1. Viruses
Virus - a submicroscopic, obligate parasite consisting primarily of protein and nucleic acid.
"Submicroscopic" - implies that the object can not be seen using a light microscope; must use electron microscope.
Viruses share traits with both living and non-living things.
Living - can replicate themselves; can infect hosts
Non-living - can be crystallized and stored.

3. Characteristics of Viruses
Very small requires electron microscope to see them.
2. Virus particles (aka. virions) are composed of:
a. Nucleic acid - 5 to 40%, depending on virus.
Nucleic acid typically is RNA, but some have DNA. Nucleic acid can be single- or double- stranded
b. Protein sheath (aka. capsid) - 60 to 95% protein, depending on virus.
Capsid protects virus from heat, enzymes, UV and other denaturing agents.

5. Characteristics of Viruses
3. Shapes
a. Rigid rods - long, straight rods with nucleic acid core and protein coat. Raw spaghetti.
Ex: Tobacco Mosaic Virus (TMV) is 15 x 300 nm.
b. Flexuous rods - long curved rods with nucleic acid core and protein coat. Cooked spaghetti.
Ex: Citrus Tristeza Virus is ca 2,000 nm long.
c. Short rods (aka. Rhabdoviruses) - truncated, bullet shaped.
Ex: Potato Yellow Dwarf Virus (PYDV) is 75 x 380 nm.
d. Eicosahedrons (20-sided isometric particles). Formerly referred to as spherical viruses, because they appeared spherical in the earliest electron micrographs with poor resolution.
Ex: Wound Tumor Virus (WTV) can vary in diameter from 17 to 60 nm

9. Viroids
They are small, low molecular weight RNAs that can infect plants, replicate themselves, and cause disease.
2. Naked nucleic acid (no protein coat). However they are stable because of nucleic acid side chains "double over" and block sites of attachment for enzymes and protect against other environmental variables.
3. Much smaller than viruses. RNA contains base compared to 4,000-20,000 bases in viruses.

10. How do viruses enter plants and cause symptoms?
They require wounds - either through vector (insects, nematodes) feeding or mechanical.
2. Once inside host cell, protein coat disassociates and nucleic acid replicates. Virus and nucleic acid spread from cell to cell.

13. Aspects of Symptom Expression
Localized – symptoms occur at the point of inoculation only; do not spread. Local lesions.
Systemic- symptoms are not limited to point of inoculation. Chlorosis, stunting, etc.

14. Insect Vectors
Aphids
Whiteflies

15. Insect Vectors
Thrips
Leafhopper Plant Hoppers

16. Tobacco Mosaic Virus Non – persistent Tobacco, tomato, and other
solanaceous plants

17. Barley Yellow Dwarf Hosts - barley,wheat,
maize, rice and other grasses
Persistent, circulative virus

18. Diagram of an aphid vector feeding on a plant host showing the internal route
of the viruses that cause BYD.

19. Nematode Vectors
Xiphinema index
Xiphinema spp.
Longidorus spp.

20. Virus Dispersal
Insects
Nematodes
Soilborne fungi
Seed
Vegetative Plant Parts – if parent is infected, cutting is infected
Parasitic Seed Plants – some viruses transmitted only through dodder.
Mechanical
Cutting and harvesting equipment
Simple touch of hand or clothing.
Carborundum – a powdered abrasive used to wound leaf surface before virus inoculation.