8.5 Viruses Flashcards
1
Q
What is the fanleaf virus?
A
- A long-standing group of diseases (also called Fanleaf degeneration) that leads to the following:
- early shoot growth is stunted
- canes can grow in distorted ways
- leaves are very pale, malformed and can look like a fan
2
Q
Name 1 variety highly susceptible to fanleaf virus.
A
Cabernet Sauvignon
3
Q
How is the fanleaf virus spread?
A
- Enormously by the move to grafted vines following phylloxera
- Spread slowly by the dagger nematode
4
Q
What are the management options for fanleaf virus?
A
- No cure for the disease
- Affected vines must be removed (costly)
- Before vineyards are replanted, soil tests should be done to check for the presence of dagger nematodes and only virus-tested, clean planting material should be used.
5
Q
What is the leafroll virus?
A
- A group of viral diseases widely present around the world that slows down the growth of roots and shoots
- Surviving fruit takes extra weeks to ripen (more acidity, less colour and lower sugar levels)
- Vine stores less carbohydrate
- Does not kill vines
- Reduce yield, affects quality
6
Q
How is the leafroll virus spread?
A
- By grafting
- By mealy bugs, a key pest in South Africa, the Mediterranean, Argentina and some parts of California.
7
Q
What are the symptoms of the leafroll virus?
A
- Symptoms are not always clear
- Vines have to be tested in a laboratory
- Some vines and rootstocks carry the virus without showing symptoms
8
Q
When does the leafroll virus occur? What happens?
A
- In Autumn
- Leaves change colour to red for black grape varieties
- and to yellow for white grape varieties.
9
Q
What are the management options for leafroll virus?
A
- No cure
- Remove unproductive vines and replant with virus-free stock
- Nurseries can screen vines for virus infections.
- Mealy bugs favour humid environments and therefore open canopies help to reduce the pest
- encourage the mealy bug’s natural predators: ladybugs, lacewings and others
10
Q
Why is control by spraying difficult for leafroll virus?
A
Because of the mealy bug’s waxy coating
