Wednesday, September 21, 2011

Pierce's Disease

Some Thoughts on the History of Pierce’s Disease
With Special Reference to Southern California Vineyards


Dr. Stephen Krebs
Napa Valley College

In the fall of 1977, I was hired as vineyard manager for San Pasqual Vineyards in Escondido, California.  We had a beautiful 100 acre vineyard that had been planted in 1974 and a nice new modern winery, a real contrast with the several other old-fashioned wineries in San Diego’s North County region at that time.  The wine industry was expanding rapidly in the first big planting boom since the repeal of Prohibition.  It was a heady time in the Southland, but trouble was looming ahead.

In the spring of 1977, I had completed my Master of Science degree in horticulture, following a Bachelor of Science in plant science the year before.  My emphasis was on grape growing and I tried to soak up as much knowledge as I could from the great professors of that era.  During my years in the Viticulture and Enology department at UC Davis, there had been a lot of discussion about Pierce’s Disease.  A grant to the university by the Napa Valley Vintners Association had funded new research on the problem.  There was hope that a solution would soon be found.

The disease had been named for USDA plant pathologist Dr. Newton Pierce.  In the 1880s, he had been summoned by the vintners in the Southern California town of Anaheim to study a mysterious problem that was devastating the vineyards in that region.  Dr. Pierce thought that a bacterium must certainly be causing the destruction and he was correct in this assumption.  What he did not know was that surrounding vegetation was harboring the deadly organism and that tiny insects known as blue-green sharpshooters were transmitting the bacterium to the nearby vineyards.  He recommended removal of dead vines.

Strangely, the disease seemed to disappear after a short time and interest in its cause waned.  While it had initially been called Anaheim Disease, later it bore the name of its first scientific researcher.  However, in the meantime, the Southern California grape industry had been seriously decimated and viticulture became a minor endeavor in the area.  Wine production shifted almost completely to the northern parts of the state.

For a long time, no one really worried much about Pierce’s Disease.  Then it suddenly reappeared in the Fresno area in the 1930s, where it did significant damage before it receded as it had years earlier in Southern California.  Then in the 1960s, it wreaked havoc in the North Coast and got the attention of the Napa Valley’s growers and winemakers.

Historically, the identity of the causal organism has been a matter of some confusion.  Dr. Pierce had thought it was a bacterium, but a review of the scientific literature shows that for a while it was believed to be a virus, later a mycoplasma and then it was finally and definitively shown to be the bacterium Xylella fastidiosa.  The scientific name means “limited to the xylem”, the water-conducting tissue of the grape vine.  It is believed to be a native of the humid and warm Gulf Coast region of the US Deep South and was accidentally introduced into Southern California in the late nineteenth century.

With the renewal of interest in viticulture during the 1970s in Southern California, many new grape growing enterprises were established in San Diego and Riverside counties, where the vine was first cultivated after its introduction into the state by the Spanish padres in the 1700s.  The first major disaster since Anaheim Disease had destroyed the local industry during the 1880s was about to occur in Escondido.

I had returned to Northern California as vineyard manager at Mayacamas Vineyards following the 1979 vintage, but was retained as a viticultural consultant by San Pasqual Vineyards after my departure.  Following a series of several wet winters, the new vineyard manager there had begun to notice odd foliar symptoms like leaf mottling and erratic bud break in the spring, followed by leaf scorching and desiccated fruit in the summer and fall.  It certainly looked like the Pierce’s Disease problem that had been merely an academic subject for me up until that time.  Shockingly, in just a few short years all of the vines had been completely killed.  A lush and productive vineyard was now a graveyard for dead grape vines.  Unfortunately, it was a scene that would be repeated fifteen years later and 30 miles north on a much larger scale in the region known as Temecula.

In the late 1980s the glassy-winged sharpshooter had been accidentally introduced into Southern California.  This was a much more formidable vector of Pierce’s Disease than the native blue-green type which had spread the bacterium through my old vineyard in Escondido.  Like Xylella fastidiosa, it was also a native of the Deep South, where it was adapted to the warm and humid climate.  While we may think of Southern California as a dry area, it is actually quite humid with its proximity to the ocean and the prevailing mild temperature conditions.  The latitude of the area is very much like the Gulf Coast area, so day length is similar.  All of this makes it a very suitable location for the glassy-winged sharpshooter.  The extensive citrus industry would also play an important role in Temecula, because the glassy-winged sharpshooter feeds and reproduces very well in the orange groves that were mixed with vineyards in the Riverside County growing area.

In 1999, a very serious outbreak of Pierce’s Disease occurred in the vineyards of Southern California.  This time it was fueled by the combination of a new and vigorous vector, more wet weather and an abundance of newly-planted vineyards.  The area was badly damaged and for a while the very existence of grape growing in the south was brought into doubt.  Statewide, the presence of this new vector caused great alarm.

Since this time, the Southern California wine industry has recovered to a large extent, driven by the removal of citrus trees and a more benign weather pattern.  However, it is worth pondering the question of why this part of the state has been the site of all of the major outbreaks of Pierce’s Disease since the late 1880s.

Based on the history of the region, it is clear that a combination of factors in the south must be very favorable to periodic large-scale development of the disease.  First of all, the climate is similar to the place of origin of the bacterium.  There is an abundant population of sharpshooter vectors, both the native blue-green and the introduced glassy-winged.  In addition, the area teems with alternate host plants which harbor the disease organism.  And in a pattern we see almost everywhere in California, aspiring vintners are busily planting the susceptible grape vine host near Pierce’s Disease habitats.  The result has repeatedly been heavy losses to this destructive scourge of the grape vine.

SK: May 2007

Dr. Stephen KrebsProgram Coordinator and Vineyard Manager
Dr. Krebs runs the viticulture and winery technology program at Napa Valley College. As part of his duties, he teaches viticulture classes and manages the student vineyard. He holds a doctorate, master of science and bachelor of science from the University of California at Davis, all focusing on viticulture. He has also served as viticulturist and manager for Sunny Slope Ranch in Glen Ellen, Matanzas Creek Winery in Santa Rosa, and Mayacamas Vineyards in Napa. Stephen performed field research on grape varieties in Europe and California for author Jancis Robinson’s book Vines, Grapes and Wine.

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