant tissue, bacterial plant
pathogens usually grow intercellularly, that is,
they grow between cells.
Physiology
of phytopathogenic bacteria
Most phytopathogenic bacteria are aerobic
(live in the presence of oxygen) and some are
facultative anaerobes which can grow with or
without oxygen.
Plant pathogenic bacterial genera
Erwinia
Pseudomonas
Ralstonia
Xanthomonas
Agrobacterium
(Rhizobium)
Clavibacter
Bacillus
Streptomyces
Xylella
Phytoplasma
There are several ways by which
bacterial plant pathogens may be
spread:
by wind, water, and soil movements;
by insect vectors;
by infected seeds;
by contaminated tools.
The bacteria enter the plant
othrough natural openings, like the stomata
of the leaves,
oor through wounds in the plant tissue.
Methods of penetration and invasion
by bacteria
Infection is generally considered to be passive, i.e. accidental,
although a few cases of plant chemoattractants have been
reported.
Artificially, bacteria are most commonly introduced into plants
by wounding.
Types of bacterial plant diseases

Foliar diseases (e.g., spot, speck, streak,
blight)

Vascular diseases (e.g., wilt, blight)

Soft rots

Developmental diseases (e.g., canker,
scab, gall)
GALLS
are characterized by abnormal outgrowths of
plant tissue.
Some of the abnormal outgrowths of plant tissue
produced in galls are large and swollen, while others
are small and divide rapidly.
Infected plants will develop smooth, light-colored galls
on its roots and stems. As the galls age, they develop
into hardened, discolored galls that eventually slough
off to make room for new, secondary galls. These
formations inhibit the plant's ability to transport
nutrients and water throughout the plant. This lack of
transport results in the plant's loss of vigor which is
also accompanied by growth stunt and branch and twig
dieback.
Xanthomonas campestris
Crown gall, in particular, may develop in
sugar beets, fruit trees, and other plants with
broad leaves and stems that come out of the
ground.
This disease is soil-borne.
Bacterial galls can be produced by the
genus Agrobacterium and certain species
of Arthrobacter, Pseudomonas, Rhizobacter
and Rhodococcus.
Bacterial wilts
Slime-producing bacteria are the causative
agents of bacterial wilts.
These bacteria plug up the plant’s vascular
system (the vessels that carry water and
nutrients) and cause the plant to wilt.
Infected plants wilt rapidly and often die.
Bacterial wilts
There are four main wilt-causing bacterial
plant pathogens : Corynebacterium
insidiosum causes wilts in alfalfa; Erwinia
tracheiphila causes wilts in cucumbers;
Erwinia stewartii produces wilts in sweet
corn; and Pseudomonas solanacearum
causes wilts in tobacco.
Leaf spots
- the most common symptom of bacterial disease
Spot-causing bacterial plant pathogens: Pseudomonas striafaciens,
Xanthomonas pruni.
The bacterial plant pathogens that produce leaf spots
may also infect the stems and fruits of the diseased
plant. The spot-causing bacterial plant pathogens
are: Pseudomonas spp., Xanthomonas spp.
Blights or deadening of tissue on leaves,
stems or tree trunks, and rots
Fire blight - Erwinia amylovora
Fungal infections
Fungi can grow on living or dead plant tissue and can
survive in a dormant stage until conditions become
favorable for their proliferation. They can penetrate
plant tissue or grow on the plant’s surface. Fungal
spores, which act like seeds, are spread by wind,
water, soil, and animals to other plants.
Warm, humid conditions promote fungal growth.
Methods of penetration and invasion by fungi
Fungal infections
While many fungi play useful roles in plant growth,
especially by forming mycorrhizal associations with the
plant’s roots, others cause such common plant
diseases as anthracnose, late blight, apple scab,
club root, black spot, damping off, and powdery
mildew.
Many fungi can attack are variety of plants, but some
are specific to particular plants.
Viruses and viroids
While more than 300 plant viruses have been
identified.
The symptoms of viral infection include yellowing,
stunted growth in some part of the plant, and plant
malformations like leaf rolls and uncharacteristically
narrow leaf growth. The mosaic viruses can infect
many plants. Plants infected with this virus have
mottled or streaked leaves; infected fruit trees produce
poor fruit and a small yield.
The viruses and viroids that attack plants are the
hardest pathogens to control.
Effective treatment of bacterial infections in
plants may involve radical treatment or
removal of the infected plant parts or removal
of the diseased plant altogether. A more
drastic measure may mean the removal of
entire communities of susceptible or diseased
plants.
Mechanical inoculation and early stages in the systemic distribution of viruses in plants
Forms and locations of survival of fungi and bacteria between crops
Control
control of plant disease begins with good soil
management
selection of disease-resistant plants
disposal of infected plants
plan crop rotation to eliminate over-wintering
disinfect pruning tools
prevent surface wounds on plants
expose plant material you wish to compost to
dry air, heat and sunlight to kill any diseasecausing bacteria
Control
control insects that might vector bacterial
diseases
if the disease is systemic, affecting the stem
as well as leaves, the plant cannot recover.
Destroy it to prevent the spread of the disease
to other plants.
Make sure the soil is healthy with plenty of
organic nutrients and beneficial
microorganisms.