8.5 Viruses Flashcards
What is the fanleaf virus? What are the symptoms?
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
Name 1 variety highly susceptible to fanleaf virus.
Cabernet Sauvignon
How is the fanleaf virus spread?
- Enormously by the move to grafted vines following phylloxera
- Spread slowly by the dagger nematode
What are the management options for fanleaf virus?
- 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.
What is the leafroll virus?
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
How is the leafroll virus spread?
- By grafting
- By mealy bugs, a key pest in South Africa, the Mediterranean, Argentina, and some parts of California
What are the symptoms of the leafroll virus?
- Symptoms are not always clear
- Vines have to be tested in a laboratory
- Some vines and rootstocks carry the virus without showing symptoms
When does the leafroll virus occur? What happens?
In Autumn
- The leaves change colour to red for (black grape varieties) and to yellow (white grape varieties)
What are the management options for leafroll virus?
- 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
Why is control by spraying difficult for leafroll virus?
Because of the mealy bug’s waxy coating
