Virus in Seeds ? Its possible! Roger
EPWS 310 - Plant Pathology
Lectures - Fall 1998
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Lecture 6
1. Inoculation- when the pathogen comes in contact with the host. Inoculum-
the pathogen (s) that come into contact with the host.
Examples of inoculum- in fungi, spores, sclerotia, mycelium. Individuals of
bacteria, viruses, viroids, mycoplasmas. In nematodes- adult, larvae, eggs.
Propagule=one unit of inoculum. Colony forming units.
A. Types of inoculum- Primary inoculum - inoculum that survives the
overwintering or over summering. The infections it causes are called
primary infections. Secondary inoculum- inoculum produced from primary
infections. The secondary inoculum in turn induces primary infections.
B. Sources of inoculum-
-plant debri, or soil
-seedborne, transplants, tubers, propagative material
-perennial weeds or alternate hosts
*Fungi, bacteria, parasitic higher plants, and nematodes either produce
their inoculum on the surface of infected plants or their inoculum reaches
the plant surface when the infected tissue breaks down. Viruses, viroids,
mycoplasmas, and fastidious bacteria produce their inoculum within the
plants; such inoculum almost never reaches the plant surface in nature and,
therefore, cannot by itself escape from one plant and spread to another.
C. Landing or Arrival of Inoculum-
-wind (most of the inoculum does not reach susceptible hosts).
-water
-insects (most efficient).
2. Penetration
A. Germination of spores and seeds-
-requires certain environmental conditions
-resting spores or germinates immediately*
*spore germination is often stimulated by exudates of the plant roots,
leaves, or fruit. Nutrients (sugars and amino acids).
*Fungistasis- Due to toxic metabolites in the soil or competition, the
spore is not able to germinate or the hyphae lyses soon after germination.
Explain hyphae and germ tube. Fungistasis is often counteracted by the root
exudates. Soil that is fungistatic is called suppressive soil.
-What affects direction to penetration sites?? moisture, temp., soil
texture, plant exudates, thigmotropic (contact) responses to the topography
of the leaf surface resulting in germ tubes growing at right angle to
cuticular ridges that generally surround stomata.
-Zoospores- chemical stimuli, zone of elongation of roots, physical
stimuli, nutrient gradient.
-Seeds penetrate by producing a radicle that penetrates or produces a
haustoria.
-Nematodes- same conditions, Carbon dioxide and some amino acids. Nematodes
are generally attached more to the roots of host plants but also go to non
host.
B. Attachment of Pathogen to host
How do they stick? Viruses, mycoplasmas, protozoa, and fastidious bacteria
are placed directly into cells of plants by their vectors. Fungi, bacteria,
and parasitic higher plants must first become attached. They have on their
surface polysaccharides, glycoproteins, polymers of hexosamines, and
fibrillar materials, when moistened becomes sticky and helps the plant to
adhere. LECTINS. Germ tubes also have this characteristic. The
muscilaginous material contains degradative enzymes that chew away the
outer cell wall.
C. Recognition between host and pathogen
Not known for sure how recognition works. If the plant responds quickly to
the pathogen the end result is often resistance.
D. Penetration-the act.
1. Natural openings-stomata, lenticels, hydathodes, nectarthodes. Bacteria,
fungi, nematodes.
2. Wounds- fruits and vegetables. Fungi, Bacteria, viruses (Vectored)
3. Direct penetration- enzymatic or pressure. Fungi, parasitic higher
plants, nematodes.
Direct penetration- Explain about appressorium, penetration peg,
intercellular hyphae, intracellular hyphae, haustoria. Penetration in
parasitic higher plants is similar to fungi. Penetration by nematodes is
accomplished by repeated back and forth thrusts of their stylets.
3. Infection- the process by which pathogens establish contact with the
susceptible cells or tissues of the host and procure nutrients form them.
During infection pathogens grow or multiply, or both, within the plant
tissues and invade and colonize the plant to a lesser or greater extent.
-one result of infections is symptoms. An infection that does not appear
right away is called a latent infection. The latency is due to
environmental conditions or maturity level of the host. Symptoms can occur
at 2 days or 2 years (mycoplasmas) depending on the pathogens, environment
and host. IN most cases, symptoms occur within two weeks of initial
invasion.
-Incubation period- the time between inoculation and the appearance of
disease symptoms.
-Invasion- Subcuticular (black spot on rose, apple scab), surface of the
plant but send haustoria into the epidermis (powdery mildew, downy mildew),
intracellular or intercellular (rusts), xylem tissue (Fusarium,
Verticillium). Bacteria invade intercellularly until the cell wall breaks
down and then the bacteria grows intracellularly. Viruses, viroids, and
mycoplasmas invade intracellularly, nematodes invade intercellularly in
most cases.
-Colonization- by fungi- can grow throughout the plant and then produce
spores by the millions.
Bacteria- divide every 20 to 30 minutes. Number becomes very large.
Fastidious bacteria and mycoplasmas reproduce much slower than bacteria and
are usually in lower numbers in the plant.
Viruses and Viroids-reproduce in the individual cells. 10 million virus
particles per cell.
Nematodes- female lays about 300-600 eggs, about 1/2 are females. Two to 12
generations produces per year. Each generation increases the number of
nematodes in the soil by 100 fold.
4. Dissemination - spread of pathogen inoculum.
Almost all dissemination of pathogens that is responsible for plant disease
outbreaks, and even for disease occurrences of minor economic importance,
is carried out passively by such agents as air, water, insects, certain
other animals, and humans.
Air-most of these spores do not contact a susceptible hosts. They have a
better chance in monculture. What would they have hit in a polyculture?
Rusts occur at several thousand meters above infected fields and can be
carried for miles.
Water- Important in disseminating pathogens in three ways:
1. Bacteria, nematodes, and spores, sclerotia and mycelial fragments of
fungi present in the soil are disseminated by rain or irrigation water that
moves on the surface re through the soil.
2. All bacteria and the spores of many fungi are exuded in a sticky liquid
and depend for their dissemination on rain or irrigation water, which
either washes them downward or splashes them in all directions.
3. Raindrops or drops from overhead irrigation pick up the fungal spores
and any bacteria present in the air and wash them downward where some of
them may land on susceptible plants.
*Water dissemination is more efficient in that the pathogens land on an
already wet surface and can move or germinate immediately.
-Insects, mites, nematodes, and other vectors-
Aphids and leafhoppers are primary vectors for viruses. Leafhoppers are the
main vectors for mycoplasmas and fastidious bacteria. The Dutch elm disease
also depends on the a bug. In these vectored diseases, the pathogen is
completely dependent on the vector.
*Very efficient method of transmission.
Humans-within a field, machinery, tools, airplane.
Examples-Dutch elm disease, white pine blister rust, downy mildew of grape.
5. Survival- When the host tissue dies, whether an annual or perennial
plant, the pathogen survives until the new season.
a. Methods of survival
1:-fungi- perennial plants, mycelium in infected tissues (Cankers, spores
on bud scales, fallen , infected leaves or fruits. Annual plants- mycelium
in infected plant debris, as resting spores, sclerotia, on seeds, tubers.
Some are soil inhabitants- able to survive indefinitely as saprophytes
(Pythium, Fusarium, Rhizoctonia). They usually have several hosts. Soil
transients- are specialized parasites that generally live in close
association with their host but may survive in the soil for relatively
short periods of time. Rusts overwintering on plants grown at warmer
temperatures and move from them to crops grown in colder climates as
temperature allows. Rust go form annual to perennial and overwinters in the
perennial.
2. bacteria-same as fungi. Many overwinter in insect vector.
3. viruses-in living plant tissue such as the tops and roots of perennial
plants, the vegetative propagating organs, and in the seeds of some hosts.
Some viruses overwinter in their vectors. TMV (cigarettes).
4. Nematodes-as eggs in the soil and in plant roots .
5. Parasitic plants- survive either as seeds or as their vegetative form on
their host.
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