Agriculture Reference
In-Depth Information
veraison. If there was an infection in the previous year, do not leave infected clusters on the
vine or on the vineyard floor.
Crown Gall
Agrobacterium tumefaciens
Vineyards in colder regions are more susceptible to this disease, which is caused by a
bacterium. Both pruning wounds and damage caused by freezing can be sites for the bac-
teria to enter. The growths, or galls, usually form on the base of the trunk near the soil line.
They eventually become dark, bulbous, and hard. They interfere with movement of water
and nutrients to the rest of the vine, resulting in poor growth and eventually death of the
vine.
CONTROL
Always inspect new vines for galls. Prevent freezing damage to vines by protecting them
from winter injury. Control other fungal diseases. Remove diseased vines and destroy them.
There is no recommended organic treatment for crown gall.
Downy Mildew
Plasmopara viticola
This fungal disease is a major problem for growers in the eastern U.S. Vinifera grapes
are more susceptible to downy mildew than American varieties, and French hybrids are
somewhat susceptible.
As with other fungal diseases, the spores for downy mildew overwinter on the vine-
yard floor. Humidity and splashing rain encourage the release of the spores, which infect
green tissues on the vines. The affected leaves turn brown, spotted, or mottled on top. On
the underside of the leaves is where you find the white, fluffy growth that is recognizable
as downy mildew. Vines may drop infected leaves; shoots and tendrils can curl and thicken.
Eventually, green tissue can die completely.
Fruit infected with downy mildew turns brown, then purple, and is covered with the
white fluffy growths. The grapes detach easily from the stems.
CONTROL
