Little is known about colonization of roots of trees by Phytophthora ramorum. We examined zoospore concentration and exposure time needed to infect six Quercus (oak) species and the inoculum produced from their roots. Sprouted acorns, exposed to zoospores (3,000/ml) for different times and transplanted to potting soil, were susceptible to infection within 1 h of exposure but root weights were not impacted after 4 weeks (P = 0.952). Roots of Quercus prinus seedlings, inoculated with sporangia, had 0.6 to 3.2% colonization of the total root mass after 5 months. Neither root lesions nor obvious root sloughing were observed. Inoculum threshold levels were tested by exposing radicles to varying zoospore concentrations for 24 h. Results showed that radicle infection occurred even at 1 zoospore/ml. To test inoculum production, roots were inoculated with sporangia and transplanted into pots. Periodically, samples of runoff were collected and plated on selective medium. Afterward, root segments were plated to calculate percent colonization. After 16 and 35 days, root colonization and inoculum production from oak was lower than that of Viburnum tinus, a positive control. This study shows that P. ramorum is able to infect sprouted oak acorns and produce secondary inoculum, which may be important epidemiologically.