Pierce’s Disease in the San Joaquin Valley

Karl Lund, UCCE Madera, Merced & Mariposa Counties

Pierce’s disease is caused by the bacterium Xylella fastidiosa. These bacteria live within xylem, the vascular tissue through which water travels in a plant. As the bacteria population grows it stimulates the plant to produce tyloses, cellular outgrowths that plug xylem vessels. The bacteria and tyloses cause vessel plugging which restricts water movement in the plant, thus causing many of the disease symptoms. These blockages will eventually lead to the vine’s death. It is estimated that Pierce’s disease costs the California grape industry $56.1 million a year in lost productivity (Tumber et al. 2014). To minimize losses, it is important to understand the biology of the disease, including the bacteria’s host range, how the bacteria moves from plant to plant, and how to identify infected plants will help growers prevent losses and control the disease.

The bacterium X. fastidiosa has a large known plant host range. The European Food Safety Authority maintains a database of known hosts for X. fastidiosa; (an updated list approved in April of 2020 can be found at https://doi.org/10.2903/j.efsa.2020.6114). Other local crop plants such as almond, citrus, olive, peach, plum, and alfalfa can all host the bacteria. None cropping trees such elm, maple, oak, sycamore, and mulberry can also serve as host to the bacteria.

Ornamental plants such as oleander, Spanish broom, and periwinkle, as well as weeds such as ragweed and wild mustard can all host to the bacteria. Overall, at least 350 host plants have been identified from over 75 plant families as hosts for X. fastidiosa. From a control standpoint once X. fastidiosa has been introduced to a geographic area it will be virtually impossible to eliminate from that location with such a wide variety of possible hosts.

X. fastidiosa does have another level of complexity. To date four distinct subspecies of X. fastidiosa have been identified.  X. fastidiosa ssp. fastidiosa is the subspecies that causes Pierce’s disease in grapevine, while X. fastidiosa ssp. multiplex is the subspecies that causes almond leaf scorch (Rapicavoli et al. 2018). In theory this would reduce the number of potential hosts for the Pierce’s disease causing form in the environment. Unfortunately, while X. fastidiosa ssp. fastidiosa is unable to cause almond leaf scorch, it is still able to survive in almond trees albeit at reduced concentrations. With the reverse being true for X. fastidiosa ssp. multiplex (Almeida and Purcell 2003). While this does mean that almond trees with almond leaf scorch would be unable to act as a source for Pierce’s disease in grapevine. It also means that almond trees would be able to host X. fastidiosa ssp. fastidiosa with no outward symptoms.

Bacteria within the xylem tissue of one plant may be spread to another plant through the feeding activities of certain xylem-feeding insects. In vineyards two groups of insects have been identified as possible vectors: sharpshooters and spittlebugs. Spittlebugs have been shown to vector X. fastidiosa in controlled settings, but their importance as a Pierce’s disease vector in vineyards is unclear. Sharpshooters on the other hand, are known to be effective vectors of Pierce’s disease in vineyards.

There are several different sharpshooters that in California vector X. fastidiosa. The most important of these in the coastal portions of California is the blue-green sharpshooter. This sharpshooter is not adapted to the hotter climate of the San Joaquin Valley (SJV). In the SJV, we have 3 other sharpshooters: the green sharpshooter (Draeculacephala minerva), the red- headed sharpshooter (Xyphon fulgida), and the glassy-winged sharpshooter (Homalodisca vitripennis). Image 1 shows all three sharpshooters, as well as contains a size comparison of the three sharpshooters in the lower corner of each image.

Image 1. From left to right the green, red-headed and glassy-winger sharpshooters. Size comparison measurements are in the bottom left of each individual picture. Photos by Jack K. Clark © Regents of the University of California

The green sharpshooter and the red-headed sharpshooter are both small and prefer to feed on grasses. The red-headed sharpshooter is specifically drawn to and reproduces on Bermudagrass. Both the green and the red-headed sharpshooters can be found in irrigated pastures and along waterways such as stream, creeks, canals, and ditches. Neither of these sharpshooters prefers to feed on grapevines, however they may do so under certain conditions and thus transmit Pierce’s disease. However, since neither of these sharpshooters prefer to feed on grapevines, they tend not to spread deeply into vineyards, so when these vectors transmit Xylella, it is usually only to grapevines along the edges of a vineyard, whereas vines in the middle, or the sides away from the green or red-headed sharpshooters’ preferred habitat, are not affected.

The glassy-winged sharpshooter is twice the size of either of the other two sharpshooters. Their large size makes them more dangerous as a vector for Pierce’s disease because they can travel further than smaller sharpshooters and feed more effectively on a wider variety of plants, including woody plants such as grapes. To date over 350 plants have been identified as hosts of glassy-winged sharp shooter: https://www.cdfa.ca.gov/pdcp/Documents/HostListCommon.pdf . Many of the hosts for glassy- winged sharpshooters are also hosts for X. fastidiosa. One of the key hosts for both glassy- winged sharpshooters and X. fastidiosa in the SJV, and for local control of Pierce’s disease is citrus. The large feeding range of the glass-winged sharpshooter also means that it can spread X. fastidiosa throughout the vineyard, instead of just to the edges.

Image 2. April of 2020 California Department for Food and Agriculture map of glassy-winged sharpshooter distribution within central and southern California. Full map available at: https://www.cdfa.ca.gov/pdcp/Maps/GWSS_Distribution.jpg

The glassy-winged sharpshooter is not a native California insect, only arriving in California in the late 1980’s (first recorded in 1989). As this non-native pest is such a dangerous vector the CDFA tracks their distribution. A portion of the 2020 map covering the San Joaquin Valley and southern California can be seen in Image 2. Most of Kern county, parts of Tulare and Fresno Counties, and a very small sliver of Madera county just over the San Joaquin River from the city of Fresno near highway 41 all host to naturalized populations of glassy-winged sharpshooters.

Identification of glassy-winged sharpshooters within, and near these areas is important for controlling both their spread, as well as the spread of Pierce’s disease. Features on its body are helpful to identify him. From the top the insect has a deep brown color with creamy white dots on the head and thorax (Image 3A). These colors and dots continue onto the abdomen, however here they are covered with transparent wings (the source of their glassy name).

Highlighting the glassy wings are red lines and patches which can be seen from both the top and side (Image 3B). The other main identifying mark is the flat white marking along the side of the abdomen. When sitting on a stem this white mark stands out under and through the wings of this sharpshooter. Younger nymph glassy-winger sharpshooters (Image 3C) have yet to develop their namesake wings. Their bodies are a lighter grayish brown with very small white dots. For this stage, the standout feature is their red eyes. The red is the same color that will soon highlight the parent’s wings. Later stage nymphs (Image 3D) have started to transition to the adult body color, and the red color in the eye is mostly lost. However, the red color has transitioned onto the wing pads in a pattern that has started to develop the adult wing’s patterning.

Image 3. Images of Glassy-Winged Sharpshooter. All images were taken with insect on top of a UC ANR business card for sizing purposes. A: Top view of adult Glassy-Winged Sharpshooter. B: Side view of adult Glassy-Winged Sharpshooter. C: Top view of young nymph Glassy-Winged Sharpshooter. D: side view of late stage nymph Glassy-Winged Sharpshooter.

Monitoring for glassy-winged sharpshooters can be done using yellow sticky cards. It is recommended to use cards that are at least 5.5” x 9” in size. One card should be placed for every 10 acres and checked weekly for recent activity. Monitoring should be done from budbreak through November. If a glassy-winged sharpshooter is found, and you are outside of a known population center, please contact your local agriculture commissioner’s office or cooperative extension office. Green and red-headed sharpshooters are not attracted to yellow sticky cards, so to monitor their populations you will need to use a sweep net. Sweep lush green grasses near and within your vineyard in April and May to assess population size. For both green and red-headed sharpshooters finding 2 adults in 50 sweeps warrants a response.

Unfortunately, as both of these sharpshooters are only incidentally on grapevines, treating the grapevines won’t help the situation. The preferred habitat (lush grassy areas) will need to be addressed. For glassy-winged sharpshooters a single find warrants a response.  A list of treatment options for glassy-winged sharpshooters can be found on the UC IPM webpage (http://ipm.ucanr.edu/PMG/r302301711.html).

Image 4. Pierce’s disease leaf symptoms. Leaves margins turn yellow (or red in red varieties) then burn back from the margins to center in patches.

Early identification of infected vines is the final step in preventing a larger problem from Pierce’s disease. Infected vines can be a source of the disease for vectors to spread to neighboring vines. They are also a strong indicator that the bacteria and a vector are present in your location. The leaves of infected vines will turn yellow (for green varieties) or red (for red varieties) along the margins. This discoloration will then work inwards from the margin with the discoloration quickly turning to brown/dried dead tissue. This often happen unevenly or in sections (Image 4). Affected leaves eventually fall off but will sometimes leave the petiole still attached to the shoot (Image 5). Shoot tissue also shows an uneven maturation process leaving green islands within lignified brown tissue (Image 6). Not all these symptoms will be found on every infected vine. If you suspect a vine is infected with Pierce’s disease you can contact your counties viticulture advisor for corroboration. However, ultimately a diagnostic analysis is required to confirm the presence of X. fastidiosa in the suspected vine. Table 1 lists laboratories within California that offer Pierce’s disease testing.

Pierce’s Disease Testing Laboratories (Checked June 2020)
Lab Name	City	Phone	Website
Agri-Analysis Associates	Davis, CA	800-506-9852	http://www.agri-analysis.com/
AL&L Crop Solutions	Vacaville, CA	530-387-3270	https://allcropsolutions.com/
CSP Labs	Pleasant Grove, CA	916-655-1581	https://csplabs.com/
Eurofins BioDiagnostics	Gilroy, CA	408-846-9964	https://www.eurofinsus.com/biodiagnostics/
FPS, University of California, Davis	Davis, CA	530-752-3590	https://fps.ucdavis.edu/index.cfm
Table 1 California Laboratories that offer Pierce’s disease testing

References:

Almeida, R. P. P. and Purcell, A. H. 2003 Biological Traits of Xylella fastidiosa Strains from Grapes and Almonds. Appl. Environ. Microbiol. 69(12) 7447-7452.

Rapicavoli, J., Ingel, B., Blanco-Ulate, B. Cantu, D. and Roper, C. 2018. Xylella fastidiosa: an examination of a re-emerging plant pathogen. Mol. Plant Pathol., 19(4), 786–800.

Tumber, K. P, Alston, J. M, & Fuller, K. 2014. Pierce’s disease costs California $104 million per year. California Agriculture, 68(1-2).