2.3.1
FATE OF A PIERCE'S DISEASE STRAIN OF XYLELLA FASTIDIOSA IN RIPARIAN WOODLAND PLANTS IN NORTHERN CALIFORNIA

AH PURCELL and SR SAUNDERS

Division of Insect Biology, University of California, Berkeley, CA 94720-3112, USA

Background and objectives
Xylella fastidiosa is a xylem-inhabiting bacterium that causes Pierce's disease (PD) of grapevines and plant diseases in numerous perennial plants [1]. Most evidence suggests that PD in California spreads to cultivated grapes from outside the vineyards, so the fate of X. fastidiosa in natural vegetation near vineyards may be important. Studies in the 1940s showed that vectors could acquire the causal pathogen of PD from 75 out of 100 plant species tested (reviewed in [1]). More recently, X. fastidiosa multiplied in four out of five species studied but moved systemically within the plant in only two and with many fewer bacteria than in grape. Threshold populations for vector acquisition of X. fastidiosa are about log 4 live cells per gram of plant tissue [2].

The fate of X. fastidiosa was studied in over 30 species of perennial, riparian (stream bank) plants. We selected the most common species from areas where the incidence of PD in Napa Valley, California was highest.

Materials and methods
Field plants were inoculated in April to early June, 1995-96 by introducing cultured cells of a PD strain of X. fastidiosa via needle puncture. Seedlings or rooted cuttings of the same plants were inoculated with the same PD strain using insects in small cages [2], then kept the plants in a glasshouse. Samples were taken immediately adjacent to the needle puncture or from the caged section, usually 3-4 months after inoculation. Samples were also taken 20 cm from the inoculation site to detect systemic movement. Some plants were sampled 1 year later to evaluate over-winter survival of the infection. Bacterial populations per gram of plant tissue [2] were estimated. Homogenized tissue samples (0.1 g/2 ml water) from most of the species tested were evaluated for their inhibition of the growth of X. fastidiosa in vitro.

Results and conclusions
X. fastidiosa multiplied to concentrations of log 4 to, occasionally, log 6 cells/g, but with no evidence of systemic movement for both field and laboratory tests in the following plants: California laurel (Umbellularia), coast live oak and valley oak (Quercus), spicebush (Calycanthus), Oregon ash (Fraxinus), Algerian ivy (Hedera), coyote brush (Baccharis), California mugwort (Artemisia), stinging nettle (Urtica) and poison oak (Toxicodendron). Higher populations (log 6 or greater) and regular or occasional systemic movement of X. fastidiosa occurred in Himalayan blackberry and California blackberry (Rubus), big-leaf maple (Acer), California buckeye (Aesculus), French broom (Genista), greater periwinkle (Vinca), elderberry (Sambucus), poison hemlock (Conium), and umbrella sedge (Cyperus). No bacteria were cultured from coffeeberry (Rhamnus), California black walnut (Juglans), Fremont cottonwood (Populus), toyon (Heteromeles), white alder (Alnus), or three species of willow (Salix). No X. fastidiosa from in vitro subcultures grew in log 4 dilutions of homogenized samples from coffeeberry or black walnut. California laurel homogenates inhibited growth by over 60% (most other species ranged from 5 to 20%). We readily cultured X. fastidiosa from alder and three species of willows 1-4 weeks after vector inoculation but rarely after 12 weeks, and with no evidence of systemic movement, which explains our initial failure to culture from these plants. We conclude that PD strains of X. fastidiosa multiply to low levels (below log 6/gram) without systemic movement and eventually die out in most woody plants. Such plants would support diffuse, transient populations of X. fastidiosa. Systemic hosts generally supported the highest populations of X. fastidiosa and could best serve as reservoirs for vector acquisition by expanding acquisition sites and harboring the bacterium for longer periods.

References
1. Purcell AH, Hopkins DL, 1996. Annual Review of Phytopathology 34, 131-51.
2. Hill BL, Purcell AH, 1997. Phytopathology 87, 1197-201.