PCC_MayJun_2019_e

Pierce’s Disease and
Glassy-winged Sharpshooter:
Still a Threat to California Viticulture
By STEPHEN VASQUEZ | Technical Viticulturist, Sun-Maid Growers
First known as Anaheim grapevine
disease or California vine
disease, Pierce’s disease (PD) has
impacted California’s grape production
since the late 1880’s. Grape
growers had been losing vineyards
to an unidentifiable disease, which
prompted the US Government to hire
Newton B. Pierce, the first United States
Department of Agriculture (USDA)
plant pathologist and namesake of the
disease. Pierce had spent considerable
time walking vineyards in Los Angeles,
San Bernardino and Orange Counties
where Muscat of Alexandria used for
raisins and Mission and other wine
grape varieties were dying at alarming
rates (2). At the time, approximately
25,000 acres had been infected and or
lost to an unknown “malady”. Pierce
also spent time traveling to France, Italy
and other Mediterranean grape growing
regions studying plant disease symptom
expression and declining grapevines
to compare with those found in California.
After researching all aspects
of California viticulture production,
including pests, diseases and associated
symptoms, Pierce could never correctly
identify the cause of California vine disease.
For many years, a plant virus was
thought to be the culprit of PD. It wasn’t
until the mid-1970’s when University
of California (UC) researchers isolated
a bacterium from diseased vines. Once
isolated, the bacteria were reintroduced
to healthy grapevine plants that developed
Pierce’s disease symptoms within
two to four months (1). The bacterium
is known to move from vine to vine
with the help of insect vectors representing
the sharpshooter (Cicadellidae)
and spittlebug (Cercopidae) families.
Infected insects ease of movement into
a vineyard can be devastating in a few
seasons when a PD susceptible grape
variety is planted.
Although Pierce’s disease outbreaks
occurred in California vineyards from
time to time since being ID’d it was not
a primary issue for the grape industry.
Major raisin, table and wine grape
growing regions had moved north
into the San Joaquin and Sacramento
Valleys and the central and north
coast. The mild, southern California
weather was the perfect environment
for the pathogen, vectors and disease
development. In contrast, the
seasonality of the interior valleys and
coastal grape growing regions seemed
to result in a lower PD incidence year
to year. However, there were PD “hot
spots” located near riparian areas
(i.e. Napa and Kings Rivers) or alfalfa
planting that experienced significant
vine deaths. Those hot spots were costly
to individual farming operations but
was not a concern for the industry.
That changed when the disease/vector
dynamics shifted.
In 1999, the nonnative PD vector,
glassy-winged sharpshooter (GWSS),
arrived in Temecula Valley. At that time
a once thriving southern California
wine industry was experiencing rapid
vine death. GWSS turned out to be an
effective vector and superior flyer when
compared to native sharpshooters.
Additionally, vineyards planted
next to citrus proved to be a deadly
combination. GWSS used citrus groves
to feed, breed and for protection from
potential predators from late fall to early
spring. California grape growers were
concerned about their future as they
watched Temecula Valley vineyards
die. As GWSS spread to other parts of
California, PD became a much greater
concern and problem. According to K.P.
Tumber et. al (3) California growers are
48 Progressive Crop Consultant May/June 2019