The single greatest threat to the long-term survivability of susceptible
cultivars is Pierce's disease. Pierce's disease (PD) is caused by a xylem-limited
bacterium that clogs the vascular tissue of susceptible grape cultivars.
The causal organism is a gram-negative, rod-shaped bacterium named Xylella
fastidiosa that is indigenous to the Gulf Coast region of the United
States. Although different races of this organism cause similar diseases
in other crops, they appear to be host specific, i.e., the grape strain
does not appear to infect peach and the peach strain does not appear to
cause symptoms in grape. Grapevines become infected when a sharpshooter
that carries the bacterium feeds on tender tissue. These insect vectors
are very efficient at transferring the bacterium during feeding and infection
Once a grapevine is infected, the bacteria multiply and colonize the
xylem, or water conducting tissue of the plant. This vascular constriction
inhibits the movement of water through the grapevine and often results
in first visible symptoms noted during periods of heat or drought stress.
Electron micrographs of Xylella fastidiosa in xylem vessels
Photographs by Dr. Doug Cook.
It is important to distinguish between two groups of grapevines: susceptible
cultivars and tolerant cultivars. Once a susceptible cultivar is
infected, there is no known, approved method of treating the infection
and the disease will most probably be fatal to the vine. Cultivars vary
in the length of time it takes the pathogen to cause vine death. Tolerant
cultivars appear to have internal mechanisms of suppressing bacterial numbers
to the point that the vine can live and be productive even in the presence
of the bacteria. There is preliminary evidence that some non-susceptible
cultivars may in fact be resistant to infection. All native Texas species
of Vitis are believed to be tolerant of PD, which potentially makes
them carriers of the bacterium. As a consequence, removal of adjacent,
wild grapevines is imperative to disease management.
Pierce's disease infection is dependent upon the presence of a susceptible
host, a source of the bacteria, and an insect vector to inoculate the susceptible
host. In addition to native grapevines, there are other indigenous plant
species that harbor the bacteria without visual symptoms. Surveys in California
have identified several alternate host, but in our area, where the disease
is endemic, there are undoubtedly many more plant species capable of supporting
the causal agent.
Although a serious problem to commercial grape growers on the West Coast,
PD is not native to California, but was probably
introduced from the Gulf Coast through infected grapevines. Three or
four species of sharpshooters are believed to be the most important vectors
in those areas. It is likely that there are numerous species of sharpshooters
that can potentially transmit the bacteria in Texas. Work continues to
identify these insects, determine their preferred habitat, and understand
population dynamics. Sharpshooters do prefer certain habitats. Bermudagrass,
perennial rye, fescue grass, blackberry, willow, and elderberry provide
important food sources or egg-laying sites for some sharpshooters. Sources
of water are also essential to supporting sharpshooter populations, so
choosing sites away from these rivers, creeks or ponds can aid in insect
Images of several Sharpshooters know to transmit PD.
Photographs by Jim Medley, Beaumont Agricultural Research and Extension Center.
Most, if not all, sharpshooter species go through five larval, or instar
stages in which they apparently loose the ability to transmit the disease
with each molt. In areas of rampant infection, it is assumed that alternate
sources of the bacterium are widely available. Keeping vineyards and adjacent
areas free of potential alternate hosts is essential for long-term management
of Pierce's disease. Monitoring insect populations, especially after habitat
disturbance such as cutting of adjacent hay fields, can greatly assist
growers in the judicious use of insecticides.
There are numerous symptoms expressed by susceptible cultivars after
infection. The first symptom is usually uneven marginal leaf necrosis that
often appears near the point of infection. Since the disease inhibits water
movement in the vine, symptoms often appear during heat stress or near
veraison (color change) in the cluster.
Leaf scorch caused by PD
The clusters of heavily infected vines may actually collapse during
this time of high water and carbohydrate movement.
Cluster collapse at veraison caused by PD
Another diagnostic symptom of PD is the abscission of leaf blades from
shoots with retention of leaf petioles. In addition, as winter approaches,
new shoots become woody and develop periderm on one-year-old shoots. This
periderm formation usually begins at the basal portion of a shoot and progresses
toward the growing tip. In infected grapevines, periderm formation is not
uniform, usually resulting in green "islands" at the nodal area while the
internodal portion of the stem becomes brown.
Irregular patches on infected stem tissue.
While each of these symptoms can be confused with one or more other
non-related factors, the occurrence of several symptoms together provides
strong suspicion of infection in a susceptible host.
Pierce's disease Probability in Texas
In the mid 1970's, Dr. Ron Perry published a bulletin entitled A
Feasibility Study for Grape Production in Texas which included the
following figure detailing the expected presence of Pierce's disease in
Expected Probability of Pierce's Disease in Texas
At that time, it was postulated that the range of the disease was limited
by the natural range of insect vectors. Cold temperatures have been shown
to be therapeutic to plants infected with Xylella, but exact duration
and absolute temperatures have not been identified. It is now believed
that P.D. is limited to areas which do not receive severe winter temperatures.
After a series of warm winters in Texas, outbreaks of PD were confirmed
in vineyards previously thought to be in low probability areas.
The Texas Hill Country, long thought to be a transition zone between
high and low probability for PD, experienced several warm winters in the
mid 1990's after which several vineyards were found to be completely infected.
Prospective growers should realize that this disease is cyclic and that
infections are likely to occur. Vineyard survival will ultimately depend
on site selection, cultural practices that reduce the risk of widespread
economic loss, weather and to some degree, luck.
1996 Positive Elisa Tests Results for Pierce's Disease
Conventionally, an" ELISA", or antiseral reaction test is used to confirm
suspected cases of Pierce's disease in Texas. Problems encountered a few
years ago were due to defective antiserum distributed by the manufacturer.
This procedure is still recommended for growers wishing to confirm Pierce's
disease infection in grapevines.
For research purposes, polymerase chain reaction technology (PCR) is
used detect the presence of the causal bacteria. This test, which is approximately
10,000 times more sensitive that the ELISA test will be helpful in confirming
suspected insect vectors and alternate hosts of the bacteria.
MANAGEMENT OF PIERCE'S DISEASE OF GRAPE
Because there is no known control for Pierce's disease, the act of planting
susceptible cultivars in areas where P.D. is known to exist assumes an
Remove Wild Grapevines
In Texas, wild hosts of the grape pathogen have not been identified.
In other states, grape strains of Xylella fastidiosa have been isolated
from wild grape, ragweed, alfalfa and almond trees. As a precaution, it
is recommended that wild grapes be removed from around the vineyard
Remove Diseased Vines
Based on foliage and cane symptoms confirmed by laboratory diagnosis,
diseased plants should be immediately destroyed. Regardless of varietal
tolerance, any vine with symptoms of this disease should be pulled up or
cut off at the ground and removed from the vineyard. Since observations
indicate that the disease can spread from vine to vine within the vineyard,
removal of diseased vines reduces the potential sources of inoculum that
could be transmitted by insect vectors.
The disease is vectored by certain kinds of xylem-feeding insects, mainly
the leafhopper group known as sharpshooters. All of the insect species
responsible for vectoring PD in Texas are not known at this time. There
are species of leafhoppers that inhabit Texas vineyards and adjacent wild
hosts that look like sharpshooters that are not known to vector P.D. Sharpshooters
tend to be significantly larger than other species of leafhoppers found
in and adjacent to vineyards.
The difficulty of vector management as a means to manage P.D. is the
inability to identify all potential vectors within and adjacent to the
vineyard, so chemical control of vectors is tenuous at best. Nonetheless,
the current thinking in California is that vector transmission occurs primarily
from host plants adjacent to the vineyard, so California growers practice
vector control in areas adjacent to the vineyard. Growers should use caution
when choosing insecticides to insure that specific pesticide labels permit
The pattern of PD spread in Texas more closely parallels that observed
in Florida where significant vine-to-vine spread of the disease occurs.
This would indicate that insecticidal control of vectors within the vineyard
may also be needed..
Based on the best information available, the following vector control
recommendations are suggested:
Establish and maintain a 150 foot buffer (minimum) around the vineyard
through mechanical or chemical mowing or cultivation.
The California experience would indicate that the greatest danger from
transmission of PD through sharpshooter vectors is shortly after budbreak
and decreases as the season progresses. Starting at budbreak and
continuing for 6 weeks, sample the vegetation in the area outside and adjacent
to the buffer, or in the absence of a buffer, sample the vegetation adjacent
to the vineyard.
Sampling consists of using a standard sweep net and taking a minimum
of eight 25-sweep samples at least twice a week. If adult sharpshooter
numbers exceed an average of 1 per 25-sweep sample, insecticidal treatment
may be justified.
Treat a 65-foot band adjacent to the buffer or a 130-foot band adjacent
to the vineyard in the absence of a mowed buffer. If it is not possible
to treat adjacent vegetation, it might be appropriate to treat the vineyard
itself. The problem with this approach is that if the alternate host reservoir
for the sharpshooter vectors is large and the buffer is small or absent,
then within vineyard treatments may be ineffectual in keeping sharpshooters
out. Twice a week spraying for 4 to 6 weeks following budbreak may be necessary,
but only if sweep samples indicate that a threshold population has been
Care should be exercised in judiciously using insecticides. Unfortunately,
the greater the number of sprays, the more likely secondary pest outbreaks
will be created, especially with spider mites.
Use an insecticide registered for use for the target area. In most cases
(and for all sites external to the vineyard), sharpshooters are not listed
as a target pest on the label. Specific use restrictions for grapes and
alternate hosts will be found on the label.
Vineyard Floor Management
Because there is limited information as to other species may serve as
a source of the P.D. organism, many growers are utilizing clean cultivation
to eliminate any possible inoculum source within the vineyard. Weed growth
under the trellis can be controlled with cultivation, or herbicides, but
management of the vineyard floor between the rows has become problematic.
Clean cultivation can have serious drawbacks such as the potential for
serious soil loss due to erosion. The use of cover crops in vineyard row
centers has several advantages over cultivation including increased equipment
mobility, the preservation of soil structure within the vineyard and erosion
Because at this point we do not know what plant species constitute propagative
alternate hosts of Pierce's disease, the decision on what plant species
growers should plant or encourage on the vineyard floor is still only a
guess. In light of these considerations, it may be wise to plant (drill
or no-till seed) cool season, annual cover crops such as annual rye grass
or oats in October and encourage cover crop growth during the months that
grapevines are dormant. These annual plants have a low probability of contracting
the causal bacterium and would be growing during a period when transmission
to grapevines is not believed to occur.
Cover crop height can be managed by mowing and is easily controlled
during the spring with low rate glyphosate applications. This practice
keeps cover crop roots in place to support equipment traffic, helps reduce
erosion and establishes an organic material layer that inhibits the germination
of indigenous weed species. When annual rye grass is used for this purpose,
additional suppression of weed seed germination may be observed due to
the allelopathic properties of rye. Additional applications of glyphosate
or glufosinate can be used throughout the growing season to keep developing
weed populations in check. Pre-emergence herbicides can also be incorporated
into a vineyard floor management program.
The Management program was formulated by the Pierce's Disease Advisory
Panel. This is an interdisciplinary working group, made up of representatives
from the commercial grape industry, as well as members of the Department
of Plant Pathology and Microbiology, Department of Entomology and Department
of Horticulture, Texas A&M University.
of Pierce's Disease in Texas
Texas AgriLife Extension Service
Key to Leafhoppers and Sharpshooters
Texas A&M AgriLife Research
Grape Pierce's Disease
University of California Pest Management Guidelines
Pierce's Disease - Xylella fastidiosa web site
University of California, Berkeley
Glassy-winged Sharpshooter Website
Napa and Sonoma County Agricultural Commissioner's Offices