Growing grapevines is fun, even if you don’t make your own wine. The decorative vines are attractive and produce a fruit you can use, or just let birds enjoy. Fungal infections, including the grape armillaria fungus, can ruin your vines, though. Know the signs of infection and what to do to prevent or manage it.
What is Armillaria Root Rot of Grapes?
Armillaria mellea is a fungus that is naturally found in trees in California and that is commonly called oak root fungus. It can be a real problem for vineyards in California, attacking and killing vines from the roots up.
Although native to California, this fungus has also been found in vines in the southeastern U.S., Australia, and Europe.
Grape Armillaria Symptoms
Armillaria on grapes can be very destructive, so it is important to know the signs of an infection and to recognize them as early on as possible:
- Shoots that are dwarfed or stunted, getting worse each year
- Premature defoliation
- Yellowing of leaves
- Death of vines in late summer
- White fungal mats under the bark just at the soil line
- Rotting of the root beneath the fungal mat
The white fungal mats are the diagnostic signs of this particular infection. As the disease progresses, you may also see mushrooms form in the soil around the vines in winter as well as rhizomorphs near the roots. These look like dark strings.
Managing Armillaria Root Rot
A grapevine with armillaria root rot is difficult or impossible to successfully treat. If you are able to catch the infection early, you can try exposing the upper roots and crown to let them dry out. Dig the soil down to 9 to 12 inches (23-31 cm.) to expose the roots in the spring. If the disease has already severely stunted the vine, this won’t likely work.
If you are growing vines in an area that has armillaria, prevention before you plant is the best strategy. You can fumigate the soil with an appropriate fungicide, but if you do this, be sure you also remove any roots left in the soil, down to a depth of about 3 feet (1 m.).
These two measures together are largely effective in preventing armillaria infections. If a site is known to be infected with armillaria, it is not worth planting grapevines there at all, and there are no rootstocks that are resistant.