Part C - Plant viruses

Question 1

List the means by which plant viruses can be transmitted within an agricultural crop, and briefly compare and contrast non-circulative with circulative, non-propagative aphid transmission.

Answer 1

Aphids
Mechanical transmission, caused by contact of infected tissue to other plants
Soil-borne transmission, some viruses can survive in the soil for long periods of time.
Seed transmission: Some viruses can be transmitted from plant to plant through seeds.
Pollen transmission: Some viruses can be transmitted from plant to plant through pollen.

The main difference between non-circulative and circulative non-propagative transmission is that the virus does not spread to the next generation of aphids in circulative, non-propagative transmission. This means that the virus can only be transmitted by a single generation of aphids.

Question 2

List factors which can lead to serious plant virus disease problems in crops and briefly describe strategies used to control plant viruses.

Answer 2

Host susceptibility;
Virus strain;
Environmental conditions &
Vectors.

Strategies:
Crop rotation; Rotating crops that are susceptible to the same virus.
Sanitation; Removing infected plants and plant debris from the field.
Chemical control; Pesticides use to control propagation.
Biological control; predators and parasites of insect vectors.
Virus-resistant genes; modified crops that are resistant to viruses.

Question 3

List the strategies that can be employed to control plant viruses AND give an example of how some of these might be applied to a particular named plant virus.

Answer 3

Tomato spotted wilt virus (TSWV): TSWV is transmitted by thrips, small insects that feed on plant sap. To control TSWV, growers can use a combination of strategies, including:
Plant resistant cultivars. There are a number of tomato cultivars that are resistant to TSWV.
Use virus-free planting material. Certified seed is often virus-free.
Control thrips populations. Thrips can be controlled with insecticides or by using cultural practices that discourage them, such as planting trap crops.

Question 4

Outline the characteristics and mechanisms involved in circulative, nonpropagative aphid transmission of plant viruses.

Answer 4

• The virus is not replicated within the aphid vector.
• The virus particles are retained in the aphid’s salivary glands for a
  relatively short period of time, typically only a few hours.
• The virus can only be transmitted if the aphid feeds for a relatively
  long period of time (usually at least 15 minutes).
• The virus can only be transmitted if the aphid has previously fed on an
  infected plant.

Question 5

Describe the essential characteristics of viruses that differentiate them from other plant pathogenic micro-organisms, AND list the means by which viruses can spread to infect new plant hosts.

Answer 5

Viruses are not cells. They have DNA and RNA, surrounded by protein
coat.
Viruses are obligate parasites (Cannot reproduce on their own).
Viruses are very small. The smallest viruses are about 10 nanometers in diameter, which is about 100 times smaller than the smallest bacteria.

Spread:
Mechanical transmission, touching
Vector transmission, insects such as aphids
Soilborne transmission, some viruses can survive in the soil for a long
period of time
Seed transmission