Six common fungal diseases growers encounter in the vineyard, discussing causes and management options for each.
1. Powdery Mildew
What it is: Wouldn’t you know it? A fungal disease that’s caused by another fungus, namely Eryisiphe necator. This is a grapevine-specific fungus (so no chance of getting it between your toes) and is actually one of the most widespread vine diseases in the world today. Some varieties are more susceptible, such as Chardonnay and Cabernet Sauvignon, while others are less so, such as Riesling and Pinot Noir.
This nasty fungus overwinters in buds and on canes. Then, when the growing season starts, it attacks the young green parts of the vine (new shoots, developing leaves, etc.). It creates grey patches that then transitions to black patches, damaging the young green material as well as developing inflorescences, thus reducing yield. The grapes that do make it are at risk of splitting during veraison, thus becoming more susceptible to other forms of diseases and infections.
How it spreads:The growth rate of Powdery Mildew is dependent on temperature—it’s most cozy at 77°F and in shady conditions. Despite what you may think, it does not require humidity, so can thrive in dry conditions.
- Keep an open canopy to avoid shade
- Sulfur, applied between budburst and veraison, can help prevent and treat the fungus. It’s noted that it’s better to spray earlier than later, as the fungus is easier to prevent than treat once it’s become established in the vineyard.
2. Downy Mildew
What it is: A water mold that lives inside the vine tissue NOT the surface. It attacks the green part of the plants—defoliating young leaves and flowers and thus reducing overall yield. It’s noted that grapes can also be affected, but that grape infection is less important than that of the green parts.
How it spreads: It’s a water mold so, you guessed it, it needs rainfall as well as warm temperatures—it’s cozy at or above 68°F. High risk periods include warm springs and summers that also include precipitation.
You’ll know you got the DMs when you see yellow, circular spots on the underside of your leaves. They’ll eventually turn white and, well, downy.
- Copper sprays called Bordeaux mixture (combination of copper sulfate and lime) is the “traditional” treatment. Bad news: This treatment only lasts up to 20mm worth of rain.
- Fungicides applied a month after budburst are known to treat the fungus.
- Keep an open canopy and good drainage in the vineyard to avoid humidity/moisture within the canopy.
3. Grey Rot
What it is: Not as sexy as “noble rot,” this does come from the same fungus, Botrytis cinerea. It causes a significant loss of yield as well as quality, affecting color, body, and aroma and flavor compounds.
How it spreads: Grapes that have been broken, split, or punctured are the most vulnerable. If attacked at the flower stage, the fungus can actually ghost until the grapes form and then re-emerge post-veraison. Creepy. Varieties that innately form tight clusters are most at risk, for example, Semillon, Sauvignon Blanc, and Pinot Noir.
It’s common in all growing areas—there is no where to hide. The sneaky spores are usually present in the vineyard and become active in periods of high humidity.
- Grape varieties with small berries and thick skins, like Petit Verdot (swoon), are less susceptible. Maybe plant these instead.
- Keep an open canopy to prevent humidity within the vine, especially near the fruit zone.
- Bad news: Traditional sulfur and copper sprays? Yeah, they don’t work. Other fungicides can be used, but should be applied at “key points” within the growing season, i.e. when flowering is nearly complete, at the end of grape formation, at bunch closure, and at veraison. BUT, fungicides will eventually become ineffective, as Grey Rot will build up a resistance. (Super sneaky.)
- Biological control: Using fungus to fight fungus.
What it is: A fungal trunk disease that causes rotten wood throughout the vine and can eventually affect an entire vineyard or even series of vineyards. No surprise that this seriously reduces yields, but it also can kill vines over a ten-year period if not dealt with.
How it spreads: Spores. And these spores can travel over long distances via wind. Vines are infected through pruning wounds. (Don’t worry, these vines aren’t being abused. This is just what you call the section of exposed wood after it’s been trimmed.) It can survive in moderate temperatures, but particularly like the rain. You’ll know you have ED (hehe) when you’ve got stunted shoot growth in the spring that sprout yellow leaves. Susceptible varieties include Grenache, Cabernet Sauvignon, and Sauvignon Blanc.
- Prune late (preferably not during the rainy season) and apply fungicide to any wounds.
- Cut back any affected trunks (about 5 to 10cm beyond the visual symptoms) and treat with fungicide. NOTE: Dead wood must be burned to avoid the spores from spreading. (See chart above.)
- Biological control—fungus fighting power!
- If a plant is severely damage by Eutypa Dieback, you may need to retrain from a sucker or remove and replant completely. Oi.
What it is: “A fungal disease that causes a reduction in crop.” (Well, these all kind of do in a way, don’t they?)
How it spreads: It’s most prevalent in years with cool, wet springs followed by humid conditions and moderate temperatures. You’ll know you have PCLS when your infected canes turn white and break off easily. The shoots that develop on these infected canes tend to form brown cracks at the base. Leaves, if growth gets that far along, can also become infected. It’s noted that Grenache is particularly susceptible to the fungus, while Cab Sauv is less so.
- Fungicides—applied three weeks after budburst and again every two weeks as wet conditions continue.
- Remove and burn diseased wood.
- Prune early or late—whatever helps you avoid the rainy weather.
What it is: Besides the one on the list that kind of freaks me out the most, Esca is a “complex fungal disease caused by a group of organisms.” They like warmer, drier climates like, oh yeah, California.
How it spreads: Like Eutypa Dieback, it enters the vine through pruning wounds. You know you have the E when your leaves get all stripe-y and your wood gets all spotty.
Oh yeah, besides reduction of yield, Esca does result in vine death after a couple of years.
- Sorry, there are no chemical controls
- Source disease-free stock
- Use less injury-inducing pruning techniques
- Do not prune in the rain
- Removing prunings promptly from the vineyard and disinfect those pruning wounds.
- Possibly biological control? “Research is continuing into using biological agents such as Bacillus subtilis.”
Thanks again for studying with me!
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