Xylella fastidiosa causes Pierce’s disease, almond leaf scorch, alfalfa dwarf and oleander leaf scorch. With the arrival of the glassy-winged sharpshooter, scientists became concerned that Xylella fastidiosa would be spread to new hosts, or that new strains of Xylella might show up on new hosts. Both appear to have happened.

Xylella in other species

Cooksey reported detecting the bacterium in or near vineyards in some plant species that had not been reported as hosts before. Cooksey and UCR entomologist Heather Costa also found possible new strains of Xylella in Southern California occurring in diseased liquidamber, olive, mirror plant and ornamental plum. Diseased plants of all four species tested positive for the bacterium, whereas nearby healthy plants were negative.

Cooksey cautioned, “The detection of Xylella fastidiosa does not necessarily mean that the bacterium is causing disease in these hosts; other pathogens or abiotic factors may be causing the observed symptoms. We are still doing genetic tests to determine whether the olive or other hosts harbor ‘new’ strains of Xylella, or if it is just existing strains now showing up on these new hosts.” 

They also detected X. fastidiosa in the weeds Spanish broom and wild mustard. UCB entomologist Sandy Purcell noted that Research Associate Tina Wistrom had found that most of the weeds in the San Joaquin Valley that were tested proved to support some multiplication of Xylella. The researchers are trying to identify which weeds might be important to control to prevent Pierce's disease spread.  

PD in Coachella Valley

The table grape-growing region of Coachella Valley has been home to the glassy-winged sharpshooter since the mid 1990s but didn't show signs of Pierce's disease until last summer. UCR entomologist Tom Perring said that 13 vines from two adjacent vineyards tested positive for Pierce's disease in July 2002. The infected vines were removed.

Historically, the region had experienced an outbreak of Pierce's disease in 1985, thus Perring said the bacteria likely is present at a low incidence throughout the Valley.  However, with increasing numbers of the glassy-winged sharpshooter, a vector of the bacterium that causes the disease, there is reason for grape growers to keep a vigilant watch for infected vines.

Because some of the mature vines were systemically infected, Perring said, "they probably have been infected for more than two years." Perring, along with Riverside County viticulture advisor Carmen Gispert, will continue to monitor PD in the area's vineyards.