Agriculture Reference
In-Depth Information
Pierce's Disease
Xylella fastidiosa
Pierce's disease is a deadly bacterial disease of grapevines. The bacteria is spread by in-
sects, most notoriously the glassy-winged sharpshooter. Once in the vine, the bacteria plugs
up the xylem, which carries water through the plant. Leaves may have yellow or red mar-
gins, and eventually the leaf margins dry out in concentric zones. Fruit will shrivel, and the
wood on new canes develops irregularly. In subsequent years, leaves become chlorotic and
scorched. Shoots and cane tips die. The disease is more prevalent in hot areas and less so
in areas with cold winters. Some varieties are more susceptible than others. Chenin Blanc,
Sylvaner, Ruby Cabernet, and White Riesling seem to be the most resistant.
CONTROL
There is no treatment for Pierce's disease. If the disease is confirmed, remove and replant
vines to maintain production. Plant resistant varieties on resistant rootstocks. Control insect
vectors, especially sharpshooters.
Powdery Mildew
Uncinula necator
This is the most serious and widespread fungal disease in many grape-growing regions
of the world, especially humid areas. Despite treatment, powdery mildew in some years
can result in losses. The degree of mildew varies between different varieties, with vinifera
and French hybrids being more susceptible. But even within this group, certain varieties are
more prone to powdery mildew. For instance, Carignan, Chardonnay, Cabernet Sauvign-
on, and Chenin Blanc are more seriously affected, while Petite Sirah, Zinfandel, and White
Riesling are less susceptible.
Powdery mildew can affect all the tissues of a grapevine. It appears as a white
powdery covering of leaves and, when severe, on the grapes. Leaves may curl, especially
in hot weather. Late in the season small, spherical, black fruiting bodies may form on the
leaves, shoots, canes, and grapes. Severe mildew can cause berries to crack and split, al-
lowing rot organisms to enter. Powdery mildew cannot grow on dormant grape tissue but
can survive the winter under infected buds.
CONTROL
