8.3 Fungal Diseases Flashcards
What is powdery mildew?
- Fungal disease that attacks young, green parts of the vine
- Starts as dull grey patches and become black patches as they advance.
- Patches can damage young shoots, inflorescences and grapes, reducing yield.
- Grapes can also split at véraison and become targets for other infections.
In what conditions does powdery mildew thrive?
- Optimum temperature around 25°C (77°F)
- Shady conditions (dense, shady canopies)
- NOTE: Does not require high humidity, can thrive in dry conditions.
What fungus causes powdery mildew?
Erysiphe necator (aka Oidium tuckeri)
What species of grapes is more vulnerable to powdery mildew?
V. vinifera.
- American species is less vulnerable
Does powdery mildew affect crops other than grape vines?
No
What are the main management options for powdery mildew?
- Keeping an open canopy to reduce shade and the density of leaves is now regarded as the preferred approach.
- Applications of sulfur (prevention & treatment)
- Systemic fungicides
When spraying sulphur to combat powdery mildew, when is it important to spray?
- It is important to spray early in the season as the disease is easier to prevent than to contain if it gets established.
- A couple of weeks after budburst and up to véraison.
What is a pro and con of using systemic fungicides to combat mildews?
Pro: They penetrate the green tissue, are not washed off by rain
Con: Can become resistant to some fungicides, so only a limited number of applications can be made in one year.
What is downy mildew?
Fungal disease that causes yellow, circular ‘oil spots’ and then white, downy fungal growth on the underside of leaves.
In what conditions does downy mildew thrive? What are high risk periods?
- Rainfall and warm temperatures (20°C / 68°F)
- Warm springs; stormy but warm summers
What causes downy mildew?
Peronospora, a water mould that lives within vine tissue, not on the surface
Where did downy mildew originate?
It was introduced from North America in the last quarter of 1800s and is now common in most wine region.
What are the main management options for downy mildew?
- Traditionally, sprays made from copper salts (Bordeaux mixture) (Only lasts until 20 mm of rain has fallen)
- Fungicides
- Good drainage and an open canopy that dries quickly are helpful to avoid the fungus developing and spreading
What is the Bordeaux mixture?
- A combination of copper sulfate and lime
- Became the standard treatment for downy mildew in the 1880s
What is grey rot? What is another name for grey rot?
- Botrytis cinerea
- Botrytis bunch rot (noble rot)
- A fungus that can cause significant damage to fruit: loss of yield and drop of quality in the wine (colour, body and aroma/flavour)
What makes a grapes more vulnerable to grey rot?
- If there are any points of entry (e.g. grapes having rubbed against each other in tight bunches or punctured by birds/insects)
What varieties are particularly susceptible to powdery mildew? What varieties are particularly less prone?
- More Susceptible: Chardonnay, Cabernet Sauvignon
- Less Prone: Pinot Noir and Riesling
What does the winery do with grey rot-affected grapes?
At harvest, the grapes are sorted and discarded
What varieties are particularly susceptible to grey rot? Why? Name a variety that has a high level of resistance.
- Semillon, Sauvignon Blanc, Pinot Noir
- Varieties with tight bunches or thin skins
- Resistance: Petit Verdot
What happens if the flowers are affected with grey rot?
The fungus can stay dormant in the grape and re-emerge after véraison
In what conditions does grey rot thrive?
The spores are typically present in the vineyard and become active in periods of rainfall and high humidity.
What are the management options for grey rot?
- Selecting grape varieties that have small grapes with thick skins
- Protecting the grapes against other pests (which could split skins)
- Keeping an open canopy
- Removing the leaves around bunches
- Fungicides
- Antagonistic bacteria (bacilius subtilis)
When should fungicides be used to combat grey rot?
- Applied at key points in the season:
- when flowering is nearly complete
- at the end of grape formation
- at bunch closure (when the grapes in a bunch get large enough so that they touch each other)
- véraison
What is eutypa dieback? What is another name for this?
- “Dead arm”
- A fungal trunk disease that leads to rotten wood in vines, can affect whole vineyards
- Reduces yields significantly
- Kills vines over a ten-year period if not tackled.
