8.5 Viruses

Question 1

What is the fanleaf virus?

Answer 1

• 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

Question 2

Name 1 variety highly susceptible to fanleaf virus.

Answer 2

Cabernet Sauvignon

Question 3

How is the fanleaf virus spread?

Answer 3

• Enormously by the move to grafted vines following phylloxera
• Spread slowly by the dagger nematode

Question 4

What are the management options for fanleaf virus?

Answer 4

• 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.

Question 5

What is the leafroll virus?

Answer 5

• 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

Question 6

How is the leafroll virus spread?

Answer 6

• By grafting

- By mealy bugs, a key pest in South Africa, the Mediterranean, Argentina and some parts of California.

Question 7

What are the symptoms of the leafroll virus?

Answer 7

• Symptoms are not always clear
• Vines have to be tested in a laboratory
• Some vines and rootstocks carry the virus without showing symptoms

Question 8

When does the leafroll virus occur? What happens?

Answer 8

• In Autumn
• • Leaves change colour to red for black grape varieties
• • and to yellow for white grape varieties.

Question 9

What are the management options for leafroll virus?

Answer 9

• 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

Question 10

Why is control by spraying difficult for leafroll virus?

Answer 10

Because of the mealy bug’s waxy coating