Symptoms of Pierce's disease on leaves, cane and fruit of a red grape
variety (eg. "leaf scorch").
Pierce's disease (PD), a lethal disease of grapevine, is caused
by the bacterium Xylella fastidiosa and is spread by certain kinds
of leafhoppers known as sharpshooters. Pierce's
disease is only known from North America through Central America and has
been reported from some parts of northwestern South America. It is present
in some California vineyards every year, with
the most dramatic losses occuring in the Napa Valley and in parts of the
San Joaquin Valley. During severe epidemics, losses to PD may require major
replanting. In Florida and other southeastern states, PD has precluded commercial
production of European varieties, but some muscadine grapes and hybrids of American wild
grape species wtih European grapes (Vitis vinifera) are tolerant or resistant to PD.
Pierce's disease, named after N.B. Pierce,
seems to be restricted to portions of North America with mild winters.
The disease is less prevalent where winter temperatures are colder, such
as at higher altitudes, farther inland from ocean influences, and at more
northern latitudes. It has been found in all southern states that raise
grapes commercially; from Florida to California, and in Mexico and Central
America. In the southeastern states, from Florida through Texas, PD is
the single most formidable obstacle to the growing of European-type (Vinifera)
grapes. Since the mid-1970s, many other strains of
Xylella fastidiosa
have been discovered, and almost all of these cause leaf scorching of woody
perennials such as American elm, maple, mulberry, or plum. In some plants,
such as peach and alfalfa, the bacterium slows and stunts plant's growth.
• Other diseases caused by Xylella in other plants and regions.
The first evidence of PD infection usually is a drying or "scorching"
of leaves (see picture on top of page). The leaves become slightly yellowed
(chlorotic) along the margins before drying, or the outer leaf may dry
suddenly while still green. Typically, the leaf dries progressively over
a period of days to weeks, leaving a series of concentric zones of discolored
and dead tissue. About mid-growing season, when foliar scorching begins,
some or all of the fruit clusters may wilt and dry up. "Scorched" leaves
detach from the distal end of the petiole (leaf stem) rather than from
the base of the petiole, leaving the bare petioles attached to canes, often
well after normal leaf fall. The bark on affected canes often matures
unevenly, leaving islands of mature (brown) bark surrounded by immature
(green) bark or the reverse.
Multiplication of Xylella fastidiosa in four plant species
(Other Host Plants)
One of the key features of grape strains of X. fastidiosa is
that grape strains infect a wide range of plant
species
(Freitag 1951). Recent studies
(Hill
and Purcell 1995) have shown that the fate of X. fastidiosa
can vary greatly from one plant species to another. The bacteria can move
internally from cell to cell in blackberry as in grape, but not within California
mugwort. All of these plants are important breeding plants of the blue-green
sharpshooter in Northern California.