Management of Riparian Woodlands for Control of Pierce’s Disease in Coastal

The third year of this continuing project evaluated reducing Pierce’s disease (PD) of grapevines while restoring stream bank woodlands in north coastal California. By replacing wild grape, blackberry, and other plants used by BGSS for breeding with plants that are not favored by the BGSS for reproduction or feeding, the numbers of blue-green sharpshooters (BGSS) that we detected in yellow sticky traps were again reduced to significantly lower levels compared to undisturbed controls. Our tests of buffer strip plantings of redwood and Douglas fir between vineyards and riparian woodlands as a barrier to reduce the influx of BGSS into vineyards during spring have thus far been disappointing because of the long times required to generate buffer plants of a suitable size. Our second goal is to reduce the percentage of BGSS that are infective with the Pierce’s disease bacterium (Xylella fastidiosa) by replacing plants that support the multiplication, within-plant movements, and year-round survival of X. fastidiosa with plants that do not. Since June, 1997 we continued to record significantly decreased trap catches of BGSS in the removal and replanting treatments. Overall sticky trap catches of BGSS were reduced over 99%in the treatment in which vegetation was removed and replanted at our first site. At our second site (Napa River), where vegetation was first removed beginning in the growing season of 1996, we found that BGSS counts were reduced by over 70%, but most of the catches in the treated site were at one trap location, located near hillside vegetation that connected to the riparian zone. We established pretreatment (baseline) records of BGSS activity at a third site on Maacamas Creek in Sonoma County, where we began vegetation removal in late 1997, continued through the spring of 1998. Spring catches of BGSS in all sites were sharply lower than in previous years, probably because of cooler temperatures and rain. Effects of reduced numbers of BGSS on the spread of Pierce’s disease to adjacent vineyards will have to await the establishment of new vineyards being planted along some of the experimental areas.