The fate of Xylella fastidiosa, the bacterium that causes Pierce's disease of grape, was assessed in 29 species of plants associated with vineyards in California's San Joaquin Valley. Bacterial populations and movement in greenhouse-grown plants were measured 1, 3, and 9 weeks after mechanical and insect inoculation. X. fastidiosa was recovered in 27 of 29 species in greenhouse tests, with common sunflower (Helianthus annuus), cocklebur (Xanthium strumarium), annual bur-sage (Ambrosia acanthicarpa), morning glory (lpomoea purpurea), horseweed (Conyza canadensis), sacred datura (Datura wrightii), poison hemlock (Conium maculatum), and fava bean (Vicia faba cv. Aquadulce) being infected in more than 50% of inoculation attempts. Twenty-three species supported bacterial populations in excess of 10(4) CFU/g of plant tissue. X. fastidiosa populations increased for 9 weeks after inoculation in six species, and were static or declined in 16 species. Although the blue-green sharpshooter (Graphocephala atropunctata) was more efficient than mechanical inoculation for infecting plants, median populations and systemic movement for the two methods did not differ significantly. X. fastidiosa colonization of greenhouse and field-grown plants was compared in five alternate hosts. X. fastidiosa was recovered from 23 and 34% of field-grown plants, compared with 49 and 67% of greenhouse-grown plants, in the winter and summer, respectively, with at least 10 times fewer bacteria in field plants in the winter. Although X. fastidiosa has a wide host plant range, its fate in most species is variable and heavily influenced by field conditions.