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Fungi
• Diverse and widespread
• Filamentous (hyphae) form a network of
mycelium (lots of hyphae)
• Recognized by reproductive structures
(mushrooms, rusts, conks, etc.)
• Most of the 100,000 spp. are saprophytes
– Live on dead organic matter
• Approximately 8,000 species attack plants
– Plant pathogens
Bacteria
• Prokaryotic microscopic organisms
– Free living single cells, or
– Filamentous colonies
• Reproduce via binary fission
– 2 daughter cells are identical to mother cell
• Don’t usually produce resistant resting spores
– Need host or growth medium to survive
• For rapid spread, plant infecting bacteria usually
require:
– Warmth
– Moist conditions
Fungal Diseases
• Reproduction by sexual and asexual means
• Spread through a variety of methods
– wind/water blown spores
– rhizomorphs
– Sclerotia (overwintering)
• Include organisms from Kingdom Protista, that are
now classified outside the Kingdom Fungi:
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Downy mildews
Pythium
Phytophthora
Clubroots
Bacterial Diseases
• Less common than fungal or viral diseases
• They can be either:
– parasites, saprophytes (live off dead material), or autotrophs
(photosynthesis or Chemosynthesis)
• Symptoms include:
– Cankers, Wilts, Shoot Blights, Leaf Spots,
Scabs, Soft Rots, & Galls
• Generally, cannot invade healthy tissue; need wound or
opening to infect.
• Control methods usually cultural in nature (don’t use
antibiotics on large scale)
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Bacterial Diseases
• Bacterial galls: In some cases, toxic materials are
produced that cause plant tissues of roots, stems
or leaves to grow abnormally as in crown gall.
• Bacterial leaf spot disease: The bacteria usually
enter through leaf stomata.
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• Symptoms include water-soaking, slimy texture,
fishy or rotten odor, confined initially between leaf
veins resulting in discrete spots that have straight
sides and appear angular.
Disease Development
• Infections occur through leaf scars and wounds. These give
rise to small cankers in which the bacteria survive the
winter.
• Rain or water splash, and pruning tools spread the
bacterium.
• Bacteria overwinter in active cankers, in infected buds, and
on the surface of infected and healthy trees and weeds.
• The bacterium reproduces best between 21ºC and 25ºC.
• Generally disease seems to be more severe after cold
winters and prolonged spring rains.
Bacterial infections
• The infected head tissue
often takes on a tan color
– Becomes moist and mushy
– Develops a foul odour.
• The leaf undergoes HR
response
– Results in classic spotting
of leaves.
– Reduces photosynthesis and
cell respiration of plant
material
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Viruses
• Viruses are "submicroscopic" entities that infect
individual host plant cells.
• Viruses are obligate parasites: They can only
replicate themselves within a host's cell.
• In the virus infected plant, production of chlorophyll
may cease (chlorosis, necrosis)
• Cells may either grow and divide rapidly or may grow
very slowly and be unable to divide
Viral Diseases
• > 400 viruses infect plants; few are
economically important pathogens
• The infection remains forever
• Viruses are transmitted from plant to plant
by living factors: insects, mites, fungi and
nematodes
• Or non-living factors: rubbing, abrasion or
other mechanical means (including grafting or
other forms of vegetative propagation)
• Occasionally transmitted in seed.
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Plant Viral Reproduction
1. Attachment--this
requires specialized
envelope proteins. These
proteins make viruses
specific for different
cells.
• 2. Penetration--viral
particles enter the cell,
the caspid is removed and
genetic material enters
the nucleus.
• 3. Replication--the virus
uses the host replication
machinery to make many
copies of itself
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Cultural Practices to Minimize
Spread of Disease
• Favorable irrigation practices (drip vs
overhead)
Moisture
• Timing of Planting
management
• Wider row spacings
• Eradicate alternate hosts for viruses
Temperature
• Heat for soil sterilization
• Hot water treatment of planting
material
• Solarization
Important to minimize
water and humidity to
limit disease spread
Management of Plant Disease
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Sanitation
Fungicides
Host plant resistance
Crop rotation
Cultural practices
Temperature
• Biological control – Rhizobacteria may interfere
with colonization of plant roots by fungi and
bacteria
• Organic amendments (avoid diseased plants in
mulch, etc.)
• Improved plant health and nutrition
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