By Natalie Swinhoe, Anthropology and Evolution, Ecology, and Biodiversity, 2015
Pierce’s Disease in grapevines is a major threat to California’s viticultural economy. Caused by the bacterial strain Xylella fastidiosa, the disease blocks water transfer in the xylem of stems, leading to water stress and eventual death. Until the 1990s, the only carriers for the disease were native Blue-Green Sharpshooters, Graphocephala atropunctata. However, between 1994 and 2000, a devastating outbreak occurred in Southern California, destroying more than 1000 acres of vineyards (Ringenberg et al., 2014). This epidemic was caused by a new nonnative vector- the Glassy-Winged Sharpshooter, Homalodisca vitripennis (Tumber et al, 2013). Compared to the Blue-Green Sharpshooter, the Glassy-Winged Sharpshooter has a much greater capacity to spread Pierce’s Disease because it can fly further and feed on a larger variety of plant parts (Alston et al., 2013, Baccari and Lindow, 2010).
Transmission of Xylella in grapevines occurs in the water-conducting xylem tissue inside the vine stem (Ringenberg et al.,2014). Sharpshooters feed on this tissue and water loss to the plant may occur but the fruit remains undamaged in non-infected plants (Tumber et al., 2013). When the Xylella bacteria make contact with a vine stem, through the saliva of an infected sharpshooter, it blocks the xylem by attaching to the walls and producing biofilms (Navarrete and De La Fuente, 2014). This biofilm is a form of growth in which bacteria cells aggregate in an adhesive polysaccharide matrix and attach to the xylem surface, resulting in reduced water transport and prolonged water stress (Bi et al., 2007). A few weeks after grape xylem has been infected with Xylella, vines show symptoms of chronic water depression including: leaf chlorosis, marginal scorching, fruit shriveling, and vine dwarfing (Navarrete and De La Fuente, 2014, Baccari and Lindow 2010). The Xylella pathogen has the ability to move both axially and radially in xylem vessels, increasing the number of colonies as the disease progresses (Baccari and Lindow, 2010). As vessel colonization increases, the plant is less able to mobilize water to its upper extremities, eventually leading to progressive water deprivation and death (Lindow et al., 2013). When a vector feeds on a plant infected with Xylella, the bacteria will attach to the insect’s mouthparts and foregut, enabling it to continue spreading the disease as it feeds on different vines (Navarrete and De La Fuente, 2014, Tumber et al., 2013). This is an effective transmission mechanism that may lead to large outbreaks of the disease.
In 2000, the Pierce’s Disease Control Program was introduced in California to minimize the impact of infections (Tumber et al., 2013). Since then, more than $48 million has funded government agencies, nursery industries, and the University of California system to find a solution to Xylella outbreaks (Tumber et al., 2013). Current research in preventing transmission of Pierce’s Disease falls into two categories, vector management and disease management.
Sharpshooters are the primary vectors of Xylella fastidiosa as they are xylem sap feeding specialists. Adult sharpshooters carry the pathogen in their foregut as a monolayer and cells are released in saliva into the plant xylem tissue during feeding (Navarrete and De La Fuente, 2014, Tumber et al., 2013, Almeida et al., 2005). The Glassy-Winged Sharpshooter is the most important vector in Pierce’s Disease transmission because of its high mobility, extreme polyphagy, and wide distribution on susceptible crops (Alston et al., 2013, Baccari and Lindow, 2010, Almeida et al., 2005). As a result, vector management has become an important area of research in order to reduce incidence of Pierce’s Disease. Management tactics focus on reducing the number of vectors in a large area, keeping them out of vineyards, and manipulating their interactions with grapevines (Almeida et al., 2005).
One major class of vector management is chemical sprays. These include biocontrol agents such as insecticides (neonicotinoids) and repellents (Almeida et al., 2005). Insecticides rely on lethal and sub-lethal doses over time to control insect populations (Castle et al., 2005). Studies of neonicotinoid effects on Glassy-Winged Sharpshooter populations have occurred using irrigation systems equipped with injection sites (Castle et al., 2005). A statistically significant decline in sharpshooter densities was recorded over six weeks after being treated by the neonicotinoid imidacloprid (Castle et al., 2005). This treatment has been effective in reducing the density of nymph sharpshooters, however, the insecticide is not effective at keeping adult sharpshooters away from vines (Castel et al., 2005). Research has also demonstrated that spraying nearby agricultural fields may help stop the spread of adult Glassy-Winged Sharpshooters from moving onto grape plots (Prabhaker et al., 2007). Kaolin application is another well-studied method of chemical barriers. Using kaolin on grapevines creates a particle film that disrupts insect behavior (Almeida et al., 2005). In one field study, Almeida et al. found that combining neonicotinoids and kaolin resulted in a 50-75% reduction in infection incidence versus untreated controls (2005).
A second method of vector management involves the use of physical barriers to increase spatial distribution or to restrict vector movement. One way to increase spatial distribution is to remove infected grapevines to prevent healthy vines from being exposed to the pathogen (Almeida et al., 2005). This can reduce the spread of Pierce’s Disease from vine to vine. However, by the time the grapevine shows symptoms of the disease, the removal of the plant may not be an effective enough method to reduce Xylella transmission (Almeida et al., 2005). Other physical methods have also been employed to restrict sharpshooter movements. Five-meter high screen barriers were tested as a method to reduce vector migration (Almeida et al., 2005). However, establishing and maintaining fences and screens can be time consuming and expensive, especially around land that encompasses thousands of acres (Alston et al., 2013). Although these physical barriers are less implemented, they manage the vectors by increasing the distance between feeding sites which reduces the ability of sharpshooters to find a continuous food supply (Yoon et al., 2014).
Disease management focusses on exploring ways in which Xylella fastidiosa can have a less severe effect on grapes. Research in this area is ongoing and trials require years of preparation and many seasons of grape harvesting in order to study the implications of Pierce’s Disease. Currently, there are two major areas of disease management under investigation. These are transmission studies using diffusible signal factors and genetic trials.
Diffusible Signal Factor Studies
When Xylella fastidiosa attaches to a grapevine’s xylem, it produces an unsaturated fatty acid signal molecule which has been named diffusible signal factor (DSF) (Lindow et al., 2011). The accumulation of this signal molecule suppresses the virulence of Xylella in grapes because it disrupts the development of cell-to-cell signaling (Lindow et al., 2011). Researchers have been able to increase the DSF levels in laboratory tests by transforming the grape with rpfF genes (Lindow et al., 2011). In one study, Lindow et al. found that grape plants possessing certain rpfF genes were much less susceptible to the Xylella pathogen (2011). Another study artificially elevated DSF levels in grapes and discovered that DSF accumulation is associated to regulation of motility, biofilm formation, and virulence (Beaulieu et al., 2013). Virulence is reduced because the biofilm increases in adhesiveness- greatly reducing its ability to spread throughout the plant (Beaulieu et al., 2013).
New research has shown that DSF also has an influence on Xylella’s ability to adhere to the mouthparts of sharpshooter vectors (Baccari et al., 2013). Using green fluorescent protein (GFP) bioluminescent markers, Baccari et al. demonstrated that pathogen cells with high levels of DSF are less able to attach to the sharpshooters’ mouths (2013). Creating a method for artificially inflating the DSF levels in grape populations may be a significant step in reducing the infection rates of Pierce’s Disease.
Many genetic studies surrounding the Xylella pathogen use transgenes from wild grape genomes. Wild grapes are not used in commercial wines because of their low quality; however, they are resistant to Pierce’s Disease. The molecular basis of their resistance is still unknown. Using plasmid cloning, laboratories are trying to isolate genes from wild grapes that accounts for their lack of susceptibility. This type of research can take years to clone genes, transform DNA, and grow grape stem cells into plants. Nevertheless, experiments with transgenes are slowly progressing towards genetically modified plants that may be completely resistant to infection. In one ongoing transgene study, six anti-Programmed Cell Death genes were screened for ability to suppress Pierce’s Disease, and when expressed in Thompson Seedless grape plants, two grape sequences were able to suppress symptoms and reduce bacterial levels in inoculated plants (Gilchrist and Lincoln, 2011). Both DNA sequences were found to reduce the bacterial titre to a level found in resistant wild grapes (Gilchrist and Lincoln, 2011).
Pierce’s Disease is a serious threat to California’s $3 billion agriculture economy. The disease continues to cost California’s grape and wine industries more than $104 million per year, with $50 million spent per year on infestation management (Alston et al., 2013). Fortunately the outbreak in California during the 1990’s has not spread north of Temecula Valley. If the Glassy-Winged Sharpshooter manages to migrate and infest the Napa Valley and Sonoma Valley regions, California’s wine industries could lose more than $185 million per year (Alston et al, 2013). In a state-wide outbreak model, Alston et al. calculated that the Napa-Sonoma regions could stand to lose more than 100,000 vines if Glassy-Winged Sharpshooters migrated north (2013). Decreasing the virulence of Xylella fastidiosa by protecting existing vines and creating transgenic plants might be the key to preventing a statewide outbreak and protecting California’s wine industries.
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Photo credit: Jill Wellington (jill111) licensed by CC BY 2.0