EPWS 310 - PLANT PATHOLOGY
Reading: Chapter 14
A virus is a subcellular, submicroscopic, infectious obligate parasite
composted of nucleic acid within a protein coat that uses the host cellular
machinery to replicate itself.
A viroid is a low molecular weight covalently closed
circular RNA molecule that may cause disease. Viroids are ssRNA, but have a
high level of secondary folding. An example is potato spindle tuber viroid.
II Properties of viruses
- Do not
divide or multiply on their own.
by inducing the host cell to form more virus.
- Do not
consist of cells and therefore have no organelles.
penetrate cells on their own - enter through wounds or vectors
not self-motile - depend upon various types of vectors for transmission.
virus nucleic acid is encapsidated by a protein. The protein surrounding
the nucleic acid is referred to as the coat protein or capsid which is
formed by protein subunits.
majority of plant viruses are made up of RNA. At least 80 viruses contain
of the virus is made of protein and the remaining 5-40% is nucleic acid.
RNA or DNA is what carries the genetic information.
Plant viruses are named on the basis of the
symptoms induced on the first reported host plant.
viruses belong to the kingdom Viruses. Some viruses are separated into
families, ex.Comoviridae, Rhabdoviridae, Potyviridae. Within the families are what was previously
considered virus groups and now is called the virus genus. Examples include
Comovirus, Cytorhabdovirus, Potyvirus.
Within the genus are the plant viruses, ex. cowpea mosaic virus, lettuce
necrotic yellows virus, lettuce mosaic virus.
abbreviated by first initial of each word of the name, i.e., tobacco
mosaic virus = TMV.
become confusing because viruses infect more than one host and viruses may
cause more than one symptom on a host, or may cause distinctly different
symptoms on two different hosts.
IV Taxonomic criteria: (see
1. type, size and number of nucleic acid units
2. architecture, symmetry, size and number of
4. replication strategy
5. genome organization
1a) Nucleic acid
ssRNA : i. e. Tobacco mosaic virus.
ssRNA: i. e. Tomato spotted wilt virus.
i. e. Wound tumor virus
i.e. Beet curly top virus
i. e. Cauliflower mosaic virus
1b) Genome segments
(genome in one segment)
(genome divided in two segments)
(genome divided in three or more segments)
Composition and Structure:
1) Particle morphology:
rod particles : i.e. Tobacco mosaic virus
or filamentous particle i.e. Potato virus X
spherical or polyhedral particles: Cucumber mosaic virus
particles: i.e. Tobacco yellow dwarf virus
i.e. Broccoli necrotic yellow virus
- Virus Vectors:
-virus can be found external and/or internal to
-Example: Big vein of lettuce is vectored by the
-Transmit 27 known viruses
-Nematode-virus relationships tend to be specific.
-Transmit by feeding - virus carried on the
higher plants - dodder
-Dodder can transmit viruses through twining stems
- Dodder makes vascular connection with the plant,
so virus can move through the phloem.
aphids, leafhoppers, whiteflies, thrips, beetles, mealybugs, treehoppers
aphids, leafhoppers and whiteflies. - Most common and important of the insect
mouthparts - makes virus transmission more efficient as virus is placed
directly into the plant.
relationships can be general in which the insect is able to transmit a number
of viruses, or may be specific, where the insect only transmits one virus.
of insect transmission:
or non-persistent - the virus is carried on the stylet. It takes
only a few seconds for the insect to acquire the virus. When the vector
moves to the next healthy, susceptible plant and begins feeding, the
virus is transmitted. The virus does not accumulate in the vector and
the vector can only transmit the virus for a short period of time.
or stylet/foregut-Long feeding periods (several minutes to few
hours) are required before they accumulate enough virus for
transmission. These viruses persist in the vector for a few (1-4) days.
the virus accumulates internally in the vector and is circulated
throughout the insect. In general, it takes several hours for the insect
to acquire the virus; however, the virus can be transmitted until all
the acquired virus has been transmitted.
the virus actually replicates in the insect. Again, it generally takes
several hours for the insect to acquire the virus and days before the
insect can begin transmitting. The insect transmits virus throughout its
VII Virus Replication
A. Infectious virus RNA acts like mRNA, + sense RNA
RNA polymerase copies + sense RNA into complementary strand
strands, - strand of RNA acts as a template for replication of + strand
strand RNA serves as messenger RNA for translation of viral proteins,
including capsid protein.
proteins polymerizes on the viral (+) RNA -> virion produced capsid -
Transmission, mechanical or vector
Nucleic acid analysis
IX Physiology of virus -
the production of some growth regulators: gibberellin and auxin.
the production of other growth regulators: ethylene and ABA (Absisic
is generally increased immediately after infection as the plant works to
replicate the virus. Respiration may then remain high, return to normal
or may decrease.
-Symptoms produced by viruses may be difficult to
distinguish between symptoms caused by other organisms, nutrient deficiencies,
chemical toxicities, etc.
-Occur on the entire plant, on leaves, stems,
fruit, and roots.
-Reduced growth - stunting, dwarfing
-Necrotic or chlorotic spots (local lesions)
-Mosaics - yellow and green mottling of foliage,
occur in what can appear to be a pattern or may appear as streaks.
-Ringspots - chlorotic rings on leaves, fruit,
-Vein clearing - the veins turn yellow or white
while the rest of the leaves stay green. May be the first symptom which occurs
and as the infection ages, the vein clearing turns into a mosaic type symptom
or general chlorosis.
-Leaf and stem distortion - curling, rolling,
twisting, rippling, small leaves, bushiness at terminals, enations.
-Fruit distortion - bumps, deformation, streaks.
-Stem pitting - cankers
-Viruses can infect locally (local lesions) or may
be systemic (move throughout the vascular system in the plant).
-Viruses can infect plants causing visible
symptoms. In this situation the virus is called a latent virus
and the host is known as a symptomless carrier. In addition,
plants which usually develop symptoms may be temporarily symptomless under
certain environmental conditions. In this case, the symptoms are masked.
- remove diseased plants
stock or seed
Mosaic alfamovirus - aphids.
Mosaic cucumovirus. - aphids, seed-borne, wide host range
yellow mosaic potyvirus - aphids, narrow host range
curly top geminivirus. - beet leafhopper, wide host range
mosaic potyvirus - aphids, seed borne.
Mottle potyvirus. - pepper - aphids
spotted Wilt tospovirus. - pepper - thrips.
yellow dwarf luteovirus - aphids, persistent-circulative
yellows cloterovirus - aphids
tristeza closterovirus - aphids
- Alfalfa Mosaic Virus (AMV) on pepper - Calico virus:
solanaceous, some woody hosts
vectors - ~ 14 spp. Can act as vectors - non-persistent, stylet borne.
- borne estimate 10 - 50% alfalfa seed contamination
- white blotching, stunting.
- avoid alfalfa.
Cucumber Mosaic Virus (CMV):
Wide host range.
in perennial weeds usually in the roots.
into new spring foliage for pick up by aphids.
+ resistant varieties.
- Beet Curly-Top Virus (BCTV) on peppers:
host range: tomatoes, beans. Tumbleweeds, mustards.
by Beet leaf hopper.
is usually very stiff - due to phloem damage.
- remove weeds, diseased plant.:
-Pepper Mottle Virus. - aphid.:
but not as chlorotic as AMV
-Tomato Spotted Wilt Virus - Thrips:
in perennial weeds.
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